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Forest decline events have increased worldwide over the last decades, being holm oak one of the tree species with the most worrying trends across Europe. Previous research identified drought and soil pathogens as the main causes behind holm oak decline. However, despite tree health loss is a multifactorial phenomenon where abiotic and biotic factors interact in time and space, there are some abiotic factors whose influence has been commonly overlooked. Here, we evaluate how land use (forests versus savannas), topography, and climate extremes jointly relate to spatiotemporal patterns of holm oak defoliation over almost three decades (1987-2014) in Spain, where holm oak represents the 25% of the national forested area. We found an increasing defoliation trend in 119 of the total 134 holm oak plots evaluated, being this increase rate significantly higher in forests compared to savannas. Moreover, we have detected that the interaction between topography and summer drought can explain trends of holm oak decline across the Mediterranean region. While a higher occurrence of dry summers increases defoliation trends in complex terrains where forests dominate, an inverse relationship was found at flatter terrains where savannas are located. Our results contribute to growing evidence of the influence of local topography, tightly linked to potential soil water availability, on forest functioning, as it can shape forest vulnerability against climate extremes. The present work could assist the identification of potential tree decline hotspots over the Mediterranean region. Moreover, our findings suggest that forest adaptive management will be key to guarantee the health and future stability of Mediterranean oak ecosystems, especially in the topographically more complex areas where tree vulnerability to climate extremes may be greater. This work was supported by a Juan de la Cierva-Incorporación postdoctoral contract IJC2020-045630-I and the MANAGE4FUTURE project (TED2021-129499A-I00) both funded by MCIN/AEI /10.13039/501100011033 and the European Union NextGenerationEU/PRTR. Ana-Maria Hereş was financed by the REASONING (PNIII-P1-1.1-TE-2019-1099) project through UEFISCDI (link; Romanian Ministry of Education and Research). This research was supported by the BERC 2018-2021 (Basque Government), and BC3 María de Maeztu Excellence Accreditation 2018-2022, Ref.MDM-2017-0714 (Spanish Ministry of Science, Innovation and Universities). defoliation coppices dehesas climatic anomalies tree decline multidecadal trends Spain Mediterranean forest Published

As drought-induced tree defoliation and mortality (i.e. tree decline) in the Mediterranean is expected to worsen with ongoing climate change, it is of paramount importance to understand how, why, and when tree decline affects soil respiration (Rs). We carried out a novel study exploring the interacting effects of climatic variability (i.e. season and year) and tree decline on soil water content (SWC), soil temperature (Tsoil), and Rs temporal variability in a Mediterranean holm oak woodland. The study further explores the effects of tree decline on the main controls of Rs at the stand scale (i.e. plant variables, SWC, Tsoil, and soil physicochemical variables). We monitored Rs, Tsoil, and SWC under the canopy of 30 holm oak trees with different defoliation degrees (healthy, affected, and dead) during two years of contrasting precipitation patterns. We estimated different plant structural variables (e.g. height, canopy diameter) on those selected trees under whose canopies we also collected soil samples to analyze different soil physicochemical variables. Our study provides, up to our knowledge, the first observational (i.e. in situ) evidence that tree decline might decrease the positive response of Rs to increased precipitation and drying-rewetting cycles. It also suggests that tree decline can significantly increase SWC and decrease Rs but largely depending on the declining stage, the year, and the season. Finally, tree decline affected the relative importance of the different drivers of Rs, with both SWC and Tsoil gaining importance as trees defoliate and die. Altogether, our results point towards a negative impact of drought-induced tree decline on soil carbon (C) content and cycling, particularly under forecasted climate change scenarios with dryer and more intense precipitation regimes., This research was supported by the Spanish National Research Council (CSIC) in the JAE-doc modality co-financed by the European Social Fund (ESF), the ATLANTIS (PID2020–113244GB-C21) projects funded by the Spanish Government, the Basque Government through the BERC 2022–2025 program, and the Spanish Ministry of Science and Innovation through the BC3 María de Maeztu excellence accreditation (MDM-2017–0714). J.D. and A.R. acknowledge support from the FCT (2020.03670.CEECIND and SFRH/BDP/108913/2015, respectively), as well as from the MCTES, FSE, UE, and the CFE (UIDB/04004/2021) research unit financed by FCT/MCTES through national funds (PIDDAC)., With funding from the Spanish government through the "Severo Ochoa Center of Excellence" accreditation MDM-2017–0714.

Climate change and pathogen outbreaks are the two major causes of decline in Mediterranean holm oak trees (Quercus ilex L. subsp. ballota (Desf.) Samp.). Crown-level changes in response to these stressful conditions have been widely documented but the responses of the root systems remain unexplored. The effects of environmental stress over roots and its potential role during the declining process need to be evaluated. We aimed to study how key morphological and architectural root parameters and nonstructural carbohydrates of roots are affected along a holm oak health gradient (i.e. within healthy, susceptible and declining trees). Holm oaks with different health statuses had different soil resource-uptake strategies. While healthy and susceptible trees showed a conservative resource-uptake strategy independently of soil nutrient availability, declining trees optimized soil resource acquisition by increasing the phenotypic plasticity of their fine root system. This increase in fine root phenotypic plasticity in declining holm oaks represents an energy-consuming strategy promoted to cope with the stress and at the expense of foliage maintenance. Our study describes a potential feedback loop resulting from strong unprecedented belowground stress that ultimately may lead to poor adaptation and tree death in the Spanish dehesa., This research was mainly funded by the Spanish Government through the IBERYCA project (CGL2017-84723-P), its associated FPI scholarship BES- 2014-067971 (ME-V) and SMARTSOIL (PID2020- 113244GB-C21). It was further supported by the BC3 Marıa de Maeztu excellence accreditation (MDM-2017-0714; the Spanish Government) and by the BERC 2018–2021 and the UPV/EHUGV IT-1018-16 programme (Basque Government). Additionally, this research was further supported through the ‘Juan de la Cierva programme’ (MV; IJCI-2017-34640; the Spanish Government) and one project funded by the Romanian Ministry of Research, Innovation and Digitization through UEFISCDI (A-MH; REASONING, PN-III-P1-1.1-TE-2019-1099).

New knowledge on soil structure highlights its importance for hydrology and soil organic matter (SOM) stabilization, which however remains neglected in many wide used models. We present here a new model, KEYLINK, in which soil structure is integrated with the existing concepts on SOM pools, and elements from food web models, that is, those from direct trophic interactions among soil organisms. KEYLINK is, therefore, an attempt to integrate soil functional diversity and food webs in predictions of soil carbon (C) and soil water balances. We present a selection of equations that can be used for most models as well as basic parameter intervals, for example, key pools, functional groups’ biomasses and growth rates. Parameter distributions can be determined with Bayesian calibration, and here an example is presented for food web growth rate parameters for a pine forest in Belgium. We show how these added equations can improve the functioning of the model in describing known phenomena. For this, five test cases are given as simulation examples: changing the input litter quality (recalcitrance and carbon to nitrogen ratio), excluding predators, increasing pH and changing initial soil porosity. These results overall show how KEYLINK is able to simulate the known effects of these parameters and can simulate the linked effects of biopore formation, hydrology and aggregation on soil functioning. Furthermore, the results show an important trophic cascade effect of predation on the complete C cycle with repercussions on the soil structure as ecosystem engineers are predated, and on SOM turnover when predation on fungivore and bacterivore populations are reduced. In summary, KEYLINK shows how soil functional diversity and trophic organization and their role in C and water cycling in soils should be considered in order to improve our predictions on C sequestration and C emissions from soils., This article is based upon work from COST Actions FP1305 (BioLink) and ES1406 (KEYSOM), supported by COST (European Cooperation in Science and Technology), and their Short Term Scientific Mission (STSM) programs. Omar Flores’ work was funded by FPU PhD grant program of the Spanish Ministry of Science, Innovation and Universities. Jorge Curiel Yuste received funding from the Spanish Ministry of Economy and Competitiveness (MINECO) under projects IBERYCA (CGL2017-84723-P) and the BC3 María de Maeztu excellence accreditation (MDM-2017-0714). Jorge Curiel Yuste also received funding from the Basque Government through the BERC 2018-2021 program., We also acknowledge institutional support from the Unit of Information Resources for Research at the Unit of Information Resources for Research at the "Consejo Superior de Investigaciones Científicas" (CSIC) for the article-processing charges contribution.

In this paper, site-specific allometric biomass models were developed for European beech (Fagus sylvatica L.) and silver fir (Abies alba Mill.) to estimate the aboveground biomass in Șinca virgin forest, Romania. Several approaches to minimize the demand for site-specific observations in allometric biomass model development were also investigated. Developing site-specific allometric biomass models requires new measurements of biomass for a sample of trees from that specific site. Yet, measuring biomass is laborious, time consuming, and requires extensive logistics, especially for very large trees. The allometric biomass models were developed for a wide range of diameters at breast height, D (6&ndash, 86 cm for European beech and 6&ndash, 93 cm for silver fir) using a logarithmic transformation approach. Two alternative approaches were applied, i.e., random intercept model (RIM) and a Bayesian model with strong informative priors, to enhance the information of the site-specific sample (of biomass observations) by supplementing with a generic biomass sample. The appropriateness of each model was evaluated based on the aboveground biomass prediction of a 1 ha sample plot in Șinca forest. The results showed that models based on both D and tree height (H) to predict tree aboveground biomass (AGB) were more accurate predictors of AGB and produced plot-level estimates with better precision, than models based on D only. Furthermore, both RIM and Bayesian approach performed similarly well when a small local sample (of seven smallest trees) was used to calibrate the allometric model. Therefore, the generic biomass observations may effectively be combined with a small local sample (of just a few small trees) to calibrate an allometric model to a certain site and to minimize the demand for site-specific biomass measurements. However, special attention should be given to the H-D ratio, since it can affect the allometry and the performance of the reduced local sample approach.

Dehesas, human-shaped savannah-like ecosystems, where the overstorey is mainly dominated by the evergreen holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.), are classified as a global conservation priority. Despite being Q. ilex a species adapted to the harsh Mediterranean environmental conditions, recent decades have witnessed worrisome trends of climate-change-induced holm oak mortality. Holm oak decline is evidenced by tree vigour loss, gradual defoliation and ultimately, death. However, before losing leaves, trees undergo leaf-level physiological adjustments in response to stress that may represent a promising field to develop biochemical early markers of holm oak decline. This study explored holm oak photoprotective responses (pigments, tocopherols and photosynthetic performance) in 144 mature holm oak trees with different health statuses (i.e., crown defoliation percentages) from healthy to first-stage declining individuals. Our results indicate differential photochemical performance and photoprotective compounds concentration depending on the trees' health status. Declining trees showed higher energy dissipation yield, lower photochemical efficiency and enhanced photoprotective compounds. In the case of total violaxanthin cycle pigments (VAZ) and tocopherols, shifts in leaf contents were significant at very early stages of crown defoliation, even before visual symptoms of decline were evident, supporting the value of these biochemical compounds as early stress markers. Linear mixed-effects models results showed an acute response, both in the photosynthesis performance index and in the concentration of foliar tocopherols, during the onset of tree decline, whereas VAZ showed a more gradual response along the defoliation gradient of the crown. These results collectively demonstrate that once a certain threshold of leaf physiological damage is surpassed, that leaf cannot counteract oxidative stress and progressive loss of leaves occurs. Therefore, the use of both photosynthesis performance indexes and the leaf tocopherols concentration as early diagnostic tools might predict declining trends, facilitating the implementation of preventive measures to counteract crown defoliation.

Altres ajuts: COST Actions BIOLINK (FP1305) and KEYSOM (ES 1406). Jorge Curiel Yuste received funding by the Basque Government through the BERC 2018-2021 program. The relatively poor simulation of the below-ground processes is a severe drawback for many ecosystem models, especially when predicting responses to climate change and management. For a meaningful estimation of ecosystem production and the cycling of water, energy, nutrients and carbon, the integration of soil processes and the exchanges at the surface is crucial. It is increasingly recognized that soil biota play an important role in soil organic carbon and nutrient cycling, shaping soil structure and hydrological properties through their activity, and in water and nutrient uptake by plants through mycorrhizal processes. In this article, we review the main soil biological actors (microbiota, fauna and roots) and their effects on soil functioning. We review to what extent they have been included in soil models and propose which of them could be included in ecosystem models. We show that the model representation of the soil food web, the impact of soil ecosystem engineers on soil structure and the related effects on hydrology and soil organic matter (SOM) stabilization are key issues in improving ecosystem-scale soil representation in models. Finally, we describe a new core model concept (KEYLINK) that integrates insights from SOM models, structural models and food web models to simulate the living soil at an ecosystem scale.

12 Pags.- 34Figs.- 3 Tabls., The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species remain unclear. We used an extensive network of tree‐ring width (RWI, ring‐width index) records of 16 tree species from 567 forests, and high‐resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982‒2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended‐only RWI and NDVI series (i.e. after removing long‐term growth trends) and pre‐whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity. We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first‐year drought legacies were negative and different from random in 9% and 5% of the RWI and NDVI series respectively. The number of significant second‐ and third‐year legacies was substantially lower. Differences between RWI and NDVI legacies indicate that canopy greenness and radial growth responses to drought are decoupled. We found variations in legacies between tree species with gymnosperms presenting larger first‐year drought legacies than angiosperms, which were exposed to less severe droughts. Greater growth variability can explain the presence of first‐year RWI legacies in gymnosperms from dry sites despite that the relationship between growth variability and legacies was complex. Synthesis. Accounting for species and site responses to drought provides a better understanding of the magnitude and duration of drought legacies on forest growth and productivity. Despite the widespread occurrence of growth reductions in the years during and after drought occurrence, significant legacies were not very common, mostly lasted one year, and were more widespread in gymnosperms. These are relevant factors to be considered in the future when studying the consequences of drought on forest productivity and tree growth., This study was financially supported by: Xunta de Galicia, Grant/Award Number: PGIDIT06PXIB502262PR, GRC GI‐1809; INIA, Grant/Award Number: RTA2006‐00117; CANOPEE, 2014‐2020‐FEDER funds; and Spanish Science Ministry: RTI2018‐096884‐B‐C31, RTI2018‐096884‐B‐C33, AGL2017‐83828‐C2‐2R and ENV4‐CT97‐0641 projects. GSB was supported by a ‘Juan de la Cierva Formación’ grant from MINECO (FJCI 2016‐30121). This research was supported by the BERC 2018‐2021 program, and by the Spanish Ministry of Science, Innovation and Universities through the BC3 María de Maeztu excellence accreditation (MDM‐2017‐0714) and the IBERYCA (CGL2017‐84723‐P) project. R.S.S. was supported by VULBOS project (UPO‐1263216, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014‐2020), and A.H. by PinCaR project (UHU‐1266324, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014‐2020). K.N. was supported by the Ministry of Education, Science and Sports (Republic of Slovenia, Raziskovalci‐2.1‐UL‐BF‐ C3330‐19‐952011).

Andalusian regional government project GEOCARBO [P08-RNM-3721]; European Union ERDF funds; Spanish Ministry of Science and Innovation project SOILPROF [CGL2011-15276-E]; Spanish Ministry of Science and Innovation project CARBORAD [CGL2011-27493]; Spanish Ministry of Science and Innovation project GEISpain [CGL2014-52838-C2-1-R]; Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme, DIESEL project [625988]

Mediterranean forests will experience more frequent and intense drought periods and extreme rainfall events in the coming decades. Concomitantly, drought-induced forest die-off is likely to increase. Changes in rainfall patterns and forest die-off directly influence soil microbial communities and activity and, consequently, carbon (C) and nitrogen (N) turnover, but their interactive effects have not yet been explored. We investigated the short-, and the long-term interactive effects of forest die-off and drying-rewetting cycles on soil C and N mineralization rates of a Mediterranean woodland. Soil samples collected under and out of the influence of holm oak (Quercus ilex) trees with different defoliation degrees (six healthy, six affected and six dead) were incubated under two contrasting water regimes (i.e. drying-rewetting cycles vs. constant soil moisture). Potential soil C and N mineralization responded differently to water regimes, with an overall 55% increase in C mineralization and a 22% decrease in N mineralization in the drying-rewetting cycle treatment compared to the constant moisture treatment. Holm oak decline decreased the response of C mineralization while increased the response of N mineralization to the drying-rewetting cycles at both the short- and the long-term. Moreover, N turnover showed a higher sensitivity to these environmental changes than that of C during most of the year. Our study provides solid evidence that an intensification of the drying-rewetting regimes can result in a decoupling of soil C and N cycles in Mediterranean forests and that forest die-off might enhance this decoupling at both the short- and the long-term, with important implications for the ecosystem functioning., This study was supported by the International Laboratory of Global Change (LINCGlobal, CSIC4540), the Spanish Ministry of Economy and Competitiveness grant VERONICA (CGL2013-42271-P), the Community of Madrid grant REMEDINAL3-CM (S2013/MAE-2719) and the FCT/MEC through national funds and the co-funding by the FEDER (PT2020 Partnership Agreement and COMPETE 2020, UID/BIA/04004/2013). The authors are especially grateful to David López-Quiroga and Ana Prado Comesaña for their excellent help in the field and laboratory and to Aldo Barreiro for his assistance with data and statistical analysis. AR was supported by the Spanish National Research Council (CSIC) in the JAE-doc modality co-financed by the European Social Fund (ESF) and by a Postdoctoral Grant of the Portuguese Science and Technology Foundation (SFRH/BDP/108913/2015).

Research Highlights: Tree decline can alter soil carbon cycling, given the close relationship between primary production and the activity of roots and soil microbes. Background and Objectives: We studied how tree decline associated to old age and accelerated by land-use change and increased drought in the last decades, affects soil properties and soil respiration (Rs). Materials and Methods: We measured Rs over two years around centennial European beech (Fagus sylvatica L.) trees representing a gradient of decline in a sub-Mediterranean forest stand, where the number of centennial beech trees has decreased by 54% in the last century. Four replicate plots were established around trees (i) with no apparent crown dieback, (ii) less than 40% crown dieback, (iii) more than 50% crown dieback, and (iv) dead. Results: Temporal variations in Rs were controlled by soil temperature (Ts) and soil water content (SWC). The increase in Rs with Ts depended on SWC. The temperature-normalized Rs exhibited a parabolic relationship with SWC, suggesting a reduced root and microbial respiration associated to drought and waterlogging. The response of Rs to SWC did not vary among tree-decline classes. However, the sensitivity of Rs to Ts was higher around vigorous trees than around those with early symptoms of decline. Spatial variations in Rs were governed by soil carbon to nitrogen ratio, which had a negative effect on Rs, and SWC during summer, when drier plots had lower Rs than wetter plots. These variations were independent of the tree vigor. The basal area of recruits, which was three times (although non-significantly) higher under declining and dead trees than under vigorous trees, had a positive effect on Rs. However, the mean Rs did not change among tree-decline classes. These results indicate that Rs and related soil physico-chemical variables are resilient to the decline and death of dominant centennial trees. Conclusions: The development of advanced regeneration as overstory beech trees decline and die contribute to the Rs homeostasis along forest succession. © 2019 by the authors. We are thankful to Nicolas Martin-St.Paul and Pepa Aroca for insightful comments on data analyses, and Eva Miranda for technical assistance. J.R.-C. acknowledges the support of the Ministry of Economy, Industry and Competitiveness through the “Ramón y Cajal” Programme. J.C.Y. is indebted to the Spanish projects IBERYCA (CGL2017-84723-P), the BC3 María de Maeztu excellence accreditation (MDM-2017-0714) funded by the Ministry of Economy and Competitiveness (MINECO) and the Basque Government through the BERC 2018-2021 program. R.S. is supported by the FWO and European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie fellowship (grant agreement no. 665501).

Woody encroachment is a widespread phenomenon resulting from the abandonment of mountain agricultural andpastoral practices during the last century. As a result, forests have expanded, increasing biomass and necromass car-bon(C) pools. However,the impact on soil organic carbon(SOC) isless clear.The main aim of this study wasto in-vestigate the effect of woody encroachment on SOC stocks and ecosystem C pools in six chronosequences located along the Italian peninsula, three in the Alps and three in the Apennines. Five stages along the chronosequences were identified in each site. Considering the topsoil (0–30 cm), subsoil (30 cm-bedrock) and whole soil profile, the temporal trend in SOC stocks was similar in all sites, with an initial increment and subsequent decrement inthe intermediate phase. However, thefinal phase of the woody encroachment differed significantly between the Alps (mainly conifers) and the Apennines (broadleaf forests) sites, with a much more pronounced increment inthe latter case. Compared to the previous pastures, after mature forest (N62 years old) establishment, SOC stocksincreased by: 2.1(mean) ± 18.1(sd) and 50.1 ± 25.2 Mg C·ha−1in the topsoil, 7.3 ± 17.4 and 93.2 ±29.7 Mg C·ha−1in the subsoil, and 9.4 ± 24.4 and 143.3 ± 51.0 Mg C·ha−1in the whole soil profile in Alps and Apennines, respectively. Changes in SOC stocks increased with mean annual air temperature and average minimum winter temperature, and were negatively correlated with the sum of summer precipitation. At the same time, all other C pools (biomass and necromass) increased by 179.1 ± 51.3 and 304.2 ± 67.6 Mg C·ha−1in the Alps and theApennines sites, respectively. This study highlights the importance of considering both the subsoil, since deep soillayers contributed 38% to the observed variations in carbon stocks after land use change, and the possible repercussions for the carbon balance of large areas where forestsare expanding, especially under pressing global warming scenarios., The project of this work is part of the research activities of the PhD in science, technology and biotechnology for sustainability. The first author received a fully founded scholarship partially by the University of Tuscia (Viterbo - Italy) and partially by the University of Molise. Tommaso Chiti participated in the project by conducting his work with the funding obtained through the LIFE MediNet project (grant number LIFE15 PRE IT/732295). Jorge Curiel Yuste was financed in part by the Basque Government through the BERC 2018-2021 (grant code) program and by Spanish Ministry of Economy and Competitiveness (MINECO) through BC3 María de Maeztu excellence accreditation MDM-2017-0714. I.C (grant code).

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems., This work was supported in part by the Spanish Ministry of Economy and Competitiveness projects SOILPROF (CGL2011-15276-E), GEISpain (CGL2014-52838-C2-1-R), CARBORAD (CGL2011-27493), VERONICA (CGL2013-42271-P), IBERYCA (CGL2017-84723-P), Junta de Andalucía project GLOCHARID, including European Union ERDF funds; and by the European Commission project DIESEL (PEOPLE-2013-IOF-625988). RV acknowledges support from the University of Delaware, Universidad de Almería, Universidad de Granada, and Estación Experimental de Zonas Áridas (EEZA-CSIC) to visit the study site, and support from NSF (Award #1652594) while writing this manuscript.

This study examines the effects of climate and the degree of forest fragmentation legacies on response of oak to drought. A microcosm approach was set up with holm oak seedlings from three provenances grown in soils coming from two regions of contrasting climate (drier vs. wetter), and three scenarios of forest fragmentation (low, mid, and high agricultural matrix influence). We measured different indicators of the plant-soil system functioning such as ecosystem respiration, net ecosystem exchange, gross primary productivity, stomatal conductance, quantum yield, biomass allocation, and mycorrhization. Legacies of the bioclimatic region and the degree of forest fragmentation on soil properties drove the response to drought of an experimental plant-soil system, masking the effects of seedling provenance. The system was functionally more resistant to drought in soils from forest fragments with more agricultural influence and from the drier region. Our results indicate that the degree of forest fragmentation and bioclimatic legacies on soil properties exerted a much more decisive effect on the response of the plant-soil system to drought than holm-oak seedling provenance., This work was supported by the Spanish Ministry for Economy and Competitiveness (MINECO) with the projects VULGLO (grant number CGL2010 22180 C03 03), VERONICA (grant number CGL2013-42271-P) and MyFUNCO (grant number CGL2011-29585-C02-02), and by the Comunidad de Madrid with the project REMEDINAL 3-CM (grant number S2013/MAE-2719). DFR held a pre-doctoral fellowship awarded by the Mexican Council of Science and Technology (CONACyT, grant number 310849).

Many tree species from Mediterranean regions have started to show increased rates of crown defoliation, reduced growth, and dieback associated with the increase in temperatures and changes in the frequency and intensity of drought events experienced during the last decades. In this regard, Quercus ilex L. subsp. ballota [Desf.] (Holm oak), despite being a drought-tolerant species widely distributed in the Mediterranean basin, it has recently started to show acute signs of decline, extended areas from Spain being affected. However, few studies have assessed the role of climatic variability (i.e., temperature, precipitation, and drought) on the decline and resilience of Holm oak. Here, we measured secondary growth of seventy Holm oaks from a coppice stand located in central Spain. Sampled trees had different stages of decline, so they were classified into four vigour groups considering their crown foliar lost: healthy (0%), low defoliated (, This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) with the project VERONICA (CGL2013-42271-P), and by the Spanish Ministry for Innovation and Science with the grant Consolider-Montes (CSD2008_00040).

Aboveground plant activity influences fine roots and rhizosphere activity, which is reflected on soil respiration (SR). However, it is still unclear and poorly understood the nature of plant activity control over SR, especially under drought conditions. We studied the plant activity-SR relationship at different timescales in a water-limited mixed Mediterranean forest where Scots pines (Pinus sylvestris L.) are undergoing drought-induced die-off and are being replaced by the more drought-resistant Holm oak (Quercus ilex L.). Half-hourly sap flow (SF), as a proxy of photosynthesis, coupled with measures of SR using solid-state CO2 sensors, were monitored during nine months in four different trees, representative of the diversity and health condition of the forest. SF was strongly associated with SR at both daily and seasonal timescales. At daily timescales, almost no lags were found between SF and SR, indicating a fast control of photosynthesis on SR. However, the association between SF and SR weakened during the summer drought. These temporal patterns were not constant across the trees representing the die-off and replacement processes. SR beneath living pines was highly controlled by SF at daily scale, whereas Holm oak seemed to be more controlled by SF at seasonal scale. The relationship between SF and SR measured beneath dead pine and Holm oak at the daily and seasonal scales was consistent with the colonization of soil gaps by holm oak roots following Scots pine death and suggests that surviving Scots pines are unable to expand their root system in these gaps. Our results collectively show how drought modulates the link between canopy photosynthesis and soil respiration, and increase our understanding on how belowground processes may be affected by the successional dynamics following drought-induced forest mortality. © SISEF. The authors thank D. Riveros-Iregui for providing technical details for the set-up of the CO2 sensors and M. Mencuccini and R. Vargas for their valuable comments. This study was supported by the Spanish Government projects SECASOL (CGL2009-081 01).

We studied key mechanisms and drivers of soil functioning by analyzing soil respirationand enzymatic activity in Mediterranean holm oak forest fragments with different influence of the agricultural matrix. For this, structural equation models (SEM) were built including data on soil abiotic (moisture, temperature, organic matter, pH, nutrients), biotic (microbial biomass, bacterial and fungal richness), and tree-structure related (basal area) as explanatory variables of soil enzymatic activity and respiration. Our results show that increased tree growth induced by forest fragmentation in scenarios of high agricultural matrix influence triggered a cascade of causal-effect relations, affecting soil functioning. On the one hand, soil enzymatic activity was strongly stimulated by the abiotic (changes in pH and microclimate) and biotic (microbial biomass) modifications of the soil environment arising from the increased tree size and subsequent soil organic matter accumulation. Soil CO2 emissions (soil respiration), which integrate releases from all the biological activity occurring in soils (autotrophic and heterotrophic components), were mainly affected by the abiotic (moisture, temperature) modifications of the soil environment caused by trees. These results, therefore, suggest that the increasing fragmentation of forests may profoundly impact the functioning of the plant-soil-microbial system, with important effects oversoil CO2 emissions and nutrient cycling at the ecosystem level. Forest fragmentation is thus revealed as a key albeit neglected factor for accurate estimations of soil carbon dynamics under global change scenarios. Fil: Flores Rentería, Dulce. Consejo Nacional de Ciencia y Tecnología; México. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados; México. Consejo Superior de Investigaciones Cientificas. Museo Nacional de Ciencias Naturales; España Fil: Rincón, Ana. Instituto de Ciencias Agrarias; España. Consejo Superior de Investigaciones Científicas; España Fil: Morán López, Teresa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio de Ecotono; Argentina. Consejo Superior de Investigaciones Cientificas. Museo Nacional de Ciencias Naturales; España Fil: Heres, Ana María. Transilvania University of Brasov; Rumania. Universidad del País Vasco; España Fil: Pérez Izquierdo, Leticia. Consejo Superior de Investigaciones Científicas; España. Instituto de Ciencias Agrarias; España Fil: Valladares, Fernando. Consejo Superior de Investigaciones Cientificas. Museo Nacional de Ciencias Naturales; España. Universidad Rey Juan Carlos; España Fil: Curiel Yuste, Jorge. Universidad del País Vasco; España. Instituto Vasco de Investigación y Desarrollo; España

The chapter IV of the thesis: FLORES-RENTERÍA, Dulce. Effects of forest fragmentation on the plant‐soil‐microbial interactions = Efectos de la fragmentación del encinar en las interacciones planta‐suelo‐microorganismos (2016), is published in: Flores‐Rentería, D., J. Curiel Yuste, A. Rincón, F. Brearley, J. García‐Gil and F. Valladares (2015). Habitat Fragmentation can Modulate Drought Effects on the Plant‐soil‐microbial System in Mediterranean Holm Oak (Quercus ilex) Forests. Microbial Ecology: 1‐15. doi: 10.1007/s00248‐015‐0584‐9. http://hdl.handle.net/10261/128289, El capítulo IV de la tesis doctoral: FLORES-RENTERÍA, Dulce. Efectos de la fragmentación del encinar en las interacciones planta‐suelo‐microorganismos = Effects of forest fragmentation on the plant‐soil‐microbial interactions (2016, está publicado en: Flores‐Rentería, D., J. Curiel Yuste, A. Rincón, F. Brearley, J. García‐Gil and F. Valladares (2015). Habitat Fragmentation can Modulate Drought Effects on the Plant‐soil‐microbial System in Mediterranean Holm Oak (Quercus ilex) Forests. Microbial Ecology: 1‐15. doi: 10.1007/s00248‐015‐0584‐9. http://hdl.handle.net/10261/128289, © 2015, Springer Science+Business Media New York. Ecological transformations derived from habitat fragmentation have led to increased threats to above-ground biodiversity. However, the impacts of forest fragmentation on soils and their microbial communities are not well understood. We examined the effects of contrasting fragment sizes on the structure and functioning of soil microbial communities from holm oak forest patches in two bioclimatically different regions of Spain. We used a microcosm approach to simulate the annual summer drought cycle and first autumn rainfall (rewetting), evaluating the functional response of a plant-soil-microbial system. Forest fragment size had a significant effect on physicochemical characteristics and microbial functioning of soils, although the diversity and structure of microbial communities were not affected. The response of our plant-soil-microbial systems to drought was strongly modulated by the bioclimatic conditions and the fragment size from where the soils were obtained. Decreasing fragment size modulated the effects of drought by improving local environmental conditions with higher water and nutrient availability. However, this modulation was stronger for plant-soil-microbial systems built with soils from the northern region (colder and wetter) than for those built with soils from the southern region (warmer and drier) suggesting that the responsiveness of the soil-plant-microbial system to habitat fragmentation was strongly dependent on both the physicochemical characteristics of soils and the historical adaptation of soil microbial communities to specific bioclimatic conditions. This interaction challenges our understanding of future global change scenarios in Mediterranean ecosystems involving drier conditions and increased frequency of forest fragmentation., This work was supported by the Spanish Ministry for Economy and Competitiveness (formerly known as Innovation and Science) with the project VULGLO (CGL2010 22180 C03 03); and the projects REMEDINAL 2 (CM S2009 AMB 1783) and MICROAMBIENTE (S2009/AMB-1511), funded by the Comunidad de Madrid.

Background and aims: The occurrence of drought-induced forest die-off events is projected to increase in the future, but we still lack complete understanding of its impact on plant-soil interactions, soil microbial diversity and function. We investigated the effects of holm oak (Quercus ilex) decline (HOD) on soil microbial community and functioning, and how these effects relate to changes in the herbaceous community. Methods: We selected 30 holm oak trees with different defoliation degrees (healthy, affected and dead) and analyzed soil samples collected under the canopy (holm oak ecotype) and out of the influence (grassland ecotype) of each tree. Results: HOD increased potential nitrogen (N) mineralization and decreased inorganic N concentrations. These results could be partially explained by changes in the herbaceous composition, an increased herbaceous abundance and changes in soil microbial functional diversity and structure, with HOD favoring bacteria against fungi. Moreover, herbaceous abundance and microbial functional diversity of holm oak and grassland ecotypes converged with HOD. Conclusions: Our results show that HOD triggers a cascade effect on plant understory and soil microbial communities, as well as a plant succession (savannization) process, where understory species colonize the gaps left by dead holm oaks, with important implications for ecosystem C and N budgets., This study was supported by the International Laboratory of Global Change (LINCGlobal), the Spanish Ministry of Economy and Competitiveness grant VERONICA (CGL2013-42271-P), the Community of Madrid grant REMEDINAL3-CM (S2013/MAE-2719) and the FCT/MEC through national funds and the co-funding by the FEDER, within the PT2020 Partnership Agreement and COMPETE 2020 (UID/BIA/04004/2013). AR was supported by the Spanish National Research Council (CSIC) in the JAE-doc modality co-financed by the European Social Fund (ESF)

Dryland ecosystems are considered the largest global carbon sink. However, extreme climate phenomena like the El Nino events (EN) may change soil respiration (Rs) – the CO2 emitted from soils resulting from biological activity and the largest outgoing flux of carbon from terrestrial ecosystems. Our aim was to study the effect of the EN on Rs in the North Peruvian dryland forest, and its interaction with soil temperature and the tree canopy. Our results indicate that Rs during the EN years increased by a factor of 100 compared to normal years, but this effect was exacerbated by the proximity to trees. Only under trees and during the EN event temperature exerted a positive control over daily Rs fluctuations. Our results, indicate how in these dryland forests the expected increase in the EN frequency and intensity could affect soil CO2 emissions, and hence ecosystem carbon budgets, but that this effect would very much depend on tree density and tree spatial distribution.

The Cerrado is the largest savanna of South America and its physiognomy varies from savanna to woodlands. There are two main types of woodlands in the Cerrado: dystrophic woodlands, dominated by N-fixing leguminous trees (LEG), and mesotrophic woodlands dominated by non-leguminous drought-tolerant trees (DRY), which are calcicoles and sensitive to Al The working hypothesis is that LEG and DRY are functional antagonists in terms of the acidification/alkalization processes involving different forms of inorganic nitrogen and pH, Ca and Al in soil. Tree species basal area and soil properties were used to investigate the antagonism between LEG and DRY using generalized linear models. The results suggest that LEG and DRY are antagonists. The LEG were positively associated with Al, NO and NH4+ content and negatively related to increasing Ca content, whereas the DRY were negatively associated with Al, NO and NH4+ and positively associated with increasing Ca content. The upper soil layer in plots dominated by LEG species became more acidic and the upper soil layer in plots dominated by DRY species became more alkaline. The results suggest that LEG and DRY are functional antagonists and their preferences for NH or NO might influence the way the Cerrado woodland changes.

How forests cope with drought-induced perturbations and how the dependence of soil respiration on environmental and biological drivers is affected in a warming and drying context are becoming key questions. The aims of this study were to determine whether drought-induced die-off and forest succession were reflected in soil respiration and its components and to determine the influence of climate on the soil respiration components. We used the mesh exclusion method to study seasonal variations in soil respiration (RS) and its components: heterotrophic (RH) and autotrophic (RA) [further split into fine root (RR) and mycorrhizal respiration (RM)] in a mixed Mediterranean forest where Scots pine (Pinus sylvestris L.) is undergoing a drought-induced die-off and is being replaced by holm oak (Quercus ilex L.). Drought-induced pine die-off was not reflected in RS nor in its components, which denotes a high functional resilience of the plant and soil system to pine die-off. However, the succession from Scots pine to holm oak resulted in a reduction of RH and thus in an important decrease of total respiration (RS was 36 % lower in holm oaks than in non-defoliated pines). Furthermore, RS and all its components were strongly regulated by soil water content-and-temperature interaction. Since Scots pine die-off and Quercus species colonization seems to be widely occurring at the driest limit of the Scots pine distribution, the functional resilience of the soil system over die-off and the decrease of RS from Scots pine to holm oak could have direct consequences for the C balance of these ecosystems., This study was supported by the Spanish government projects SECASOL (CGL2009-08101), DRIM (CGL2010-16373), VULGLO (CGL2010-22180-C03-03), SECADIN (CGL2012-32965) and VERONICA (CGL2013-42271-P); by the Government of Catalonia grants (2009-SGR-00247 and 2014-SGR-453); and by a Community of Madrid grant REMEDINAL 2 (CM S2009/AMB-1783)., J. B. was supported by FPI (BES-2010-036558) and EEBB (EEBB-I-13-07002) scholarships from the Spanish Ministry of Economy and Competitiveness.

Research on the subterranean CO dynamics has focused individually on either surface soils or bedrock cavities, neglecting the interaction of both systems as a whole. In this regard, the vadose zone contains CO-enriched air (ca. 5% by volume) in the first meters, and its exchange with the atmosphere can represent from 10 to 90% of total ecosystem CO emissions. Despite its importance, to date still lacking are reliable and robust databases of vadose zone CO contents that would improve knowledge of seasonal-annual aboveground-belowground CO balances. Here we study 2.5 years of vadose zone CO dynamics in a semiarid ecosystem. The experimental design includes an integrative approach to continuously measure CO in vertical and horizontal soil profiles, following gradients from surface to deep horizons and from areas of net biological CO production (under plants) to areas of lowest CO production (bare soil), as well as a bedrock borehole representing karst cavities and ecosystem-scale exchanges. We found that CO followed similar seasonal patterns for the different layers, with the maximum seasonal values of CO delayed with depth (deeper more delayed). However, the behavior of CO transport differed markedly among layers. Advective transport driven by wind induced CO emission both in surface soil and bedrock, but with negligible effect on subsurface soil, which appears to act as a buffer impeding rapid CO exchanges. Our study provides the first evidence of enrichment of CO under plant, hypothesizing that CO-rich air could come from root zone or by transport from deepest layers through cracks and fissures., These data were funded by the Andalusian regional government project GEOCARBO (P08-RNM-3721), including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects SOILPROF (CGL2011-15276-E), CARBORAD (CGL2011-27493), and GEISpain (CGL2014-52838-C2-1-R). This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme, DIESEL project (625988).

The aim of this study was to understand how drought-induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought-induced die-off, is being replaced by Holm-oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and 13C solid-state Nuclear Magnetic Resonance (CP-MAS 13C NMR) to soils within areas of influence (defined as an surface with 2-m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close-chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below-ground before above-ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r-strategic bacteria) further gives indications of how drought-induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils., The authors are indebted to the Spanish government projects CONSOLIDER INGENIO MONTES (CSD2008- 00040), SECASOL (CGL2009-08101), DRIM (CGL2010- 16373) the Catalonian government project 2009 SGR 247 and the “Proyecto de Excelencia de la Junta de Andalucia de referencia (P08-CVI-03549)". JCY was awarded a “Ramón y Cajal” research Fellowship while writing this manuscript.

We investigated the effects of volatile organic compounds (VOCs) emitted by pine litter, specifically terpenes, on soil microbial biomass carbon and nitrogen and heterotrophic soil respiration under different microclimatic scenarios of water availability and temperature. Soil in glass jars (0.6 L headspace) was exposed to pine needle litter, avoiding any physical contact between soils and litter. Treatments were subjected to two moisture levels, control and drought (20 % and 10 % gravimetric soil water content respectively) and to different temperatures (temperature response curve from 5 °C to 45 °C). In control soils, exposure to litter was associated with a significant decrease in microbial biomass carbon and ninhydrin extractable organic nitrogen, and with a significant increase in heterotrophic respiration (up to 46 %) under optimum temperature (25 °C). Drought, on the other hand, restricted the effects of litter exposure on heterotrophic respiration but exposure to litter was associated with a significant increase in microbial biomass nitrogen. We did not detect significant overall microbial consumption of terpenes in this study. These results suggest either that other VOCs not measured in the study were being consumed and/or that VOCs emissions were triggering strong changes in the composition and functioning of soil microbial communities. More studies under field conditions are needed to assess the magnitude of litter VOCs effects on carbon and nitrogen cycles.

11 páginas, 9 figuras, 1 tabla., Ecosystem respiration (Reco) is highly variable in semiarid ecosystems. After a period of drought, Reco can jump to a high value in response to rain events, and afterward it decays exponentially with time. To better understand the timing, size, and duration of rain-induced respiration pulses, we examined 57 rain events from over 23 site-years of eddy covariance measurements at an open annual grassland, a woodland understory, and a peatland pasture in California, USA. To explain these findings, we conducted a factorial litter-watering experiment on grass and soil plots exposure to varying amounts of sunlight and water. To interpret and compare the population of rain-induced respiration pulses, we fitted the data with a two pool, 4-parameter exponential decay model. At the field scale, the total amounts of CO2 emission emanating from a single and sustained respiration pulse (mean ± standard deviation) were 44.4 ± 38.0, 24.2 ± 17.8, and 94.6 ± 45.8 gC m−2 at the open grassland, the woodland understory, and the peatland pasture, respectively. The large variations in precipitation of these respiration-triggering rain events were associated with 73–84% of variation in the total amount of CO2 emission. Litter that experienced antecedent photodegradation tended to enhance respiration pulses. Biotic and abiotic processes involved in the dynamic of respiration pulses were pulse-specific, and we summarized them into two pulse scenarios. This study illustrates an emerging connection of ecosystem processes, and these findings may help to improve models on the dynamics of ecosystem CO2 cycling., This research was conducted at a site that is a member of the AmeriFlux and FLUXNET networks, and this research was supported in part by the Office of Science (BER), the U.S. Department of Energy (DE-FG02-03ER63638), and the Western Regional Center of the National Institute for Global Environmental Change under Cooperative Agreement (DE-FC02-03ER63613). This study is also partially supported by the National Science Foundation (DEB- 0317139), the Kearney Soil Science Foundation, and the California Agricultural Experiment Station.

The contribution of decomposing soil organic carbon (SOC) to total annual soil respiration (SR) was evaluated by radiocarbon measurements at a Scots pine stand growing on a plaggen soil in the Belgian Campine region. Two approaches were used to estimate the contribution of different C pools to SR. In the first approach, the variations in (14)C content of soil CO(2) efflux were monitored during one year (2003) and compared to the atmospheric and SOC (14)C signatures to determine the contribution of "fast" (root respiration and fast decomposing SOC) and "slow" cycling C pools to total SR. In the second approach an estimate of the total heterotrophic soil respiration (Rh), comprising the slow cycling C and the heterotrophic part of the fast-cycling C pools, was derived applying a box model based on the amount of the bulk SOC pool and its (14)C-derived mean residence time (MRT). The quantification of the Rh and the decomposition rate of the slow-cycling SOC allows to indirectly determining the contribution of the heterotrophic C that decompose within a year. Measurements of total SR performed in the field allowed assessing the contribution of the different C pools to total soil C efflux. On an annual basis, the fast-cycling C was the main contributor to SR, about 85%, while the contribution of the slow-cycling C (with MRT > 1 yr) to total SR was 15%. Total annual Rh was 36% of total SR, which is in the lower range reported for temperate coniferous forests. The comparison of Rh with other estimates for the same site (47-50% of total SR) suggest a possible underestimation of the C flux from the mineral soil. In fact, the "very old" C contained in the plaggen horizon strongly affects the signature of the mostly young C leaving the soil. In conclusion, our results indicate that the contribution of SOC decomposition to total soil CO(2) flux in this forest is less than 40%, and at least half of it comes from organic compounds less than 1 year old.

Microbial-mediated decomposition of soil organic matter (SOM) ultimately makes a considerable contribution to soil respiration, which is typically the main source of CO2 arising from terrestrial ecosystems. Despite this central role in the decomposition of SOM, few studies have been conducted on how climate change may affect the soil microbial community and, furthermore, on how possible climate-change induced alterations in the ecology of microbial communities may affect soil CO2 emissions. Here we present the results of a seasonal study on soil microbial community structure, SOM decomposition and its temperature sensitivity in two representative Mediterranean ecosystems where precipitation/throughfall exclusion has taken place during the last 10 years. Bacterial and fungal diversity was estimated using the terminal restriction fragment length polymorphism technique. Our results show that fungal diversity was less sensitive to seasonal changes in moisture, temperature and plant activity than bacterial diversity. On the other hand, fungal communities showed the ability to dynamically adapt throughout the seasons. Fungi also coped better with the 10 years of precipitation/throughfall exclusion compared with bacteria. The high resistance of fungal diversity to changes with respect to bacteria may open the controversy as to whether future ‘drier conditions’ for Mediterranean regions might favor fungal dominated microbial communities. Finally, our results indicate that the fungal community exerted a strong influence over the temporal and spatial variability of SOM decomposition and its sensitivity to temperature. The results, therefore, highlight the important role of fungi in the decomposition of terrestrial SOM, especially under the harsh environmental conditions of Mediterranean ecosystems, for which models predict even drier conditions in the future.

We seek to understand how biophysical factors such as soil temperature (Ts), soil moisture (theta), and gross primary production (GPP) influence CO2 fluxes across terrestrial ecosystems. Recent advancements in automated measurements and remote-sensing approaches have provided time series in which lags and relationships among variables can be explored. The purpose of this study is to present new applications of continuous measurements of soil CO2 efflux (F0) and soil CO2 concentrations measurements. Here we explore how variation in Ts, theta, and GPP (derived from NASA's moderate-resolution imaging spectroradiometer [MODIS]) influence F0 and soil CO2 production (Ps). We focused on seasonal variation and used continuous measurements at a daily timescale across four vegetation types at 13 study sites to quantify: (1) differences in seasonal lags between soil CO2 fluxes and Ts, theta, and GPP and (2) interactions and relationships between CO2 fluxes with Ts, theta, and GPP. Mean annual Ts did not explain annual F0 and Ps among vegetation types, but GPP explained 73% and 30% of the variation, respectively. We found evidence that lags between soil CO2 fluxes and Ts or GPP provide insights into the role of plant phenology and information relevant about possible timing of controls of autotrophic and heterotrophic processes. The influences of biophysical factors that regulate daily F0 and Ps are different among vegetation types, but GPP is a dominant variable for explaining soil CO2 fluxes. The emergence of long-term automated soil CO2 flux measurement networks provides a unique opportunity for extended investigations into F0 and Ps processes in the near future.

The chapter III of: FLORES-RENTERÍA, Dulce. Effects of forest fragmentation on the plant‐soil‐microbial interactions = Efectos de la fragmentación del encinar en las interacciones planta‐suelo‐microorganismos (2016), is relationed with the paper: Agricultural matrix affects differently the alpha and beta structural and functional diversity of soil microbial communities in a fragmented Mediterranean holm oak forest. Soil Biology and Biochemistry 92: 79-90 (2016). http://hdl.handle.net/10261/151312, El capítulo III de la tesis doctoral: FLORES-RENTERÍA, Dulce. Efectos de la fragmentación del encinar en las interacciones planta‐suelo‐microorganismos = Effects of forest fragmentation on the plant‐soil‐microbial interactions (2016, está relacionado con el artículo: Agricultural matrix affects differently the alpha and beta structural and functional diversity of soil microbial communities in a fragmented Mediterranean holm oak forest. Soil Biology and Biochemistry 92: 79-90 (2016). http://hdl.handle.net/10261/151312, Given the increase in habitat fragmentation in the Mediterranean forests, understanding its impacts over the ecology of soil microbial communities, responsible for many ecosystem functions, and their capacity to metabolize different substrates from soil organic matter, is of upmost importance. We evaluated how the influence of the agricultural matrix, as one of the main consequences of forest fragmentation, may affect both the composition and the functioning of soil microbial communities in Mediterranean holm oak forests. We determined structural and functional alpha and beta-diversity of microbial communities, as well as microbial assemblages and metabolic profiles, by using a commonly used fingerprinting technique (Denaturing Gel Gradient Electrophoresis) and a community level physiological profiles (CLPP) technique (EcoPlate). Key drivers of soil microbial structure and metabolism were evaluated by using structural equation models (SEM) and multivariate ordination (envfit) approaches. Our results pointed out that forest fragmentation affects microbial community structure and functioning through a complex cascade of causal-effect interactions with the plant–soil system, which ultimately affects the nutrient cycling and functioning of forest soils. We also found a strong scale-dependency effect of forest fragmentation over the ecology of microbial communities: fragmentation increases the local (alpha) diversity, but affected negatively microbial diversity at the landscape scale (beta diversity). This homogenization of the microbial communities and their metabolism at landscape scale resulting from habitat fragmentation may have unknown potential consequences on the capacity of these communities, and hence these ecosystems, to respond to the climate change. Finally, we found a consistent relation between the structure and functional diversity of bacterial community, which further showed the important role that the assemblage of microbial communities might have over their functioning., This work was supported by the Spanish Ministry for Economy and Competitiveness (formerly known as Innovation and Science) with the projects VULGLO (CGL2010 22180 C03 03), VERONICA (CGL2013-42271-P) and MyFUNCO (CGL2011-29585-C02-02), and the project REMEDINAL 3-CM (S2013/MAE-2719) funded by the Comunidad de Madrid.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms., Microorganisms are vital in mediating the earth's biogeochemical cycles; yet, despite our rapidly increasing ability to explore complex environmental microbial communities, the relationship between microbial community structure and ecosystem processes remains poorly understood. Here, we address a fundamental and unanswered question in microbial ecology: 'When do we need to understand microbial community structure to accurately predict function?' We present a statistical analysis investigating the value of environmental data and microbial community structure independently and in combination for explaining rates of carbon and nitrogen cycling processes within 82 global datasets. Environmental variables were the strongest predictors of process rates but left 44% of variation unexplained on average, suggesting the potential for microbial data to increase model accuracy. Although only 29% of our datasets were significantly improved by adding information on microbial community structure, we observed improvement in models of processes mediated by narrow phylogenetic guilds via functional gene data, and conversely, improvement in models of facultative microbial processes via community diversity metrics. Our results also suggest that microbial diversity can strengthen predictions of respiration rates beyond microbial biomass parameters, as 53% of models were improved by incorporating both sets of predictors compared to 35% by microbial biomass alone. Our analysis represents the first comprehensive analysis of research examining links between microbial community structure and ecosystem function. Taken together, our results indicate that a greater understanding of microbial communities informed by ecological principles may enhance our ability to predict ecosystem process rates relative to assessments based on environmental variables and microbial physiology.

[Aims]: Drought-induced forest die-off and subsequent species replacement may modify environmental conditions and eventually affect litter decomposition. We aimed to disentangle the effects of tree species and die-off state on litter decomposition in a mixed forest where Pinus sylvestris populations experiencing severe drought-induced die-off are being replaced by Quercus ilex., [Methods]: Litter bags with leaves and fine roots from both species were placed under canopies representing three habitats of the die-off and replacement process (healthy and dead P. sylvestris and healthy Q. ilex). Mass was assessed over 3 years., [Results]: Species-specific chemistry of litter (C:N ratio) had a direct effect on mass loss, but also indirect effects, attributed to the decomposer microbial community associated with a given habitat-species. In their respective original habitats, oak leaves decomposed 44 % faster than pine needles, whereas oak roots decomposed 46 % slower than pine roots., [Conclusions]: Forest die-off and species replacement affected litter decomposition. This effect can have great implications in forest functioning, particularly if drought-induced die-off worsens in the next decades, according with the trend observed in the studied system., This study was supported by the Spanish Government projects CGL2009-08101, CGL2010-16373, CGL2012-32965 and CGL2013-42271-P, by the Government of Catalonia grants (2009-SGR-00247 and 2014-SGR-453) and by the TRY initiative on plant traits (http://www.try-db.org). JB was supported by an FPI scholarship (BES-2010-036558) from the Spanish Ministry of Economy and Competitiveness. JCY acknowledges the support of the “Ramon y Cajal” programme.

Altres ajuts rebuts per dur a terme aquest projecte: ICTS-Reserva Biológica de Doñana (projects 38/2007, 27/2009, and 11/2013) i Goverment of Madrid (REMEDINAL 2; CM S2009 AMB 1783) This study reports the relationship between the diversity and functioning of fungal and bacterial soil communities with vegetation in Mediterranean woodland that experienced severe die-off after a drought episode. Terminal restriction fragment length polymorfism (TRFLP) was used to describe microbial community structure and diversity five years after the episode in different habitats (Juniperus woodland, shrubland, grassland), when the vegetation had not yet recovered. Vegetation diversity was positively related to TRF bacterial richness under unaffected canopies and was higher in diverse grassland. Fungal TRF richness correlated with vegetation type, being greater in Juniperus woodland. Microbial respiration increased in grassland, whereas microbial biomass, estimated from soil substrate-induced respiration (SIR), decreased with bacterial diversity. Die-off increased bacterial richness and changed bacterial composition, particularly in Juniperus woodland, where herbaceous species increased, while fungal diversity was reduced in Juniperus woodland. Die-off increased microbial respiration rates. The impact on vegetation from extreme weather episodes spread to microbial communities by modifying vegetation composition and litter quantity and quality, particularly as a result of the increase in herbaceous species. Our results suggest that climate-induced die-off triggers significant cascade effects on soil microbial communities, which may in turn further influence ecosystem C dynamics.

Twenty chambers for measurement of soil CO2 efflux were compared against known CO2 fluxes ranging from 0.32 to 10.01 mumol CO2 m(-2) s(-1) and generated by a specially developed calibration tank. Chambers were tested on fine and coarse homogeneous quartz sand with particle sizes of 0.05-0.2 and 0.6 mm, respectively. The effect of soil moisture on chamber measurements was tested by wetting the fine quartz sand to about 25% volumetric water content. Non-steady-state through-flow chambers either underestimated or overestimated fluxes from -21 to +33% depending on the type of chamber and the method of mixing air within the chamber's headspace. However, when results of all systems tested were averaged, fluxes were within 4% of references. Non-steady-state non-through-flow chambers underestimated or overestimated fluxes from -35 to +6%. On average, the underestimation was about 13-14% on fine sand and 4% on coarse sand. When the length of the measurement period was increased, the underestimation increased due to the rising concentration within the chamber headspace, which reduced the diffusion gradient within the soil. Steady-state through-flow chambers worked almost equally well in all sand types used in this study. They overestimated the fluxes on average by 2-4%. Overall, the reliability of the chambers was not related to the measurement principle per se. Even the same chambers, with different collar designs, showed highly variable results. The mixing of air within the chamber can be a major source of error. Excessive turbulence inside the chamber can cause mass flow of CO2 from the soil into the chamber. The chamber headspace concentration also affects the flux by altering the concentration gradient between the soil and the chamber. (Less)

The global forest carbon (C) stock is estimated at 662 Gt of which 45% is in soil organic matter. Thus, comprehensive understanding of the effects of forest management practices on forest soil C stock and greenhouse gas (GHG) fluxes is needed for the development of effective forest-based climate change mitigation strategies. To improve this understanding, we synthesized peer-reviewed literature on forest management practices that can mitigate climate change by increasing soil C stocks and reducing GHG emissions. We further identified soil processes that affect soil GHG balance and discussed how models represent forest management effects on soil in GHG inventories and scenario analyses to address forest climate change mitigation potential. Forest management effects depend strongly on the specific practice and land type. Intensive timber harvesting with removal of harvest residues/stumps results in a reduction in soil C stock, while high stocking density and enhanced productivity by fertilization or dominance of coniferous species increase soil C stock. Nitrogen fertilization increases the soil C stock and N2O emissions while decreasing the CH4 sink. Peatland hydrology management is a major driver of the GHG emissions of the peatland forests, with lower water level corresponding to higher CO2 emissions. Furthermore, the global warming potential of all GHG emissions (CO2, CH4 and N2O) together can be ten-fold higher after clear-cutting than in peatlands with standing trees. The climate change mitigation potential of forest soils, as estimated by modelling approaches, accounts for stand biomass driven effects and climate factors that affect the decomposition rate. A future challenge is to account for the effects of soil preparation and other management that affects soil processes by changing soil temperature, soil moisture, soil nutrient balance, microbial community structure and processes, hydrology and soil oxygen concentration in the models. We recommend that soil monitoring and modelling focus on linking processes of soil C stabilization with the functioning of soil microbiota. This review has been supported by the grant Holistic management practices, modelling and monitoring for European forest soils – HoliSoils (EU Horizon 2020 Grant Agreement No 101000289) and the Academy of Finland Fellow project (330136, B. Adamczyk). In addition to the HoliSoils consortium partners, Dr. Abramoff contributed on this study and her work was supported by the United States Department of Energy, Office of Science, Office of Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the United States Department of Energy under contract DE-AC05- 00OR22725.