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The transformation of primary forestlands into lands under cultivation exerts an impact on the physical and chemical properties of the soils, leading to loss of carbon from the soil. The aims of this study were (1) to estimate the impact of different land use on organic carbon stock (OC-stock) and (2) to determine the clay percentage, the dispersion factor, and bulk density with regard to soil management. Thirty soil profiles were sampled as follows: four under primary forests with 80-100 years (old growth); two under secondary wood forests with 45-50 years; eight under secondary young forests with 15-25 years; four under pasture land with 15-20 years; and 12 under continuous cultivation for > 50 years. We determined the changes in OC-stock in the soils under different land-use conditions in relation to the primary forest variant. There was a difference in the OC-stock of the continuous cultivation variant compared to the remaining land-use variants for all soil-thickness layers. The greatest loss of cultivated soil was in the 0-20 cm layer of the primary forest OC-stock. In the 50-100 cm layer, the OC-stock of the cultivated soils diminished significantly. As regards physical properties, such as clay percentage, dispersion factor, and bulk density, we observed significant differences between the cultivated soils compared to the remaining land-use variants.

Nonstructural carbon (NSC) reserves act as buffers to sustain tree activity during periods when carbon (C) assimilation does not meet C demand, but little is known about their age and accessibility; we designed a controlled girdling experiment in the Amazon to study tree survival on NSC reserves. We used bomb-radiocarbon (14C) to monitor the time elapsed between C fixation and release (‘age’ of substrates). We simultaneously monitored how the mobilization of reserve C affected δ13CO2. Six ungirdled control trees relied almost exclusively on recent assimilates throughout the 17 months of measurement. The Δ14C of CO2 emitted from the six girdled stems increased significantly over time after girdling, indicating substantial remobilization of storage NSC fixed up to 13–14 yr previously. This remobilization was not accompanied by a consistent change in observed δ13CO2. These trees have access to storage pools integrating C accumulated over more than a decade. Remobilization follows a very clear reverse chronological mobilization with younger reserve pools being mobilized first. The lack of a shift in the δ13CO2 might indicate a constant contribution of starch hydrolysis to the soluble sugar pool even outside pronounced stress periods (regular mixing). © 2018 The Authors. New Phytologist © 2018 New Phytologist Trust

34 páginas, 2 tablas, 4 figuras, [EN] Current knowledge on tree carbon (C) allocation to wood is particularly scarce in plants subjected to disturbance factors, such as browsing, which affects forest regeneration worldwide and has an impact on the C balance of trees. Furthermore, quantifying the degree to which tree rings are formed from freshly assimilated vs. stored carbohydrates is highly relevant for our understanding of tree C allocation. We used (13)C labelling to quantify seasonal allocation of stored C to wood formation in two species with contrasting wood anatomy: Betula pubescens Ehrh. (diffuse-porous) and Quercus petraea [Matt.] Liebl. (ring-porous). Clipping treatments (66% shoot removal, and unclipped) were applied to analyse the effect of browsing on C allocation into tree rings, plus the effects on tree growth, architecture, ring width and non-structural carbohydrates (NSCs). The relative contribution of stored C to wood formation was greater in the ring-porous (55-70%) than in the diffuse-porous species (35-60%), although each species followed different seasonal trends. Clipping did not cause a significant depletion of C stores in either species. Nonetheless, a significant increase in the proportion of stored C allocated to earlywood growth was observed in clipped birches, and this could be explained through changes in tree architecture after clipping. The size of C pools across tree species seems to be important in determining the variability of seasonal C allocation patterns to wood and their sensibility to disturbances such as browsing. Our results indicate that the observed changes in C allocation to earlywood in birch were not related to variations in the amount or concentration of NSC stores, but to changes in the seasonal availability of recently assimilated C caused by modifications in tree architecture after browsing., S.P. was funded by an MEC-SEUI postdoctoral fellowship and a JAE-Doc (CSIC) contract. This study was partly funded by projects CGL2008-04847-C02-01/BOS (MCI-Spain) and GA-LC- 011/2008 (DGA-La Caixa). E.P., A.S., A.H. and P.M. thank the Scottish Government’s Rural and Environmental Research and Analysis Directorate for funding.

Seasonal and inter-annual dynamics of growth, non-structural carbohydrates (NSC) and carbon isotope composition (δ 13 C) of NSC were studied in a beech forest of Central Italy over a 2-year period characterized by different environmental conditions. The net C assimilated by forest trees was mainly used to sustain growth early in the season and to accumulate storage car- bohydrates in trunk and root wood in the later part of the season, before leaf shedding. Growth and NSC concentration dynamics were only slightly affected by the reduced soil water content (SWC) during the drier year. Conversely, the carbon isotope analysis on NSC revealed seasonal and inter-annual variations of photosynthetic and post-carboxylation fractionation processes, with a significant increase in δ 13 C of wood and leaf soluble sugars in the drier summer year than in the wetter one. The highly significant correlation between δ 13 C of leaf soluble sugars and SWC suggests a decrease of the canopy C isotope discrimination and, hence, an increased water-use efficiency with decreasing soil water availability. This may be a relevant trait for maintaining an acceptable plant water status and a relatively high C sink capacity during dry seasonal periods. Our results suggest a short- to medium-term homeostatic response of the Collelongo beech stand to variations in water avail- ability and solar radiation, indicating that this Mediterranean forest was able to adjust carbon-water balance in order to prevent C depletion and to sustain plant growth and reserve accumulation during relatively dry seasons.

Winter-drought induced forest diebacks in the low-latitude margins of species' distribution ranges can provide new insights into the mechanisms (carbon starvation, hydraulic failure) underlying contrasting tree reactions. We analysed a winter-drought induced dieback at the Scots pine's southern edge through a dual-isotope approach (Δ13C and δ18O in tree-ring cellulose). We hypothesized that a differential long-term performance, mediated by the interaction between CO2 and climate, determined the fates of individuals during dieback. Declining trees showed a stronger coupling between climate, growth and intrinsic water-use efficiency (WUEi) than non-declining individuals that was noticeable for 25 years prior to dieback. The rising stomatal control of water losses with time in declining trees, indicated by negative Δ13C-δ18O relationships, was likely associated with their native aptitude to grow more and take up more water (suggested by larger tracheid lumen widths) than non-declining trees and, therefore, to exhibit a greater cavitation risk. Freeze-thaw episodes occurring in winter 2001 unveiled such physiological differences by triggering dieback in those trees more vulnerable to hydraulic failure. Thus, WUEi tightly modulated growth responses to long-term warming in declining trees, indicating that co-occurring individuals were differentially predisposed to winter-drought mortality. These different performances were unconnected to the depletion of stored carbohydrates. © 2013 John Wiley & Sons Ltd., This work was partially supported by the DGI projects CGL2009-13079-C02-01 and CGL2011-26654 and grant FP7-PEOPLE-2009-RG-246725 (SMARTREES). We acknowledge R. Hernández, M. Maestro, V. Pérez-Fortea and P. Sopeña for field and technical assistance. J.P.F. is supported by a postdoctoral contract from the ‘Ramon y Cajal’ programme (MCINN). We also thank two anonymous reviewers and the editor for their useful comments.

We studied the effect of ecologically relevant ammonium concentrations and light on several morphological and physiological properties, nitrogen metabolism and carbon reserves of eelgrass Zostera marina L. Eelgrass was grown under mesocosm conditions at 3 levels of ammonium enrichment (target concentrations of 0, 10 and 25 μM) and 2 levels of light (low and high light). High ammonium supply combined with low light had a negative effect on several morphological and physiological response parameters, while no such effects were found when ammonium was supplied under high light. N enrichment caused an increase in the content of total N, intracellular ammonium, free amino acids and residual N in the plants and this response was more pronounced under low-light conditions than under high light. The soluble proteins content de - crea sed, in contrast with external ammonium enrichment. The accumulation of free amino acids and residual N in NH4 +-enriched plants was followed by a substantial drop in carbohydrate reserves (sucrose and starch), which was larger in plants grown under low-light conditions. Our results indicate that N enrichment increases the demand for C skeletons and energy, and that photosynthesis cannot supply enough C and energy to cover that demand under low-light conditions. Eelgrass plants exposed to reduced light conditions, for example close to their depth limit or when covered by drift macroalgae, may thus be especially susceptible to enhanced ammonium concentrations. Our study demonstrates that ammonium toxicity may explain why eelgrass and other seagrasses deteriorate under nutrient-rich, low-light conditions.

Insect outbreaks of increasing frequency and severity in forests are predicted due to climate change. Insect herbivory is known to promote physiological changes in forest trees. However, little is known about whether these plant phenotypic adjustments have cascading effects on tree microbial symbionts such as fungi in roots and foliage. We studied the impact of defoliation by the pine processionary moth in two infested Pinus nigra forests through a multilevel sampling of defoliated and non-defoliated trees. We measured tree growth, nutritional status and carbon allocation to chemical defenses. Simultaneously, we analyzed the putative impact of defoliation on the needle endophytes and on the soil fungal communities. Higher concentrations of chemical defenses were found in defoliated trees, likely as a response to defoliation; however, no differences in non-structural carbohydrate reserves were found. In parallel to the reductions in tree growth and changes in chemical defenses, we observed shifts in the composition of needle endophytic and soil fungal communities in defoliated trees. Defoliated trees consistently corresponded with a lower biomass of ectomycorrhizal fungi in both sites, and a higher alpha diversity and greater relative abundance of belowground saprotrophs and pathogens. However, ectomycorrhizal alpha diversity was similar between non-defoliated and defoliated trees. Specific needle endophytes in old needles were strongly associated with non-defoliated trees. The potential role of these endophytic fungi in pine resistance should be further investigated. Our study suggests that lower biomass of ectomycorrhizal fungi in defoliated trees might slow down tree recovery since fungal shifts might affect tree-mycorrhizal feedbacks and can potentially influence carbon and nitrogen cycling in forest soils. J.J.C. acknowledges the support of the CGL2015-69186-C2-1-R and RTI2018-096884-B-C31 projects (Spanish Ministry of Science), and L.S. and R.Z. acknowledge support from MCIU/AEI/FEDER/UE grants FUTURPIN AGL2015-68274-C3-2-R and RESILPINE RTI2018-094691-B-C33 and from Xunta de Galicia‐GAIN grant IN607A2016/013. J.O. and J.G.A. were supported by a Ramón y Cajal fellowship (RYC-2015-17459; RYC-2016-20528, respectively) and J.A.B. was supported by the Serra-Hunter Program-Generalitat de Catalunya.