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Eutrophication usually impacts biodiversity, species composition, and functioning of grassland communities. Whether such effects propagate to influence the stability of these community aspects is unknown. Using standardized experiments across 55 global grasslands, we quantified the effects of nutrient addition on five stability facets (i.e., temporal invariability and resistance during and recovery after dry and wet growing seasons) for three community aspects (i.e., aboveground biomass, community composition, and species richness). Nutrient addition reduced the temporal invariability and resistance of species richness and community composition, but not biomass, during dry and wet growing seasons. Temporal invariability and resistance during, but not recovery after, dry and wet growing seasons were strongly positively correlated in both ambient and eutrophic conditions. This indicates that maintaining and restoring the stability of plant communities requires increasing resistance rather than recovery. Harnessing the complexity of ecological stability provides new insights for grassland ecosystem sustainability in a changing world.

Background and aims: A synergistic response of aboveground plant biomass production to combined nitrogen (N) and phosphorus (P) addition has been observed in many ecosystems, but the underlying mechanisms and their relative importance are not well known. We aimed at evaluating several mechanisms that could potentially cause the synergistic growth response, such as changes in plant biomass allocation, increased N and P uptake by plants, and enhanced ecosystem nutrient retention. Methods: We studied five grasslands located in Europe and the USA that are subjected to an element addition experiment composed of four treatments: control (no element addition), N addition, P addition, combined NP addition. Results: Combined NP addition increased the total plant N stocks per unit area by 1.47 times compared to the N treatment, while total plant P stocks per unit area were 1.62 times higher in NP than in single P addition. Further, higher N uptake by plants in response to combined NP addition was associated with reduced N losses from the soil (evaluated based on soil δ15N) compared to N addition alone, indicating a higher ecosystem N retention. In contrast, the synergistic growth response was not associated with significant changes in plant resource allocation. Conclusions: Our results demonstrate that the commonly observed synergistic effect of NP addition on aboveground biomass production in grasslands is caused by enhanced N uptake compared to single N addition, and increased P uptake compared to single P addition, which is associated with a higher N and P retention in the ecosystem.

The amount of nitrogen (N) derived from symbiotic N2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods We evaluated symbiotic N2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15N natural abundance method. Results N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion Our results reveal that N addition mainly impacts symbiotic N2 fixation via reduced biomass of legumes rather than changes in N2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N 2 fixation in grasslands, and these effects cannot be reversed by additional P amendment. EEA Santa Cruz Fil: Vázquez, Eduardo. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Vázquez, Eduardo. Swedish University of Agricultural Sciences. Department of Soil and Environment; Suecia Fil: Schleuss, Per‑Marten. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Borer, Elizabeth T. University of Minnesota. Department of Ecology, Evolution, and Behavior; Estados Unidos Fil: Bugalho, Miguel N. University of Lisbon. Centre for Applied Ecology “Prof. Baeta Neves” (CEABN-InBIO). School of Agriculture; Portugal. Fil: Caldeira, Maria. C. University of Lisbon. Forest Research Centre. School of Agriculture; Portugal. Fil: Eisenhauer, Nico. German Centre for Integrative Biodiversity Research; Alemania Fil: Eisenhauer, Nico. Leipzig University. Institute of Biology; Alemania Fil: Eskelinen, Anu. German Centre for Integrative Biodiversity Research; Alemania Fil: Eskelinen, Anu. Physiological Diversity, Helmholtz Centrefor Environmental Research; Alemania Fil: Eskelinen, Anu. University of Oulu. Ecology & Genetics; Finlandia Fil: Fay, Philip A. Grassland Soil and Water Research Laboratory (USDA-ARS); Estados Unidos Fil: Haider, Sylvia. German Centre for Integrative Biodiversity Research; Alemania Fil: Haider, Sylvia. Martin Luther University. Institute of Biology. Geobotany and Botanical Garden; Alemania Fil: Jentsch, Anke. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Kirkman, Kevin P. University of KwaZulu-Natal. School of Life Sciences; Sudáfrica Fil: McCulley, Rebecca L. University of Kentucky. Department of Plant and Soil Sciences; Estados Unidos Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina. Fil: Peri, Pablo Luis. Universidad Nacional de la Patagonia Austral; Argentina. Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fil: Price, Jodi. Charles Sturt University. Institute for Land, Water and Society; Australia. Fil: Richards, Anna E. CSIRO Land and Water. Northern Territory; Australia. Fil: Risch, Anita C. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Suiza Fil: Roscher, Christiane. German Centre for Integrative Biodiversity Research; Alemania Fil: Roscher, Christiane. Physiological Diversity, Helmholtz Centre for Environmental Research; Alemania Fil: Schütz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research WSL; Suiza Fil: Seabloom, Eric William. University of Minnesota. Dept. of Ecology, Evolution, and Behavior; Estados Unidos Fil: Standish, Rachel J. Murdoch University. Harry Butler Institute; Australia. Fil: Stevens, Carly J. Lancaster University. Lancaster Environment Centre; Reino Unido Fil: Tedder, Michelle J. University of KwaZulu-Natal. School of Life Sciences; Sudáfrica Fil: Virtanen, Risto. University of Oulu. Ecology & Genetics; Finlandia. Fil: Spohn, Marie. University of Bayreuth. Department of Soil Ecology. Bayreuth Center of Ecology and Environmental Research (BayCEER); Alemania Fil: Spohn, Marie. Swedish University of Agricultural Sciences. Department of Soil and Environment; Suecia

SummaryEutrophication impacts grassland diversity such as species richness, functional trait diversity and composition, but whether and how these changes affect ecosystem functional stability under increasing climate extremes is unknown. We quantify the direct and diversity-mediated effects of nutrient addition on functional stability of productivity as measured by resistance during and recovery after dry and wet growing seasons, and temporal invariability across 19 grasslands spanning four continents. We use leaf traits measured in the field to construct slow-fast leaf economic spectrum and calculate functional trait composition and diversity of the slow-fast trait. Nutrient addition increased resistance during dry growing seasons mainly through its direct effects. It also increased resistance during wet growing seasons directly, but these positive effects were weakened by indirect negative effects through reducing slow-fast functional diversity. Nutrient addition had weak effects on recovery after dry growing seasons, but it decreased recovery after wet growing seasons mainly through its direct effects. Nutrient addition increased temporal invariability through resistance, but decreased it through recovery and reducing species richness, leading to overall weak net effects. Thus, processes and influencing pathways independent of plant diversity should be carefully taken into account in understanding ecosystem stability in response to increasing global changes.

The effects of altered nutrient supplies and herbivore density on species diversity vary with spatial scale, because coexistence mechanisms are scale dependent. This scale dependence may alter the shape of the species–area relationship (SAR), which can be described by changes in species richness (S) as a power function of the sample area (A): S = cAz, where c and z are constants. We analysed the effects of experimental manipulations of nutrient supply and herbivore density on species richness across a range of scales (0.01–75 m²) at 30 grasslands in 10 countries. We found that nutrient addition reduced the number of species that could co-occur locally, indicated by the SAR intercepts (log c), but did not affect the SAR slopes (z). As a result, proportional species loss due to nutrient enrichment was largely unchanged across sampling scales, whereas total species loss increased over threefold across our range of sampling scales.

The impact of various environmental stressors, such as plant invasion and extreme drought is increasing in many ecosystems around the globe. Mediterranean ecosystems have especially been affected by both stressors in recent decades. Thus, we conducted a precipitation and shrub invasion manipulation experiment in a Mediterranean cork oak (Quercus suber L.) ecosystem in Portugal. The impact of both stressors (i.e. drought and invasion) as well as their interaction on cork oak functioning was investigated, resulting in four experimental treatments: 1) control trees, 2) trees with a rain exclusion of 45%, 3) trees invaded by the shrub Cistus ladanifer and 4) trees invaded by C. ladanifer combined with rain exclusion. Each of the four treatments was replicated in three spatially separated blocks. In total 36 trees (9 per treatment) and 18 shrubs (9 per treatment) were selected randomly for measurements between October 2017 and March 2020.Invaded trees, independent of the rain exclusion, had lower leaf area index (LAI) and growth rates (trunk increment). Further changes in the relationship of pre-dawn and midday leaf water potential elucidated that competition by shrubs shifted the hydraulic behaviour of invaded trees to a more anisohydric strategy compared to non-invaded cork oaks during summer drought. These negative impacts of shrub invasion had also an effect on transpiration rates of trees, but the extent was dependent on annual precipitation. In the wet year 2018, transpiration rates of trees in all four treatments were similar due to replenished soil water resources. However, in 2019, when precipitation was strongly reduced (ambient: –25%, rain exclusion: –50%), trees under invasion and rain exclusion reduced their transpiration by 47% compared to control trees, which was stronger than the reduction caused by a single stressor (amplifying interaction). However, shrubs under the rain exclusion also suffered from the extreme drought and were not able to recover in the following autumn/winter period, releasing trees from a strong competition (buffering interaction). On the other hand, shrubs under ambient precipitation were highly competitive in the same recovery period and strongly delayed tree transpiration recovery (–51% compared to control). Consequently, extreme drought and shrub invasion interacted buffering in this recovery period after the extreme drought event. In conclusion, the lessons learned from this manipulation experiment are 1) that shrub invasion has a negative impact on the functioning of cork oak trees and 2) the magnitude of this negative impact is dependent on the predominant environmental conditions (i.e. precipitation amount), which can lead to amplifying, neutral or buffering effects of drought and invasion.

Background and aims:The amount of nitrogen (N) derived from symbiotic N2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods:We evaluated symbiotic N2 fixation in 17 grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15N natural abundance method.Results:N as well as combined N and P (NP) addition reduced legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Element addition had no significant effect on the symbiotic N2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion:Our results reveal that N addition mainly impacts symbiotic N2 fixation via reduced biomass of legumes rather than changes in N2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N2 fixation in the world’s grasslands, and these effects cannot be reversed by additional P amendment.

1- Plant damage by invertebrate herbivores and pathogens influences the dynamics of grassland ecosystems, but anthropogenic changes in nitrogen and phosphorus availability can modify these relationships. 2- Using a globally distributed experiment, we describe leaf damage on 153 plant taxa from 27 grasslands worldwide, under ambient conditions and with experimentally elevated nitrogen and phosphorus. 3- Invertebrate damage significantly increased with nitrogen addition, especially in grasses and non-leguminous forbs. Pathogen damage increased with nitrogen in grasses and legumes but not forbs. Effects of phosphorus were generally weaker. Damage was higher in grasslands with more precipitation, but climatic conditions did not change effects of nutrients on leaf damage. On average, invertebrate damage was relatively higher on legumes and pathogen damage was relatively higher on grasses. Community-weighted mean damage reflected these functional group patterns, with no effects of N on community-weighted pathogen damage (due to opposing responses of grasses and forbs) but stronger effects of N on community-weighted invertebrate damage (due to consistent responses of grasses and forbs). 4- Synthesis. As human-induced inputs of nitrogen and phosphorus continue to increase, understanding their impacts on invertebrate and pathogen damage becomes increasingly important. Our results demonstrate that eutrophication frequently increases plant damage and that damage increases with precipitation across a wide array of grasslands. Invertebrate and pathogen damage in grasslands is likely to increase in the future, with potential consequences for plant, invertebrate and pathogen communities, as well as the transfer of energy and nutrients across trophic levels. EEA Santa Cruz Fil: Ebeling, Anne. University of Jena. Institute of Ecology and Evolution; Alemania Fil: Strauss, Alex T. University of Minnesota. Department of Ecology, Evolution, and Behavior; Estados Unidos Fil: Strauss, Alex T. University of Georgia. Odum School of Ecology; Estados Unidos Fil: Adler, Peter B. Utah State University. Department of Wildland Resources and the Ecology Center; Estados Unidos Fil: Arnillas, Carlos Alberto. University of Toronto —Scarborough. Department of Physical and Environmental Sciences; Canadá Fil: Barrio, Isabel C. Agricultural University of Iceland. Faculty of Environmental and Forest Sciences; Islandia Fil: Biederman, Lori A. Iowa State University. Department of Ecology, Evolution, and Organismal Biology; Estados Unidos Fil. Borer, Elizabeth T. University of Minnesota. Department of Ecology, Evolution, and Behavior; Estados Unidos Fil: Bugalho, Miguel N. University of Lisbon. Centre for Applied Ecology (CEABN-InBIO). School of Agriculture; Portugal. Fil: Caldeira, Maria C. University of Lisbon. Forest Research Centre. School of Agriculture; Portugal. Fil: Cadotte, Marc W. University of Toronto Scarborough. Department of Biological Sciences; Canadá Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Santa Cruz; Argentina. Fil: Peri, Pablo Luis. Universidad Nacional de la Patagonia Austral; Argentina. Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fil: Blumenthal, Dana M. USDA-ARS, Rangeland Resources & Systems Research Unit; Estados Unidos

Mediterranean annual grasslands are species-diverse ecosystems of high economic and ecological value. CO2 and water fluxes in these grasslands are triggered by the first rains in autumn, after a long hot and dry summer. Climate change scenarios project altered rainfall patterns, such as prolonged dry season into the autumn, while simultaneously nitrogen (N) deposition is increasing globally. However, how these global change drivers will interact to affect Mediterranean grassland CO2, water fluxes and productivity is still unclear. In a greenhouse experiment, we subjected the seedbank of an annual Mediterranean grassland to a factorial treatment, by prolonging the dry season by 0 days (i.e. no autumn drought), 50 days and 100 days and crossing these drought treatments with two levels of N deposition: no N and N addition. A delayed onset of the rain season, i.e., a prolonged dry season, induced lower CO2 and water fluxes throughout the growing season and a lower aboveground biomass by the end of the study period. However, N addition attenuated the effects on NEE, Reco and GPP, but did not affect aboveground biomass or functional group composition. A prolonged dry season also lowered the productivity of forbs, the dominant functional group in our grassland. Our results anticipate important effects of interacting global change drivers on Mediterranean grassland functioning.

The 'two-water-worlds' hypothesis is based on stable isotope differences in stream, soil and xylem waters in dual isotope space. It postulates no connectivity between bound and mobile soil waters, and preferential plant water uptake of bound soil water sources. We tested the pool-weighted impact of isotopically distinct water pools for hydrological cycling, the influence of species-specific water use and the degree of ecohydrological separation. We combined stable isotope analysis (δ18 O and δ2 H) of ecosystem water pools of precipitation, groundwater, soil and xylem water of two distinct species (Quercus suber, Cistus ladanifer) with observations of soil water contents and sap flow. Shallow soil water was evaporatively enriched during dry-down periods, but enrichment faded strongly with depth and upon precipitation events. Despite clearly distinct water sources and water-use strategies, both species displayed a highly opportunistic pattern of root water uptake. Here we offer an alternative explanation, showing that the isotopic differences between soil and plant water vs groundwater can be fully explained by spatio-temporal dynamics. Pool weighting the contribution of evaporatively enriched soil water reveals only minor annual impacts of these sources to ecosystem water cycling (c. 11% of bulk soil water and c. 14% of transpired water).

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.

Knowledge of plant hydraulic strategies (isohydric vs anisohydric) is crucial to predict the response of plants to changing environmental conditions, such as climate-change induced extreme drought. Several abiotic factors, such as evaporative demand, have been shown to seasonally modify the isohydricity of plants, however, the impact of biotic factors, such as plant-plant interactions on hydraulic strategies has seldom been explored. Here, we investigated adaptations in hydraulic strategies for two woody species in response to seasonal abiotic conditions, experimental drought and plant invasion in a Mediterranean cork oak (Quercus suber) ecosystem with a combined shrub invasion (Cistus ladanifer) and rain exclusion experiment. From summer to winter, the degree of isohydricity shifted from partial isohydric to anisohydric in Q. suber and inversely from strict anisohydric to partial isohydric for C. ladanifer. During drought, plant invasion significantly modified the hydraulic strategy of invaded Q. suber to a higher degree of anisohydricity with severe negative consequences for tree functioning, implying progressive leaf and xylem damage. The rain exclusion alone led to a non-significant increase in anisohydricity for both species. We demonstrate that the degree of isohydricity of plants is dynamically determined by the interplay of species-specific hydraulic traits and their abiotic and biotic environment.

Nutrients and herbivores have independent effects on the temporal stability of aboveground biomass in grasslands; however, their joint effects may not be additive and may also depend on spatial scales. In an experiment adding nutrients and excluding herbivores in 34 globally distributed grasslands, we found that nutrients and herbivores mainly had additive effects. Nutrient addition consistently reduced stability at the local and larger spatial scales (aggregated local communities), while herbivore exclusion weakly reduced stability at these scales. Moreover, nutrient addition reduced stability primarily by causing changes in local community composition over time and by reducing local species richness and evenness. In contrast, herbivore exclusion weakly reduced stability at the larger scale mainly by decreasing asynchronous dynamics among local communities, but also by weakly decreasing local species richness. Our findings indicate disentangling the influences of processes operating at different spatial scales may improve conservation and management in stabilizing grassland biomass.

Interannual variability in grassland primary production is strongly driven by precipitation, nutrient availability and herbivory, but there is no general consensus on the mechanisms linking these variables. If grassland biomass is limited by the single most limiting resource at a given time, then we expect that nutrient addition will not affect biomass production at arid sites. We conducted a distributed experiment manipulating nutrients and herbivores at 44 grassland sites in 8 regions around the world, spanning a broad range in aridity. We estimated the effects of 5-11 years of nutrient addition and herbivore exclusion treatments on precipitation sensitivity of biomass (proportional change in biomass relative to proportional change in rainfall among years), and the biomass in the driest year (to measure treatment effects when water was most limiting) at each site. Grazer exclusion did not interact with nutrients to influence driest year biomass or sensitivity. Nutrient addition increased driest year biomass by 74% and sensitivity by 0.12 (proportional units), and that effect did not change across the range of aridity spanned by our sites. Grazer exclusion did not interact with nutrients to influence sensitivity or driest year biomass. At almost half of our sites, the previous year's rainfall explained as much variation in biomass as current year precipitation. Overall, our distributed fertilization experiment detected co-limitation between nutrients and water governing grasslands, with biomass sensitivity to precipitation being limited by nutrient availability irrespective of site aridity and herbivory. Our findings refute the classical ideas that grassland plant performance is limited by the single most limiting resource at a site. This suggests that nutrient eutrophication will destabilize grassland ecosystems through increased sensitivity to precipitation variation.

The impact of interacting global change stressors on terrestrial ecosystems is hard to predict due to non-linear, amplifying, neutral or even buffering interaction effects. We investigated the effects of drought and plant invasion on Mediterranean cork oak (Quercus suber L.) ecosystem functioning and recovery with a combined rain exclusion (30-45 % reduction) and shrub (Cistus ladanifer L.) invasion experiment. As key parameter, we determined tree, shrub and ecosystem transpiration in four treatments: 1) cork oak control stands, 2) cork oaks with rain exclusion, 3) cork oaks invaded by shrubs and 4) cork oaks with rain exclusion and shrub invasion. Rain exclusion and plant invasion led to moderate, but neutral reductions of tree transpiration of 18 % (compared to control) during the mild summer drought in 2018. In 2019, the rain exclusion simulated the second driest year since 1950 for Southwestern (SW) Iberia. The interaction effect of drought and plant invasion was strongly amplifying, reducing tree transpiration by 47 %. Legacy effects on shrubs under the rain exclusion treatment led to a non-linear response during recovery from the severe drought in 2019. Invaded trees showed a delayed transpiration recovery (-51 % vs. control) due to strong competition with shrubs, while invaded trees with rain exclusion recovered to 75 % of the control. This buffering interaction response was caused by a weaker competition from drought-stressed shrubs. Given the projected increase in the frequency, intensity and duration of drought, an increasing non-linear impact on Mediterranean cork oak ecosystems is expected. Our results demonstrate that abiotic stressors modulate biotic interactions thereby impacting ecosystem functioning in a highly dynamic manner. Further efforts are thus needed to model and manage the impact of interacting global change stressors on terrestrial ecosystems.

Correction to: Nature Communications https://doi.org/10.1038/s41467-020-19870-y, published online 27 November 2020.

Droughts can strongly affect grassland productivity and biodiversity, but responses differ widely. Nutrient availability may be a critical factor explaining this variation, but is often ignored in analyses of drought responses. Here, we used a standardized nutrient addition experiment covering 10 European grasslands to test if full-factorial nitrogen, phosphorus, and potassium addition affected plant community responses to inter-annual variation in drought stress and to the extreme summer drought of 2018 in Europe. We found that nutrient addition amplified detrimental drought effects on community aboveground biomass production. Drought effects also differed between functional groups, with a negative effect on graminoid but not forb biomass production. Our results imply that eutrophication in grasslands, which promotes dominance of drought-sensitive graminoids over forbs, amplifies detrimental drought effects. In terms of climate change adaptation, agricultural management would benefit from taking into account differential drought impacts on fertilized versus unfertilized grasslands, which differ in ecosystem services they provide to society.

Interaction effects of different stressors, such as extreme drought and plant invasion, can have detrimental effects on ecosystem functioning and recovery after drought. With ongoing climate change and increasing plant invasion, there is an urgent need to predict the short- and long-term interaction impacts of these stressors on ecosystems. We established a combined precipitation exclusion and shrub invasion (Cistus ladanifer) experiment in a Mediterranean cork oak (Quercus suber) ecosystem with four treatments: (1) Q. suber control; (2) Q. suber with rain exclusion; (3) Q. suber invaded by shrubs; and (4) Q. suber with rain exclusion and shrub invasion. As key parameter, we continuously measured ecosystem water fluxes. In an average precipitation year, the interaction effects of both stressors were neutral. However, the combination of imposed drought and shrub invasion led to amplifying interaction effects during an extreme drought by strongly reducing tree transpiration. Contrarily, the imposed drought reduced the competitiveness of the shrubs in the following recovery period, which buffered the negative effects of shrub invasion on Q. suber. Our results demonstrate the highly dynamic and nonlinear effects of interacting stressors on ecosystems and urges for further investigations on biotic interactions in a context of climate change pressures.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory‐based) and realized (field‐based) soil net Nmin. Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site‐specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context‐dependent knowledge for grassland management worldwide.

Microbial processing of aggregate-unprotected organic matter inputs is key for soil fertility, long-term ecosystem carbon and nutrient sequestration and sustainable agriculture. We investigated the effects of adding multiple nutrients (nitrogen, phosphorus and potassium plus nine essential macro- and micro-nutrients) on decomposition and biochemical transformation of standard plant materials buried in 21 grasslands from four continents. Addition of multiple nutrients weakly but consistently increased decomposition and biochemical transformation of plant remains during the peak-season, concurrent with changes in microbial exoenzymatic activity. Higher mean annual precipitation and lower mean annual temperature were the main climatic drivers of higher decomposition rates, while biochemical transformation of plant remains was negatively related to temperature of the wettest quarter. Nutrients enhanced decomposition most at cool, high rainfall sites, indicating that in a warmer and drier future fertilized grassland soils will have an even more limited potential for microbial processing of plant remains. Fil: Ochoa Hueso, Raúl. Universidad de Cádiz; España Fil: Borer, Elizabeth. University of Minnesota; Estados Unidos Fil: Seabloom, Eric. University of Minnesota; Estados Unidos Fil: Hobbie, Sarah E.. University of Minnesota; Estados Unidos Fil: Risch, Anita C.. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Collins, Scott L.. University of New Mexico; Estados Unidos Fil: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Bahamonde, Héctor Alejandro. Universidad Nacional de la Patagonia Austral; Argentina. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Brown, Cynthia S.. State University of Colorado - Fort Collins; Estados Unidos Fil: Caldeira, Maria C.. Universidade Nova de Lisboa; Portugal Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Dickman, Chris R.. University of Sydney; Australia Fil: Ebeling, Anne. Universitat Jena; Alemania Fil: Eisenhauer, Nico. German Centre for Integrative Biodiversity Research; Alemania. Universitat Leipzig; Alemania Fil: Esch, Ellen H.. University Of Guelph. Department Of Integrative Biology.; Canadá Fil: Eskelinen, Anu. German Centre for Integrative Biodiversity Research; Alemania. Helmholtz Centre for Environmental Research; Alemania. University of Oulu; Finlandia Fil: Fernández, Victoria. Universidad Politécnica de Madrid; España Fil: Güsewell, Sabine. Institute of Integrative Biology; Suiza Fil: Gutierrez Larruga, Blanca. Universidad Autónoma de Madrid; España Fil: Hofmockel, Kirsten. Pacific Northwest National Laboratory; Estados Unidos. University of Iowa; Estados Unidos Fil: Laungani, Ramesh. Doane University; Estados Unidos Fil: Lind, Eric. University of Minnesota; Estados Unidos Fil: López, Andrea. University of New Mexico; Estados Unidos Fil: McCulley, Rebecca L.. University of Kentucky; Estados Unidos Fil: Moore, Joslin L.. Monash University; Australia Fil: Peri, Pablo Luis. Instituto Nacional de Tecnología Agropecuaria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Tecnológica Nacional. Facultad Regional Santa Cruz. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Nacional de la Patagonia Austral. Centro de Investigaciones y Transferencia de Santa Cruz; Argentina Fil: Power, Sally A.. University of Western Sydney; Australia Fil: Price, Jodi N.. Charles Sturt University; Australia Fil: Prober, Suzanne M.. Csiro National Collections And Marine Infrastructure; Australia Fil: Roscher, Christiane. German Centre for Integrative Biodiversity Research; Alemania. University of Oulu; Finlandia Fil: Velasco Ayuso, Sergio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

AimNutrient enrichment is associated with plant invasions and biodiversity loss. Functional trait advantages may predict the ascendancy of invasive plants following nutrient enrichment but this is rarely tested. Here, we investigate (a) whether dominant native and non‐native plants differ in important morphological and physiological leaf traits, (b) how their traits respond to nutrient addition, and (c) whether responses are consistent across functional groups.LocationAustralia, Europe, North America and South Africa.Time period2007–2014.Major taxa studiedGraminoids and forbs.MethodsWe focused on two types of leaf traits connected to resource acquisition: morphological features relating to light‐foraging surfaces and investment in tissue (specific leaf area, SLA) and physiological features relating to internal leaf chemistry as the basis for producing and utilizing photosynthate. We measured these traits on 503 leaves from 151 dominant species across 27 grasslands on four continents. We used an identical nutrient addition treatment of nitrogen (N), phosphorus (P) and potassium (K) at all sites. Sites represented a broad range of grasslands that varied widely in climatic and edaphic conditions.ResultsWe found evidence that non‐native graminoids invest in leaves with higher nutrient concentrations than native graminoids, particularly at sites where native and non‐native species both dominate. We found little evidence that native and non‐native forbs differed in the measured leaf traits. These results were consistent in natural soil fertility levels and nutrient‐enriched conditions, with dominant species responding similarly to nutrient addition regardless of whether they were native or non‐native.Main conclusionsOur work identifies the inherent physiological trait advantages that can be used to predict non‐native graminoid establishment, potentially because of higher efficiency at taking up crucial nutrients into their leaves. Most importantly, these inherent advantages are already present at natural soil fertility levels and are maintained following nutrient enrichment.

Eutrophication is a widespread environmental change that usually reduces the stabilizing effect of plant diversity on productivity in local communities. Whether this effect is scale dependent remains to be elucidated. Here, we determine the relationship between plant diversity and temporal stability of productivity for 243 plant communities from 42 grasslands across the globe and quantify the effect of chronic fertilization on these relationships. Unfertilized local communities with more plant species exhibit greater asynchronous dynamics among species in response to natural environmental fluctuations, resulting in greater local stability (alpha stability). Moreover, neighborhood communities that have greater spatial variation in plant species composition within sites (higher beta diversity) have greater spatial asynchrony of productivity among communities, resulting in greater stability at the larger scale (gamma stability). Importantly, fertilization consistently weakens the contribution of plant diversity to both of these stabilizing mechanisms, thus diminishing the positive effect of biodiversity on stability at differing spatial scales. Our findings suggest that preserving grassland functional stability requires conservation of plant diversity within and among ecological communities., Eutrophication has been shown to weaken diversity-stability relationships in grasslands, but it is unclear whether the effect depends on scale. Analysing a globally distributed network of grassland sites, the authors show a positive role of beta diversity and spatial asynchrony as drivers of stability but find that nitrogen enrichment weakens the diversity-stability relationships at different spatial scales.

Mediterranean ecosystems, such as the savannah-type cork oak (Quercus suber) woodlands, are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. These ecosystems are also threatened by shrub encroachment, which might further decrease tree water availability and affect ecosystem functioning and resilience. Nevertheless, the combined effects of drought and shrub encroachment on ecosystems have seldom been investigated. A precipitation manipulation and shrub removal experiment was established in a cork oak woodland located in SE Portugal and invaded by the native shrub gum rockrose (Cistus ladanifer). Here we present and discuss the combined effects of drought and shrub encroachment on litterfall production of cork oak trees, an evergreen species, over two contrasting years, a wet year (2018) and a dry year (2019) and assess the nitrogen and phosphorus resorption efficiencies from senescent to green leaves. A previous study reported significant increases in cork oak’s nitrogen resorption efficiency in response to drought. Our preliminary results also indicate changes in nitrogen and phosphorus resorption efficiencies. An increase in nutrient resorption efficiency is likely to mitigate the limitation in nutrient uptake by the roots during drought, improving tree fitness in the short-term. However, it will probably exert a negative feedback on the nitrogen and phosphorus cycles in the long-term which might affect the ecosystem functioning under the forecasted droughts.

Mediterranean type ecosystems such as cork oak (Quercus suber) woodlands are currently threatened by extreme drought events and shrub encroachment in the Iberian Peninsula. Recently, the frequency of extreme droughts has increased with negative effects on many ecosystems. Decreasing soil water availability reduces growth and fitness of trees, and may eventually induce tree mortality. Shrub encroachment may further increase the competition for soil water, impacting tree vulnerability and resilience negatively. Yet, the synergistic effects of extreme droughts and shrub encroachment on ecosystems have rarely been investigated.We established a precipitation manipulation and shrub encroachment experiment in a cork oak stand to study the combined effects of the two environmental pressures. The cork oak woodland is located in Southeast Portugal and partially invaded by the native shrub gum rockrose (Cistus ladanifer). In December 2017, we installed rainout shelters (30 to 45% of precipitation reduction) in replicated cork oak stands invaded and uninvaded by gum rockrose, complemented by control plots with natural precipitation. In each treatment, the trees (n = 9) and shrubs (n = 9) were measured for water and carbon fluxes to reveal species-specific responses and competition effects under recurrent extreme drought.The hydrological year 2018 was characterised by above-average precipitation mainly caused by large spring rainfall events. Probably due to sufficient water supply, no clear treatment effects were evident. For example, minimum leaf water potentials (ΨPD) of the cork oak trees did not drop below −1.5 ± 0.1 MPa and maximum sap flux density was 2.1 ± 0.2 m3 m−2 day−1. Minimum ΨPD of the shrubs was three times lower (−3.5 ± 0.1 MPa) and maximum sap flux density over four-fold higher (8.8 ± 0.8 m3 m−2 day−1) than those of the trees, suggesting distinct species-specific behaviour. Reduced winter and spring precipitation, combined with a late onset of autumn rainfalls in 2019, led to a decrease in water input down to 66% (control) and 44% (drought) compared to the long-term average of 585 mm. In this dry year, negative synergistic effects of drought and shrub encroachment were expressed during the dry-down and drought period by a lower minimum ΨPD and an average sap flux density reduced by 50% (0.4 ± 0.1 m3 m−2 day−1) of invaded trees exposed to the experimental drought, compared to control trees (0.8 ± 0.1 m3 m−2 day−1). In sum, this resulted in a reduction of sap flux densities of the cork oaks by 25% (invaded), 23% (drought) and 34% (drought and invaded) over the course of the hydrological year 2019. The ongoing investigations aim to further determine the stress tolerance and critical physiological thresholds for both species and the entire ecosystem.

Increased nutrient inputs and climate change are affecting ecosystems worldwide. However, there is a dearth of knowledge on how the interacting effects of multiple nutrient inputs and climatic variability may affect ecosystem functioning including grassland species and functional diversity, productivity or resilience to disturbances. This is particularly important in the Mediterranean Basin, a hotspot of climate change, where the frequency of autumn and spring droughts is projected to increase. We conducted a 6-year nutrient addition experiment in an annual grassland site, in Portugal, that is part of a globally distributed experiment called the Nutrient Network (http://www.nutnet.org/). We added high rates of nitrogen, phosphorus and potassium to 5 × 5 m plots, following a full factorial combination in a complete randomized three block design. We established three treatments of one, two and three added nutrients and maintained control plots without addition of nutrients. We examined how a decrease in nutrient limitation and inter-annual climatic variability affected grassland productivity and diversity. We determined the community functional structure (e.g., Community Weighted Mean) and functional diversity (e.g., Function Dispersion) of key morphological and physiological leaf traits associated with the leaf economics spectrum, resource acquisition and water use strategies. Our 6-year study period was characterized by contrasting climatological years, including two dry years (2017 and 2019). We found that grassland productivity was co-limited by multiple nutrients and that species richness decreased with nutrient enrichment. Dry years reduced productivity and species richness and were a critical factor reducing functional diversity of most of the studied traits. Species with competitive characteristics dominated nutrient enriched communities and were related to ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Contrary to expectations species richness was not related to stability. This study shows that mechanisms underlying ecological functioning of Mediterranean grasslands depend on interactions of multiple nutrient addition and precipitation variability. Understanding these mechanisms is crucial to anticipate potential effects of global changes on Mediterranean grasslands.

We applied an empirical modelling approach for gross primary productivity (GPP) estimation from hyperspectral reflectance of Mediterranean grasslands undergoing different fertilization treatments. The objective of the study was to identify combinations of vegetation indices and bands that best represent GPP changes between the annual peak of growth and senescence dry out in Mediterranean grasslands. In situ hyperspectral reflectance of vegetation and CO2 gas exchange measurements were measured concurrently in unfertilized (C) and fertilized plots with added nitrogen (N), phosphorus (P) or the combination of N, P and potassium (NPK). Reflectance values were aggregated according to their similarity (r≥90 %) in 26 continuous wavelength intervals (Hyp). In addition, the same reflectance values were resampled by reproducing the spectral bands of both the Sentinel-2A Multispectral Instrument (S2) and Landsat 8 Operational Land Imager (L8) and simulating the signal that would be captured in ideal conditions by either Sentinel-2A or Landsat 8. An optimal procedure for selection of the best subset of predictor variables (LEAPS) was applied to identify the most effective set of vegetation indices or spectral bands for GPP estimation using Hyp, S2 or L8. LEAPS selected vegetation indices according to their explanatory power, showing their importance as indicators of the dynamic changes occurring in community vegetation properties such as canopy water content (NDWI) or chlorophyll and carotenoids ∕ chlorophyll ratio (MTCI, PSRI, GNDVI) and revealing their usefulness for grasslands GPP estimates. For Hyp and S2, bands performed as well as vegetation indices to estimate GPP. To identify spectral bands with a potential for improving GPP estimates based on vegetation indices, we applied a two-step procedure which clearly indicated the short-wave infrared region of the spectra as the most relevant for this purpose. A comparison between S2- and L8-based models showed similar explanatory powers for the two simulated satellite sensors when both vegetation indices and bands were included in the model. Altogether, our results describe the potential of sensors on board Sentinel-2 and Landsat 8 satellites for monitoring grassland phenology and improving GPP estimates in support of a sustainable agriculture management.

Forest certification is a conservation tool, which aims to promote the sustainable management and conservation of forest ecosystems. Establishing set-aside or lower intervention conservation zones to promote biodiversity conservation is a requisite of forest certification. We tested the effects of conservation zones on the tree biometrics and regeneration, as well as on the taxonomic, functional, and structural diversity of the shrub and herb understory, in Mediterranean oak woodlands. We also assessed how oak biometrics, regeneration and understory diversity varied among conservation zones established 10, 14, and 20 years before our sampling dates. Oak regeneration tended to be higher in conservation zones than in controls, but results varied with the age of conservation zones. For example, the abundance of oak seedlings and saplings was higher in 10-year-old conservation zones than in those established 20 years ago. Abundance of young oak trees was higher in 14-year-old conservation zones than in 10- and 20-year-old conservation zones. The understory vertical diversity was significantly higher in 14- and 20-year-old conservation zones than in controls. Functional diversity differed significantly between conservation zones and controls, with a higher abundance of late-successional shrubs, namely fleshy-fruited species in 20-year-old conservation zones. The plant species composition of the shrub and the herb understory was most dissimilar between older conservation zones and controls. Additionally, the cover and diversity of the understory herb species decreased with the age of conservation zones. Conservation zones implemented under forest certification increase habitat structural complexity of oak woodlands, which may benefit wildlife species, but there will be trade-offs with the cover and diversity of the herb understory. Forest managers must evaluate such trade-offs when establishing conservation zones in cork oak woodlands under forest certification schemes.

Global change is affecting ecosystems worldwide. Nitrogen (N) deposition is increasing globally and climate change scenarios forecast an increase of prolonged autumn droughts particularly in the Mediterranean regions. Mediterranean grasslands occupy 1.3 million km2 of the terrestrial surface, host high biodiversity values and generate important ecosystem services. Understanding the effects of N deposition and prolonged autumn drought on the productivity and dynamics of Mediterranean grasslands is crucial to support the conservation of these ecosystems in a global change context. We assessed the effects of extended autumn drought and N deposition on the productivity, species diversity, and phenology of a Mediterranean grassland through a manipulative, greenhouse, experiment. We simulated three levels of extended drought: no drought, 50 days and 100 days of drought and two levels of N deposition: no N and N addition, through a factorial design experiment. Severe drought (100 days) affected negatively grassland productivity, through a reduction of the growing season duration and accumulated growing degree days. Moderate drought (50 days) affected productivity negatively but productivity values recovered to levels similar to those of the non-drought treatment. Severe and moderate drought delayed plant phenology decreasing significantly the number of individuals with mature flowers and with fruits or seeds. Plant functional groups responded differently to drought with productivity of forbs, but not productivity of graminoids and legumes, being negatively affected by moderate and severe drought. N addition did not affect grassland productivity, diversity or phenology. Our results anticipate effects of climate change on the productivity and species composition of Mediterranean grasslands.

In the Mediterranean and temperate regions, an increase in the frequency and intensity of drought events has been recorded, probably due to climate change. In consequence, trees will more frequently experience hydric stress, a condition that can be expected to affect insect–tree interactions, while adaptation mechanisms may be further in course. The effect of tree water stress on the performance of two allochronic populations ofThaumetopoea pityocampawas here studied. Namely, we compared a unique population of this insect, in which the larvae develop in the summer (SP), with the typical population having winter larval development (WP), to test the adaptation hypothesis to host plant status. Larvae of each population were fed on needles of young pottedPinus pinasterplants under two water supply regimes: (i) well-watered (control) and (ii) subjected to 3 months of drought stress. Compared to control, stressed plants had higher amounts of soluble sugars, phenols, and higher C/N ratio, whereas water content and chlorophylls concentrations were lower. In general,T. pityocampalarvae had lower performances on water-stressed plants, as shown by lower survival rates, lower needle consumption, and longer development times. Yet, the detrimental effects of tree stress were only significant for the WP larvae, while SP larvae were able to overcome such conditions. Results demonstrate that tree water stress can negatively affectT. pityocampapopulations. Furthermore, the evidence is also provided that responses to the physiological condition of the host trees may occur at the population level, as a result of adaptation mechanisms driven by climate change.

Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs (‘consumer-controlled’). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food (‘resource-controlled’). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk., It is unclear whether terrestrial herbivores are able to consume the extra plant biomass produced under nutrient enrichment. Here the authors test this in grasslands using a globally distributed network of coordinated field experiments, finding that wild herbivore control on grassland production declines under eutrophication.

Soil nitrogen mineralisation (Nmin), the conversion of organic into inorganic N, is important for productivity and nutrient cycling. The balance between mineralisation and immobilisation (net Nmin) varies with soil properties and climate. However, because most global-scale assessments of net Nmin are laboratory-based, its regulation under field-conditions and implications for real-world soil functioning remain uncertain. Here, we explore the drivers of realised (field) and potential (laboratory) soil net Nmin across 30 grasslands worldwide. We find that realised Nmin is largely explained by temperature of the wettest quarter, microbial biomass, clay content and bulk density. Potential Nmin only weakly correlates with realised Nmin, but contributes to explain realised net Nmin when combined with soil and climatic variables. We provide novel insights of global realised soil net Nmin and show that potential soil net Nmin data available in the literature could be parameterised with soil and climate data to better predict realised Nmin., Nitrogen mineralisation (Nmin), an important index of soil fertility, is often determined in the laboratory, with an uncertain relationship to Nmin under field conditions. Here the authors show that combining laboratory measurements with environmental data greatly improves predictions of field Nmin at a global scale.

Parallel multiple aliquot calibration transfer is combined with the use of standardized optically stimulated luminescence signals and evaluation of single aliquot regenerative dose response characteristics, to produce a robust and efficient transfer protocol for mineral samples used in dating and retrospective dosimetry. Transfer was made from an IST-LPSR 60 Co primary air kerma standard to four Riso and Daybreak 90 Sr/ 90 Y irradiators, for quartz or polymineral grains of 90/100–160 μm or 160–250 μm diameter from seven samples. Grains were pretreated by activation, heating, or bleaching, and mounted on 0.5 mm thick aluminium or stainless steel disks, or 0.25 mm thick stainless steel cups. Multiple aliquot conversion coefficients based on parallel irradiations (mGy 60Co per s of 90 Sr/ 90 Y exposure) were corrected for non linearity in dose response using the regenerative measurements, and compared with “single aliquot” coefficients obtained directly from the regenerative, i.e. retrospectively measured, dose responses. Ratios of measured/given β exposure time provide controls: these were close to unity for activated and heated material, which exhibited predose sensitization, and vice versa for optically bleached samples. Parallel multiple aliquot calibration transfer, using OSL integrated over the main signal decay, was found to offer better accuracy and precision than retrospective single aliquot measurements, and was robust for polyminerals as well as quartz. Differences in conversion coefficient between grain-sizes and supports on a given irradiator ranged up to 25% and were specific to the irradiator-support-grainsize permutation. Geometric effects are quantitatively explained by the solid angle subtended at the source by the sample, and the effect of support material (and thickness) by differences in electron backscatter.

Mediterranean oak woodlands are ecosystems of high conservation and socio-economic value that occur in Southwestern Europe, North Africa and California. Oak regeneration failure is occurring in these ecosystems and may be endangering their long-term conservation. Most studies suggest that inadequate management practices may be contributing to oak regeneration failure. Forest certification is a voluntary type of certification, based on third-party auditing of compliance with performance-based sustainable management standards that has been expanding in forest ecosystems worldwide, including in Mediterranean oak woodlands. The Forest Stewardship Council (FSC) certification is the dominant certification scheme in Mediterranean oak woodlands and requires landowners to establish conservation zones in their estates. Conservation zones usually correspond to a tenth of the estate and are primarily managed for biodiversity conservation. In spite of recent studies reporting positive effects of FSC certification and conservation zones on biodiversity and forest structure in tropical regions, its effects on tree regeneration in Mediterranean oak woodlands are unknown. In this study, conducted in Southwestern Europe, we compared the abundance of cork oak (Quercus suber) regeneration and the cover, richness and diversity of Mediterranean shrublands between conservation and non-conservation zones in FSC certified cork oak woodlands. We found that in conservation zones oak regeneration was more abundant and that species richness and diversity of shrubs were significantly higher. Our results suggest that the creation of set-aside areas in cork oak woodlands, such as conservation zones, may help avert the tree regeneration crisis this ecosystem is facing.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-20997-9.

Herbivory, plant facilitation, and competition have complex impacts on tree regeneration which are seldom investigated together. Grazing exclosure experiments have allowed quantification of the effects of large herbivores on tree regeneration dynamics but have often ignored the effect of herbivorous insects. We experimentally tested how folivory (percentage of leaf damaged by insects) and microenvironment (tree canopy cover and herbs) jointly alter performance (growth and survival) of seedlings of two dominant Mediterranean oak-species within ungulate exclosures in a 3-year field study. An agroforestry system dominated by cork oak (Quercus suber) and holm oak (Q. rotundifolia) was assessed in south-east Portugal. We aimed also to determine whether the two oak species differed in the interdependences between folivory, microenvironment, covaring factors, and seedling performance. Unexpectedly, under the low–moderate insect defoliation, growth and survival of cork and holm oak seedlings were positively associated with herbivore damage. Herb removal increased oak folivory by 1.4 times. Herb removal was also positively associated with growth, directly and indirectly through its negative effect on oak folivory. Tree canopy favored insect folivory upon cork oak seedlings directly and upon holm oak indirectly via decreasing light availability. Folivory was threefold greater upon cork than upon holm oak-seedlings. Our study shows that tree canopy, herbs, and covarying factors can affect cork and holm oak-seedling performances through complex pathways, which markedly differ for the two species. The combined effect of insect herbivory and positive and negative plant-plant interactions need to be integrated into future tree regeneration efforts toward tackling the regeneration crisis of oak agroforestry systems of the Mediterranean.

Mediterranean ecosystems are hotspots for climate change, as the highest impacts are forecasted for the Mediterranean region, mainly by more frequent and intense severe droughts. Plant phenology is a good indicator of species' responses to climate change. In this study, we compared the spring phenology of cork oak trees (Quercus suber), an evergreen species, over 2 contrasting years, a mild year (2004) and a dry year (2005), which was the most severe drought since records exist. We evaluated the timing of occurrence, duration, and intensity of bud development, budburst, shoot elongation, trunk growth, and leaf senescence (phenophases) and assessed the nitrogen resorption efficiency from senescent to green leaves. The temperature was the main driver of budburst. Nevertheless, water had the main role of constraining all the other phenophases by strongly reducing the growing season length (−48 %) and consequently the tree growth. Basal area increment was the most affected growth variable (−36 %), although it occurred at a similar rate in the 2 years. Shoot elongation was also reduced (−21 %), yet elongation occurred at a higher rate in the dry year compared to the mild year. Leaf senescence during the bulk period was higher in the dry year, in which leaves were shed at the same rate over a longer period. Nitrogen concentrations in green and senescent leaves were affected by drought and nitrogen resorption efficiency increased remarkably (+22 %). Our results highlight the importance of studying different phenological metrics to improve our understanding of the ecosystem's responses to climate change. The faster dynamics observed in shoot elongation, while all other phenophases developed at the same rate, indicate that leaf area development is privileged in cork oak. Water availability was the main driver of spring growth in this Mediterranean ecosystem; however, growth may be affected by complex interplays between precipitation and temperature, such as higher temperatures during dry winters or heatwaves during spring, that are likely to result in water stress. Longer studies are needed to disentangle those interplays. Finally, a higher nitrogen resorption efficiency in response to drought appears to be an adaptive trait that mitigates the limitation in nitrogen uptake by the roots during drought and contributes to improving tree fitness in the short term but will probably exert a negative feedback on the nitrogen cycle in the long term, which might affect the ecosystem functioning under the forecasted droughts.

Herbivory, plant facilitation, and competition have complex impacts on tree regeneration which are seldom investigated together. Grazing exclosure experiments have allowed to quantify the effects of large herbivores on tree regeneration dynamics but have often ignored the effect of herbivorous insects. We experimentally tested how folivory (percentage of leaf damaged by insects), and microenvironment (tree-canopy cover and herbs) jointly alter performance (growth and survival) of seedlings of two dominant Mediterranean oak-species within ungulate exclosures. An agroforestry system dominated by cork oak (Quercus suber) and holm oak (Q. rotundifolia) was assessed in south-east Portugal. We aimed also to determine whether the two oak-species differ in the interdependences between folivory, microenvironment, covaring factors, and seedling performance. Unexpectedly, under the low–moderate insect defoliation occurred in our 3-year field study, growth and survival of cork and holm oak-seedlings were positively associated with herbivory damage. Herb removal increased oak folivory by 1.4 times. Herb removal was also positively associated with growth, directly and indirectly through its negative effect on oak folivory. Tree-canopy favored insect folivory upon cork oak seedlings directly and upon holm oak indirectly via decreasing light availability. Folivory was threefold greater upon cork than upon holm oak-seedlings. Our study shows that tree-canopy, herbs, and covarying factors can affect cork and holm oak-seedling performances through complex pathways, which markedly differ for the two species. The combined effect of insect herbivory and positive and negative plant-plant interactions need to be integrated into future tree regeneration efforts toward tackling the regeneration crisis of oak agroforestry systems of the Mediterranean.