Grazing-induced changes in plant composition affect litter quality and nutrient cycling in Flooding Pampa grasslands

Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono; Argentina.Fil: Semmartin, María. Universidad de Buenos Aires (UBA). Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA); Argentina.Fil: Chaneton, Enrique J. Universidad de Buenos Aires (UBA). Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA); Argentina.Fil: Garibaldi, Lucas Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Argentina.Fil: Semmartin, María. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Argentina.Fil: Chaneton, Enrique J. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Argentina.Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G-) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G- grasses. While no consistent differences between G+ and G- grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species' "responses" to grazing may only partially overlap with functional groups based on species "effects" on C and nutrient cycling

Grazing history effects on above-and below-ground litter decomposition and nutrient cycling in two co-occurring grasses

Fil: Semmartin, María. Universidad de Buenos Aires (UBA). Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA); Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono; Argentina.Fil: Chaneton, Enrique J. Universidad de Buenos Aires (UBA). Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA); Argentina.Fil: Chaneton, Enrique J. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Argentina.Fil: Garibaldi, Lucas Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Semmartin, María. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Argentina.Large herbivores may alter carbon and nutrient cycling in soil by changing above- and below-ground litter decomposition dynamics. Grazing effects may reflect changes in plant allocation patterns, and thus litter quality, or the site conditions for decomposition, but the relative roles of these broad mechanisms have rarely been tested. We examined plant and soil mediated effects of grazing history on litter mass loss and nutrient release in two grazing-tolerant grasses, Lolium multiflorum and Paspalum dilatatum, in a humid pampa grassland, Argentina. Shoot and root litters produced in a common garden by conspecific plants collected from grazed and ungrazed sites were incubated under both grazing conditions. We found that grazing history effects on litter decomposition were stronger for shoot than for root material. Root mass loss was neither affected by litter origin nor incubation site, although roots from the grazed origin immobilised more nutrients. Plants from the grazed site produced shoots with higher cell soluble contents and lower lignin:N ratios. Grazing effects mediated by shoot litter origin depended on the species, and were less apparent than incubation site effects. Lolium shoots from the grazed site decomposed and released nutrients faster, whereas Paspalum shoots from the grazed site retained more nutrient than their respective counterparts from the ungrazed site. Such divergent, species-specific dynamics did not translate into consistent differences in soil mineral N beneath decomposing litters. Indeed, shoot mass loss and nutrient release were generally faster in the grazed grassland, where soil N availability was higher. Our results show that grazing influenced nutrient cycling by modifying litter breakdown within species as well as the soil environment for decomposition. They also indicate that grazing effects on decomposition are likely to involve aerial litter pools rather than the more recalcitrant root compartment

Nutrient supply and bird predation additively control insect herbivory and tree growth in two contrasting forest habitats

Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono; Argentina.Fil: Kitzberger, Thomas. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche y INIBIOMA-oCONICET. Laboratorio Ecoton; Argentina.Fil: Mazía, Noemí C. Universidad de Buenos Aires (UBA). Facultad de Agronomía; Argentina.Fil: Chaneton, Enrique J. Universidad de Buenos Aires (UBA). Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA)-CONICET; Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad de Buenos Aires (UBA). Facultad de Agronomía; Argentina.It has been suggested that bottom–up and top–down forces interactively control food web dynamics. While top–down effects would increase with resource availability to plants, bottom–up effects would be stronger under low predator abundance. These predictions, however, have rarely been tested at contrasting sites while keeping the dominant plant species unchanged. Furthermore, few studies have factorially manipulated both types of forces in forest communities. For two years, we evaluated the effects of fertiliser (NPK) addition and bird exclusion on tree growth, leaf traits, insect abundance, and folivory rates in a dry/warm and a wet/cold Nothofagus pumilio forest in Patagonia, Argentina. Overall, we found no interaction between nutrient supply and bird predation, although the strength of bottom–up and top–down forces differed markedly between forest sites. Treatment effects were generally weak in the wet forest, where tree growth rates and insect herbivory were low relative to the dry forest. In the dry forest, fertilisation increased sapling growth, insect abundance and folivory, whereas bird exclusion increased leaf damage and reduced tree growth. In the wet forest, fertilisation enhanced leaf nutrient contents and folivore abundance but not sapling growth, while bird exclusion had little impact on insects or trees. These results imply that factors other than nutrients and birds were important in controlling tree growth and folivore activity in the wet forest. While treatment effect sizes varied widely among feeding guilds, in general, nutrient effects on folivores were stronger than predator effects. We conclude that, within the time‐frame of this study, tree growth and herbivory were additively affected by soil nutrients and predator presence, as bird exclusion effects did not change with elevated folivore activity on fertilised trees. We also show that both top–down and bottom–up cascades were weaker in a forest site characterised by slow‐growing juvenile trees subjected to low folivore pressure

Variable strength of top‐down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO event

Fil: Mazía, Noemí C. Universidad de Buenos Aires. Facultad de Agronomía; Argentina.Fil: Chaneton, Enrique J. Universidad de Buenos Aires (UBA). Facultad de Agronomía. IFEVA- CONICET; Argentina.Fil: Kitzberger, Thomas. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono-CONICET; Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono; Argentina.Fil: Garibaldi, Lucas Alejandro. Universidad de Buenos Aires. Facultad de Agronomía; Argentina.Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top-down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’ Nothofagus pumilio forests in the northern Patagonian Andes, Argentina.The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size.The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird-initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above-normal herbivory, and also support the view that large-scale climatic events influence the strength of trophic cascades

Global change effects on plant communities are magnified by time and the number of global change factors imposed

Komatsu, Kimberly J. Smithsonian Environmental Research Center, Edgewater. United States.Avolio, Meghan L. Johns Hopkins University. Department of Earth and Planetary Sciences. Baltimore, United States.Lemoine, Nathan P. Marquette University. Department of Biological Sciences. Milwaukee, United States.Chaneton, Enrique José. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Chaneton, Enrique José. CONICET – Universidad de Buenos Aires. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Tognetti, Pedro Maximiliano. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Tognetti, Pedro Maximiliano. CONICET – Universidad de Buenos Aires. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Yahdjian, María Laura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Yahdjian, María Laura. CONICET – Universidad de Buenos Aires. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA). Buenos Aires, Argentina.Isbell, Forest. University of Minnesota. Department of Ecology, Evolution and Behavior. Saint Paul, United States.Grman, Emily. Eastern Michigan University. Department of Biology. Ypsilanti, United States.17867–17873Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of CDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term ( minor to 10 y). In contrast, long-term (major or equal to 10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously

Environmental heterogeneity modulates the effect of plant diversity on the spatial variability of grassland biomass

Plant productivity varies due to environmental heterogeneity, and theory suggests that plant diversity can reduce this variation. While there is strong evidence of diversity effects on temporal variability of productivity, whether this mechanism extends to variability across space remains elusive. Here we determine the relationship between plant diversity and spatial variability of productivity in 83 grasslands, and quantify the effect of experimentally increased spatial heterogeneity in environmental conditions on this relationship. We found that communities with higher plant species richness (alpha and gamma diversity) have lower spatial variability of productivity as reduced abundance of some species can be compensated for by increased abundance of other species. In contrast, high species dissimilarity among local communities (beta diversity) is positively associated with spatial variability of productivity, suggesting that changes in species composition can scale up to affect productivity. Experimentally increased spatial environmental heterogeneity weakens the effect of plant alpha and gamma diversity, and reveals that beta diversity can simultaneously decrease and increase spatial variability of productivity. Our findings unveil the generality of the diversity-stability theory across space, and suggest that reduced local diversity and biotic homogenization can affect the spatial reliability of key ecosystem functions.EEA Santa CruzFil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Marinas y Costeras (IIMyC); Argentina.Fil: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Marinas y Costeras (IIMyC); Argentina.Fil: Chaneton, Enrique J. Universidad de Buenos Aires. Facultad de Agronomía; Argentina.Fil: Chaneton, Enrique J. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA); Argentina.Fil: Iribarne, Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Marinas y Costeras (IIMyC); Argentina.Fil: Tognetti, Pedro M. Universidad de Buenos Aires. Facultad de Agronomía; Argentina.Fil: Tognetti, Pedro M. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA); Argentina.Fil: Bakker, Jonathan D. University of Washington. School of Environmental and Forest Sciences; Estados UnidosFil: Borer, Elizabeth T. University of Minnesota. Department of Ecology, Evolution & Behavior; Estados UnidosFil: Bruschetti, Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Marinas y Costeras (IIMyC); Argentina.Fil: MacDougall, Andrew S. University of Guelph.Department of Integrative Biology; CanadáFil: Pascual, Jesús. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Marinas y Costeras (IIMyC); Argentina.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: Hautier, Yann. Utrecht University. Department of Biology. Ecology and Biodiversity Group; Países Bajo

Progress in creating a joint research agenda that allows networked long-term socio-ecological research in southern South America : addressing crucial technological and human capacity gaps limiting its application in Chile and Argentina

Since 1980, more than 40 countries have implemented long-term ecological research (LTER) programs, which have shown their power to affect advances in basic science to understand the natural world at meaningful temporal and spatial scales and also help link research with socially relevant outcomes. Recently, a disciplinary paradigmatic shift has integrated the human dimensions of ecosystems, leading to a long-term socio-ecological research (LTSER) framework to address the world's current environmental challenges. A global gap in LTER/LTSER only exists in the latitudinal range of 40–60°S, corresponding to Argentina and Chile's temperate/sub-Antarctic biome. A team of Chilean, Argentine and US researchers has participated in an ongoing dialogue to define not only conceptual, but also practical barriers limiting LTER/LTSER in southern South America. We have found a number of existing long-term research sites and platforms throughout the region, but at the same time it has been concluded an agenda is needed to create and implement further training courses for students, postdoctoral fellows and young scientists, particularly in the areas of data and information management systems. Since LTER/LTSER efforts in Chile and Argentina are incipient, instituting such courses now will enhance human and technical capacity of the natural science and resource community to improve the collection, storage, analysis and dissemination of information in emerging LTER/LTSER platforms. In turn, having this capacity, as well as the ongoing formalization of LTER/LTSER programs at national levels, will allow the enhancement of crucial collaborations and comparisons between long-term research programs within the region and between hemispheres and continents. For Spanish version of the entire article, see Online Supporting Information (Appendix S1).Desde 1980, más de cuarenta países han implementado programas de Investigación Ecológica a Largo Plazo (LTER por sus siglas en inglés), los cuales han mostrado su capacidad para influir sobre los avances en las ciencias básicas que permiten entender el mundo natural en escalas temporales y espaciales significativas, y también ayudar a enfocar la investigación hacia estudios socialmente relevantes. Recientemente, gracias a un cambio de paradigma en la disciplina, se integró también la dimensión humana de los ecosistemas, llevándola a un marco conceptual de Investigación Socio-Ecológica a Largo Plazo (LTSER por sus siglas en inglés) para enfrentar los desafíos medio-ambientales del mundo actual. Existe un vacío global en LTER/LTSER en el rango latitudinal de 40–60°S, correspondiente a los biomas templados/subantárticos de Argentina y Chile. Un equipo de investigadores chilenos, argentinos y estadounidenses ha trabajado por varios años para definir cuáles son la barreras que actualmente limitan la creación de una Red de LTER/LTSER en el sur de Sudamérica, no solamente en términos conceptuales, sino también a nivel práctico. Existe un buen número de sitios de investigación a largo plazo en la región, pero también concluimos que es necesario crear e implementar más cursos de capacitación para estudiantes, investigadores post-doctorales y jóvenes científicos, particularmente en las áreas de sistemas de manejo de datos e información. Considerando que los esfuerzos LTER/LTSER en Chile y Argentina son incipientes, este tipo de cursos podría mejorar la capacidad humana y técnica en la comunidad de las ciencias y los recursos naturales, así como mejorar los procesos de recolección, almacenamiento, análisis y difusión de la información. A su vez, la formalización de cursos de programas LTER/LTSER a nivel nacional para adquirir dicha capacidad de manejo de la información, permitirá un fortalecimiento crucial de las colaboraciones y comparaciones entre programas de investigación a largo plazo dentro de la región, y entre hemisferios y continentes. La versión en castellano del artículo se encuentra disponible en forma digital como Online Supporting Information S1.Fil: Anderson, Chistopher B. University of North Texas. Department of Biological Sciences; Estados UnidosFil: Celis-Diez, Juan Luis. Pontificia Universidad Católica de Valparaíso, Escuela de Agronomía; ChileFil: Bond, Barbara J.H.G. Oregon State University. Andrews Forest Long-Term Ecological Research Site. Department of Forest Ecosystems and Society; Estados UnidosFil: Martínez Pastur, Guillermo José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Cientificas; ArgentinaFil: Little, Christian. Universidad Austral de Chile. Facultad de Ciencias. Instituto de Ciencias de la Tierra y Evolución; Chile. Fundación Centro de los Bosques Nativos FORECOS; ChileFil: Armesto, Juan J. Pontificia Universidad Católica de Valparaíso, Escuela de Agronomía; ChileFil: Ghersa, Claudio Marco. 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; ArgentinaFil: Austin, Amy Theresa. 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; ArgentinaFil: Schlichter, Tomas Miguel. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche. Grupo de Ecología Forestal; ArgentinaFil: Lara, Antonio. Fundación Centro de los Bosques Nativos FORECOS; Chile. Universidad Austral de Chile. Facultad de Ciencias Forestales y Recursos Naturales. Instituto de Silvicultura; ChileFil: Carmona, Martin. Universidad de Chile. Instituto de Ecologıa y Biodiversidad; ChileFil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomia. Departamento de Recursos Naturales y Ambiente; ArgentinaFil: Gutierrez, Julio R. Universidad de La Serena. Departamento de Biología. Instituto de Ecología y Biodiversidad. Centro de Estudios Avanzados en Zonas Aridas; ChileFil: Rozzi, Ricardo. Universidad de La Serena. Departamento de Biología. Instituto de Ecología y Biodiversidad; ChileFil: Vanderbilt, Kristin University of New Mexico. Department of Biology. Sevilleta Long-Term Ecological Research Site; Estados UnidosFil: Oyarce, Guillermo University of North Texas. Library and Information Sciences; Estados UnidosFil: Fernandez, Roberto J. University of North Texas, Department of Biological Sciences; Estados Unido

Local Loss and Spatial Homogenization of Plant Diversity Reduce Ecosystem Multifunctionality

Biodiversity is declining in many local communities while also becoming increasingly homogenized across space. Experiments show that local plant species loss reduces ecosystem functioning and services, but the role of spatial homogenization of community composition and the potential interaction between diversity at different scales in maintaining ecosystem functioning remains unclear, especially when many functions are considered (ecosystem multifunctionality). We present an analysis of eight ecosystem functions measured in 65 grasslands worldwide. We find that more diverse grasslands—those with both species-rich local communities (α-diversity) and large compositional differences among localities (β-diversity)—had higher levels of multifunctionality. Moreover, α- and β-diversity synergistically affected multifunctionality, with higher levels of diversity at one scale amplifying the contribution to ecological functions at the other scale. The identity of species influencing ecosystem functioning differed among functions and across local communities, explaining why more diverse grasslands maintained greater functionality when more functions and localities were considered. These results were robust to variation in environmental drivers. Our findings reveal that plant diversity, at both local and landscape scales, contributes to the maintenance of multiple ecosystem services provided by grasslands. Preserving ecosystem functioning therefore requires conservation of biodiversity both within and among ecological communities

Out of the shadows:multiple nutrient limitations drive relationships among biomass, light and plant diversity

1. The paradigmatic hypothesis for the effect of fertilisation on plant diversity represents a one-dimensional trade-off for plants competing for below-ground nutrients ( generically) and above-ground light: fertilisation reduces competition for nutrients while increasing biomass and thereby shifts competition for depleted available light. 2. The essential problem of this simple paradigm is that it misses both the multivariate and mechanistic nature of the factors that determine biodiversity as well as their causal relationships. 3. We agree that light limitation, as DeMalach and Kadmon argue, can indeed be an important factor associated with diversity loss, and we presented it as an integral part of our tests of the niche dimension hypothesis. 4. We disagree with DeMalach and Kadmon that light is the 'main' factor explaining diversity, because this misrepresents the causal structure represented in the design of our experiment in which multiple nutrient addition was the ultimate causal driver of a suite of correlated responses that included diversity and light, and especially live and dead biomass, which are the factors that control light depletion. ]5. Our findings highlight that multiple nutrient limitations can structure plant diversity and composition independently of changes in light and biomass. For example, approximately one-third of our sites showed no significant increase in biomass with greater number of added nutrients yet still lost diversity when nutrients were added. 6. The important message is that while light limitation can be an important contributor to diversity loss, it is not a necessary mechanism

Global change effects on plant communities are magnified by time and the number of global change factors imposed

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously