• Energy Research

Aridity is a critical driver of the diversity and composition of plant communities. However, how aridity influences the phylogenetic structure of functional groups (i.e. annual and perennial species) is far less understood than its effects on species richness. As perennials have to endure stressful conditions during the summer drought, as opposed to annuals that avoid it, they may be subjected to stronger environmental filtering. In contrast, annuals may be more susceptible to interannual climatic variability. Here we studied the phylogenetic structure of the annual and perennial components of understorey plant communities, along a regional aridity gradient in Mediterranean drylands. Specifically, we asked: (1) How do species richness (S) and phylogenetic structure (PS) of annuals and perennials in plant communities respond to aridity? (2) What is the contribution of other climatic and topo-edaphic variables in predicting S and PS for both components? (3) How does the taxonomic and phylogenetic turnover of annuals and perennials vary with spatial and environmental distances? We assessed annuals' and perennials' species richness, the phylogenetic structure at deep and shallow phylogenetic levels, and taxonomic and phylogenetic turnover along spatial and environmental distances. We found no relationship between annuals' richness and aridity, whereas perennials' richness showed a unimodal pattern. The phylogenetic structure of annuals and perennials showed contrasting responses to aridity and negatively correlated with topo-edaphic variables. We found phylogenetic clustering at intermediate-to-higher aridity levels for annuals, and at lower aridity levels for perennials. Both taxonomic and phylogenetic turnover in annuals and perennials correlated with the environmental distance rather than with spatial distance between communities, suggesting adaptation to local factors. Overall, our results show a decoupling in the response of the phylogenetic structure of annual and perennial components of plant communities to aridity in Mediterranean drylands. Our findings have significant implications for land management strategies under climate change.

Le pâturage représente l'utilisation la plus étendue des terres dans le monde. Pourtant, ses impacts sur les services écosystémiques restent incertains car des interactions omniprésentes entre la pression de pâturage, le climat, les propriétés des sols et la biodiversité peuvent se produire mais n'ont jamais été traitées simultanément. En utilisant une enquête standardisée sur 98 sites sur six continents, nous montrons que les interactions entre la pression du pâturage, le climat, le sol et la biodiversité sont essentielles pour expliquer la fourniture de services écosystémiques fondamentaux dans les zones arides du monde entier. L'augmentation de la pression de pâturage a réduit la prestation de services écosystémiques dans les zones arides plus chaudes et pauvres en espèces, tandis que les effets positifs du pâturage ont été observés dans les zones plus froides et riches en espèces. La prise en compte des interactions entre le pâturage et les facteurs abiotiques et biotiques locaux est essentielle pour comprendre le sort des écosystèmes des terres arides sous le changement climatique et l'augmentation de la pression humaine. El pastoreo representa el uso más extenso de la tierra en todo el mundo. Sin embargo, sus impactos en los servicios ecosistémicos siguen siendo inciertos porque las interacciones generalizadas entre la presión del pastoreo, el clima, las propiedades del suelo y la biodiversidad pueden ocurrir, pero nunca se han abordado simultáneamente. Utilizando una encuesta estandarizada en 98 sitios en seis continentes, mostramos que las interacciones entre la presión del pastoreo, el clima, el suelo y la biodiversidad son fundamentales para explicar la prestación de servicios ecosistémicos fundamentales en las tierras secas de todo el mundo. El aumento de la presión del pastoreo redujo la prestación de servicios ecosistémicos en las tierras secas más cálidas y pobres en especies, mientras que los efectos positivos del pastoreo se observaron en las zonas más frías y ricas en especies. Considerar las interacciones entre el pastoreo y los factores abióticos y bióticos locales es clave para comprender el destino de los ecosistemas de tierras secas bajo el cambio climático y el aumento de la presión humana. Grazing represents the most extensive use of land worldwide. Yet its impacts on ecosystem services remain uncertain because pervasive interactions between grazing pressure, climate, soil properties, and biodiversity may occur but have never been addressed simultaneously. Using a standardized survey at 98 sites across six continents, we show that interactions between grazing pressure, climate, soil, and biodiversity are critical to explain the delivery of fundamental ecosystem services across drylands worldwide. Increasing grazing pressure reduced ecosystem service delivery in warmer and species-poor drylands, whereas positive effects of grazing were observed in colder and species-rich areas. Considering interactions between grazing and local abiotic and biotic factors is key for understanding the fate of dryland ecosystems under climate change and increasing human pressure. يمثل الرعي الاستخدام الأوسع للأراضي في جميع أنحاء العالم. ومع ذلك، لا تزال آثاره على خدمات النظام الإيكولوجي غير مؤكدة لأن التفاعلات المنتشرة بين ضغط الرعي والمناخ وخصائص التربة والتنوع البيولوجي قد تحدث ولكن لم تتم معالجتها أبدًا في وقت واحد. باستخدام مسح موحد في 98 موقعًا في ست قارات، نوضح أن التفاعلات بين ضغط الرعي والمناخ والتربة والتنوع البيولوجي ضرورية لشرح تقديم خدمات النظام الإيكولوجي الأساسية عبر الأراضي الجافة في جميع أنحاء العالم. أدى الضغط المتزايد للرعي إلى تقليل تقديم خدمات النظام الإيكولوجي في الأراضي الجافة الأكثر دفئًا والفقيرة بالأنواع، في حين لوحظت آثار إيجابية للرعي في المناطق الأكثر برودة والغنية بالأنواع. يعتبر النظر في التفاعلات بين الرعي والعوامل المحلية اللاأحيائية والأحيائية أمرًا أساسيًا لفهم مصير النظم الإيكولوجية للأراضي الجافة في ظل تغير المناخ وزيادة الضغط البشري.

Grazing represents the most extensive use of land worldwide. Yet its impacts on ecosystem services remain uncertain because pervasive interactions between grazing pressure, climate, soil properties, and biodiversity may occur but have never been addressed simultaneously. Using a standardized survey at 98 sites across six continents, we show that interactions between grazing pressure, climate, soil, and biodiversity are critical to explain the delivery of fundamental ecosystem services across drylands worldwide. Increasing grazing pressure reduced ecosystem service delivery in warmer and species-poor drylands, whereas positive effects of grazing were observed in colder and species-rich areas. Considering interactions between grazing and local abiotic and biotic factors is key for understanding the fate of dryland ecosystems under climate change and increasing human pressure.

Shrub encroachment influences several ecosystem services in drylands worldwide. Yet, commonly used strategies to reduce encroachment show a low medium-term success, calling for a better understanding of its causes. Previous works identified multiple drivers responsible for this phenomenon, including anthropogenic and environmental causes. However, the relative effect of climate, topography and edaphic factors on shrub encroachment is not fully understood nor has been properly quantified in Mediterranean Basin drylands. Also, understanding how these drivers lead to changes in plant communities' functional traits associated to shrub encroachment is crucial, considering traits influence ecosystem processes and associated ecosystem services. Here, we studied the understory of a Mediterranean dryland ecosystem composed of savanna-like Holm-oak woodlands, along a regional climatic gradient. We specifically assessed (i) how climatic, topographic and edaphic factors influence understory relative shrub cover (RSC) and (ii) their direct and indirect effects (via RSC) on plant functional traits. We studied the mean and diversity of 12 functional traits related to plant regeneration, establishment, and dispersal, at the community-level. We found that, under similar low-intensity land use, topographic and edaphic factors, namely slope variations and soil C:N ratio, were the most important predictors of shrub encroachment, determining communities' functional characteristics. Climate, namely summer precipitation, had a much lesser influence. Our model explained 52% of the variation in relative shrub cover. Climate had a stronger effect on a set of functional traits weakly involved in shrub encroachment, related to flowering and dispersal strategies. We show that shrub encroachment is largely predicted by topo-edaphic factors in Mediterranean drylands subject to conventional low-intensity land use. Hence, management strategies to reduce encroachment need to take these drivers into account for efficient forecasting and higher cost-effectiveness. Our results suggest that climate change might not greatly impact shrub encroachment in the Mediterranean Basin, but may affect functional structure and reduce functional diversity of plant communities, thus affecting ecosystem functioning.

Climate change and increasing socio-economic pressure is placing many ecosystems of high ecological and economic value at risk. This is particularly urgent in dryland ecosystems, such as the montado, a multifunctional savannah-like system heavily modeled by grazing. There is still an ongoing debate about the trade-offs between livestock grazing and the potential for ecosystem regeneration. While it is consensual that overgrazing hinders the development of the shrubs and trees in this system, the effects of undergrazing or grazing exclusion are unclear. This study provides the unique opportunity to study the impact of grazing on compositional and structural biodiversity by examining the ecological chronosequence in a long-term ecological research site, located in Portugal, where grazing exclusion was controlled for over 15years. As the threat of intensification persists, even in areas where climate shifts are evident, there is a critical need to understand if and how the montado might recover by removing grazing pressure. We evaluate succession on structural and compositional diversity after grazing pressure is removed from the landscape at 5, 10, and 15years post-cattle exclusion and contrast it with currently grazed plots. A LiDAR-derived structural diversity index (LHDI), a surrogate of ecosystem structure and function first developed for the pine-grassland woodland systems, is used to quantify the impact of grazing exclusion on structure and natural regeneration. The distribution of the vegetation, particularly those of the herbaceous and shrub strata (>10≤150cm), presents statistically significant changes. The LHDI closely mimics the compositional biodiversity of the shrubs, with an increase in diversity with increased years without grazing. Under present climate conditions, both shrub regeneration and the establishment of tree saplings were strongly promoted by grazing exclusion, which has important management implications for the long-term sustainability of montado systems.