Periodic Reporting for period 5 - BIODESERT (Biological feedbacks and ecosystem resilience under global change: a new perspective on dryland desertification) Reporting period: 2020-12-01 to 2022-06-30 Summary of the context and overall objectives of the project Changes in climate and grazing pressure are two main global environmental change and desertification drivers. Given the extent of drylands, they cover >40% of terrestrial surface, and the dependence of ~38% of the human population on them, it is crucial to better understand how these drivers affect the provision of ecosystem services by drylands. Land degradation in drylands already affects ~250 million people worldwide, a number that will increase because of climate change and human population growth. The BIODESERT project has four main objectives: 1) test how changes in climate and grazing pressure determine spatio-temporal patterns in ecosystem functioning and ecosystem service delivery in global drylands, 2) assess how attributes of biological communities, such as biodiversity, modulate ecosystem resilience to climate change and grazing pressure, 3) test and develop early warning indicators of desertification, and 4) forecast the onset of desertification and its ecological consequences under different climate and grazing scenarios. Key conclusions from the project are: i) considering interactions among grazing and local abiotic and biotic factors is key for understanding how drylands respond to climate change and increasing grazing pressure, ii) conserving or restoring diverse plant communities in grazed drylands can protect against land degradation and increase forage production, iii) efforts to promote more diverse grazing systems may promote reduce the negative impacts of increased grazing pressure, and iv) increases in aridity lead to systemic and abrupt changes in the structure and functioning of global drylands when particular aridity values are crossed. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In the framework of BIODESERT, we have conducted a standardized field survey carried out at 98 sites from 25 countries and six continents to evaluate how the impacts of grazing pressure on nine essential ecosystem services depend on biodiversity, climate, and soil conditions across global drylands. Interactions among grazing, climate, and biodiversity were critical to explain the delivery of ecosystem services across drylands worldwide. For instance, increasing grazing pressure reduced ecosystem service delivery in warmer and species-poor drylands, while positive effects of grazing were observed in colder and species-rich areas. We also found positive relationships among plant species richness and carbon storage, organic matter decomposition, erosion control, and both forage quality and quantity, and between belowground diversity and organic matter decomposition, irrespective of grazing pressure. Greater herbivore richness was also correlated with higher carbon storage regardless of grazing pressure. During the project we have also synthesized >50,000 observations from global drylands, has been the discovery of three ecosystem thresholds linked to aridity values of 0.54 0.69 and 0.83 respectively, which led to systemic and abrupt ecological transitions (Science 367: 787–790). These changes occurred sequentially in three phases characterized by abrupt decays in plant productivity, soil fertility and plant cover/richness. These results set the stage for future studies exploring temporal changes in dryland ecosystems, particularly in areas that will likely cross these aridity thresholds in the future. For example, it is forecasted that 18% of the terrestrial surface will cross one/several of these thresholds by 2100. Areas expected to cross the 0.8 aridity threshold are particularly sensitive as they may undergo massive vegetation collapse and species loss. We have also developed novel conceptual, analytical, modelling and synthesis approaches to achieve the objectives of the project. For example, we evaluated the relationship between biodiversity and ecosystem stability for the first time in natural ecosystems at the global scale (Proc Natl Acad Sci USA 115: 8400-8405). To do so, we coupled 14 years of temporal remote sensing measurements with field surveys in 123 drylands from six continents to show a strong climate dependency of the biodiversity–ecosystem stability relationship. Our findings suggest that land management should be adapted to the aridity conditions if we aim to secure stable plant production. Another example is the use of a novel combination of remote sensing, artificial intelligence, paleoclimatic data, and advanced statistical analyses to evaluate the drivers, extent, and future distribution of dryland forests (Nat Plants 8: 879–886). Our analyses show that mid-Holocene climates and aquifer trends are key predictors of the distribution of dryland forests worldwide. We also updated the global extent of dryland forests to 1,283 million hectares and showed that failing to consider past climates and aquifers has resulted in ignoring or misplacing up to 130 million hectares of forests in drylands. Our findings also indicate that about 11% of the current extent of dryland forests (~180 Mha) can be lost in the period 2081–2100. These results can guide restoration actions by avoiding unsuitable areas for tree establishment in a warmer and drier world.Using 10 yrs of data from a field climate change experiment, we showed that increased soil respiration rates with short-term warming returned to control levels in the longer-term (Global Change Biol 26: 5254-5266). These results highlight the importance of evaluating short- and longer-term soil respiration responses to warming to reduce uncertainties in predicting the soil carbon–climate feedback in drylands. We have also coupled results from that experiment to those from global surveys to address key scientific questions related to the objectives of the project. By doing so we observed that warmer temperatures increase the relative abundance of soil-borne potential fungal plant pathogens worldwide, with hotspots of these pathogens being present in drylands (Nature Clim Change 10: 550–554). This work advances our understanding of the global distribution of potential fungal plant pathogens and their sensitivity to global change drivers. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The global field survey that we have carried out as part of BIODESERT has gone beyond the state of the art in terms of its global coverage, number of field sites surveyed, plant/soil samples being collected, and amount of data being generated. This unique survey is allowing us to test multiple questions at the edge of multiple research fronts, including community/ecosystem ecology, global change biology and desertification.Other results obtained in the project, such as the identification of aridity thresholds, go beyond current knowledge by identifying, for the first time, three phases of abrupt ecosystem changes characterized by consecutive aridity thresholds. Our results provide a well-defined framework for sequential shifts that can inspire a new generation of multiscale models to explore ecosystem responses to climate change. They also put the focus on identifying abrupt ecosystem shifts and early warning indicators for them. In this direction, we are currently using data from BIODESERT for testing, for the first time at the global scale, different early warning metrics to anticipate abrupt ecosystem changes. Doing fieldwork in a dryland ecosystem in Morocco
