Temperate grasslands under climate extremes: Effects of plant diversity on ecosystem services

Temperate grasslands provide a broad set of ecosystem services (ES), which include both provisioning ES (e.g., yield production) and non-provisioning ES (e.g., soil carbon sequestration, weed suppression, aesthetics, recreation). Yet, ES can considerably decrease under climate extremes, potentially threatening grassland ES in the future. Meanwhile, some grassland ES were shown to increase with increasing plant diversity. However, whether plant diversity can mitigate the effects of extreme climate events on multiple ES is still unclear, as past studies frequently focused on a single ES, namely aboveground biomass production (AGB). Therefore, we conducted a systematic literature review to identify the potential mitigation effect of plant species richness against the impact of extreme drought and heat stress on multiple ES in temperate C3 grasslands, by synthesizing existing knowledge and identifying research gaps. Since the 1900s, the number of studies on plant diversity and grassland ES has increased. However, only few studies also addressed climatic extremes, despite a ten-fold increase of studies in the last two decades. Moreover, while all studies included in this review (n=31; 26 biodiversity experiments (sown and weeded), five on-farm studies) addressed provisioning ES (AGB), only 45 % of the studies investigated non-provisioning ES such as climate regulation or weed suppression. No study considered cultural ES. Overall, the positive effect of higher plant species richness on grassland provisioning ES persisted also under extreme conditions, despite reducing absolute magnitudes of ES. Since the number of studies per specific non-provisioning ES was small (n = 2 on average), the general effect of plant species richness acting as insurance against climate extremes for those ES remain largely unknown. In addition, we assessed four different indices commonly used to study biodiversity–ES relationships, but no best index for resistance, recovery, and resilience of ES against climate extremes was found. Overall, the existing evidence reviewed here suggests that maintaining or increasing plant diversity in temperate grasslands can indeed be considered as a natural insurance against current and future climate risks for AGB. However, for any non-provisioning ES, currently available research is too scarce to conclude such a mitigation effect. Closing this research gap, particularly for on-farm settings, could help advance policy and societal support for sustaina