Introduction
In the face of climate change, biodiversity may be an important driver of ecosystem stability (Eisenhauer et al. 2016; Craven et al. 2018; Pillar et al. 2018), i.e., the resistance of a given ecosystem function to change and its resilience to recover from disturbances or climate extremes (Tilman & Downing 1994; Isbellet al. 2015). Some studies have shown that plant biodiversity can provide resistance of primary productivity to climate extremes, but this relationship depends on the direction and intensity of the event (Isbellet al. 2015; Fischer et al. 2016; García-Palacios et al. 2018; Mackie et al. 2019; Biggs et al. 2020; Valenciaet al. 2020). The biotic mechanisms driving ecosystem stability under such anomalies have been mainly addressed through experimental studies based on the manipulation of species composition (Fischeret al. 2016; Craven et al. 2018; Mackie et al.2019; Biggs et al. 2020). Often the available evidence emerges from communities that were artificially assembled from scratch, which also entangles the effects of the disturbance involved in the manipulation (but see Jochum et al. 2020). Despite the importance of those findings, experimental studies may not fully translate the natural community assembly effects on the ecosystem stability under climatic anomalies.
With the predicted changes in temperature and precipitation, extreme climatic events are about to become more common (IPCC 2021). In fact, we are already experiencing it. Specifically for southeastern South America, an increase in frequency and intensity of droughts and rainfall events has been predicted (Hoover et al. 2014; Souza & Manzi 2014; Gao et al. 2019; NOAA 2020; IPCC 2021). Climate controls vegetation phenology, while the amount and distribution of annual precipitation strongly influences the annual net primary productivity of grasslands (Sala et al. 1988; Paruelo & Lauenroth 1995; Gordo & Sanz 2010). If water availability is constrained, there is an increasing influence of limited evapotranspiration for grassland biomass production (Vicente-Serrano et al. 2010). Thus, as an increase in extreme climatic events would interfere with the seasonal patterns of rainfall distribution (IPCC 2021), it is expected that this will also affect the primary productivity of plant communities. However, plant biodiversity may buffer ecosystems from the effects of these anomalies, providing resistance and resilience (Biggs et al. 2020).
According to the insurance hypothesis (Yachi & Loreau 1999), biodiversity should provide stability due to the functional redundancy of different species in nature. Functional redundancy in a community can be operationally defined as the difference between taxonomic and functional diversity based on traits driving ecosystem functions (Pillaret al. 2013), i.e., the portion of species taxonomic diversity in a community that plays similar ecosystem functions. Increased functional redundancy for a given ecosystem function implies that the community contains species that can replace each other in case of species losses due to their different environmental sensitivity (Walker et al.1999; Oliver et al. 2015). High taxonomic diversity may also imply increased functional diversity in terms of responses to drivers such as extreme climate events, i.e., increased response diversity sensu Elmqvist et al. (2003) defined by response traits sensu Lavorel & Garnier (2002). In other words, on the one hand, ecosystem stability relies on the degree of functional equivalence regarding effect traits (Lavorel & Garnier 2002) of the resident species in the community (Elmqvist et al. 2003; Violleet al. 2007; Fischer et al. 2016). On the other hand, plant functional diversity loss in terms of response traits can be associated with a decrease in ecosystem functioning stability and hence, environmental changes that potentially affect biodiversity may induce long-term changes (Hautier et al. 2015).
Closely related to the insurance hypothesis, increased species diversity may induce the “portfolio effect” (Doak et al. 1998; Tilmanet al. 1998). Beyond the simple idea of a statistical averaging of individual species contribution to biomass production, together, these two hypotheses predict a stabilizing effect of species diversity on ecosystem properties through species asynchrony. This mechanism ensures, via species richness and its different environmental responses, that the more species, the greater the probability of asynchronous species responses to environmental fluctuations, thus leading to an increased stability (Yachi & Loreau 1999; Loreau 2010).
Plant traits directly related to environmental conditions (Bruelheideet al. 2018; Testolin et al. 2021) reflect a resource acquisition and conservation trade-off known as the “leaf economics spectrum” (Wright et al. 2004; Díaz et al. 2016; Garnieret al. 2016). The conservative side of this spectrum typically comprises species that are able to store resources and use water more efficiently. So, conservative species would withstand anomalous events, providing resistance. In the acquisitive side, species use the resources to grow faster. Therefore, they would offer less resistance during anomalous events. A similar spectrum has been observed at the community level (Bruelheide et al. 2018). It is an open question, however, if communities that differ in terms of resource-use strategy would also differ regarding their stability under extreme climatic conditions. It is plausible to believe that they will. For example, under increased water availability, communities characterized by acquisitive species would benefit from resource-inputs ensuring biomass production and providing resistance. Or they could be more productive than in normal periods, leading to low resistance (Wright et al. 2015; Fischeret al. 2016). In contrast, communities defined by conservative species could maintain productivity under decreased water availability (García-Palacios et al. 2018)
In this study, we address the question whether plant communities with higher diversity provide increased ecosystem stability under climatic anomalies, and if the effects of biodiversity differ regarding the dominant resource-use strategy of the communities. We evaluate the effects of species richness and diversity, functional diversity and functional redundancy on the stability (i.e. resistance and resilience) of biomass production at the ecosystem level. From a taxonomic perspective, (i) we hypothesize that induced by compensatory dynamics, species richness (i.e. number of species) will present a positive effect on resilience, induced by a “portfolio effect” (Doak et al.1998; Tilman et al. 1998; Valencia et al. 2020), (ii) while species diversity will have a positive effect on resistance, corroborating the insurance hypothesis (Yachi & Loreau 1999; Loreau 2010). From a functional perspective, (iii) using traits that are relevant for biomass production, we hypothesize that functional redundancy (FR) will guarantee biomass production, with a positive effect on stability (resistance and/or resilience), whereas (iv) increased functional diversity in terms of response diversity (hereafter functional response diversity - FRD) will have a positive effect on stability (resistance and/or resilience).
Overall, high levels of species richness, species diversity, functional redundancy, and functional response diversity were positively related to the resistance of biomass productivity in dry and wet events, whereas resilience of biomass productivity to drought was positively related mostly to species richness.