Discussion
Despite the productivity of the native grassland communities tended to be lower than the baseline during dry events and higher than the baseline during wet events (Figures S3-5), our results showed that the effects of biodiversity on both resistance and resilience depend on the community functional structure, on the biodiversity component evaluated and the direction and intensity of anomalous climatic events.
Although the effects of biodiversity on the ecosystem resistance were mostly positive, they were closely related to the community type defined in terms of functional traits, irrespective of the direction or intensity of climatic events. To examine this pattern, we grouped the studied grassland communities by their functional relatedness (Figure 2) based on traits that are relevant for biomass production and water and nutrient balance (Lundgren et al. 2014; Engel 2017; Bruelheideet al. 2018; Testolin et al. 2021). Hence, along the first axis (Axis I; Figure 2), positive scores can be interpreted as communities dominated by acquisitive species (high SLA, Leaf N, C3), and therefore, it is expected that they would be predominantly drought sensitive (Griffin-Nolan et al. 2018). Along the same first axis, communities with negative scores indicate dominance by conservative species (high LDMC) and thus, predominantly drought resistant (Shiet al. 2016; García-Palacios et al. 2018). Our results showed that the effects of biodiversity on resistance were predominantly positive and stronger in resource-conservative communities, while they were predominantly negative and weaker in resource-acquisitive communities (Table S1). Considering biodiversity effects on the ecosystem resilience, they were more pronounced for resource-acquisitive communities and mainly positive under wet events (Table S2).
From a taxonomic perspective, species richness and species diversity were negatively correlated with species dominance in all groups. Species richness affects resistance, as we predicted and has already been shown by others (Hautier et al. 2015; Isbell et al. 2015). However, regardless of the direction or intensity of the climatic events, species richness increased ecosystem resistance in resource-conservative communities’, while it decreased resistance in resource-acquisitive communities. Considering resource-acquisitive communities under wet events, species richness negative effects on resistance may indicate an increased biomass production given water/resource inputs (Fischer et al. 2016) (Figures 4, S2 and S4). Interestingly, positive effects of species richness on resilience were only observed in communities recovering from wet events, while the effects on the recovery from dry events were negative. According to the portfolio effect (Doak et al. 1998; Tilman et al. 1998), ecosystem stability may be ensured by maintaining high levels of species richness, but our results show that this may be consistently observed only in resource-conservative communities.
Species diversity also increased the resistance of grassland communities. However, its effect was evidenced under extreme dry and extreme wet climatic events, increasing the ecosystem resistance of resource-conservative communities, thus partially supporting our hypothesis. For resource-acquisitive communities under moderate and extreme wet events, an increased species diversity had a negative effect on resistance and a positive effect on resilience. Therefore, given resource-acquisitive communities facing wet events, species diversity may induce a compensatory stabilizing mechanism of biomass production (Lehman & Tilman 2000; Grman et al. 2010; Mackie et al.2019).
From the functional perspective, we observed a consonant result between species diversity and functional redundancy effects on ecosystems’ resistance, as we predicted, according to the insurance hypothesis (Yachi & Loreau 1999). Their high positive correlation values indicate that the functional redundancy found in the communities is promoted by species diversity more than by the number of species per se. Functional redundancy had a strong positive effect on the resistance of the most resource-conservative communities facing extreme dry and wet events. In turn, ecosystems’ resilience of resource-acquisitive communities was increased by functional redundancy while recovering from moderate dry and extreme wet events.
The effect of functional response diversity on resistance was dependent on the community functional structure, as it was mainly observed in resource-conservative communities and was independent from the anomalous climatic events. However, given the resource-conservative communities, we observed contrasting effects. While in communities where species richness and functional response diversity were positively related, in general functional response diversity had a positive effect on ecosystem resistance, and in communities where the relationship was negative, its effect was negative. Thus, we may conclude that a positive relationship between species richness and traits reflecting functional response diversity (Lavorel & Garnier 2002; Elmqvist et al. 2003) plays an important role on the resistance, according to the insurance hypothesis (Yachi & Loreau 1999).
As already highlighted (Isbell et al. 2015; Wright et al.2015), when analyzing empirical data, it may be difficult to completely decouple resistance and resilience, especially when disturbances are recurrent. This implies a challenge to distinguish resistance and resilience over a temporal time series once both may be skewing each other. However, it should be further explored in future studies, for example, by looking for periods when productivity no longer depends on the previous anomalous event. Additionally, this is an important issue considering the already known dangerous effects of climate change on ecosystem stability (Hautier et al. 2015; Craven et al.2018; García-Palacios et al. 2018; IPCC 2021).
Here we have shown that the ecosystem stability measured as resistance and resilience of native grasslands depend on the biodiversity metric evaluated in the communities and the direction and intensity of the climatic anomaly. In addition, more than a single taxonomic or functional component, communities’ functional structure plays an important role in driving the effects of biodiversity on ecosystem resistance and resilience under different anomalous climatic events. This emphasizes the importance to consider different components of biodiversity when investigating ecosystem services under climate change. There is no unified measure able to synthesize all the possible answers of the ecosystems under climate change. Moreover, our results stressed the importance of biodiversity to guarantee grassland biomass productivity under disturbance events generated by climate change. As climate change models indicate that the intensity and frequency of wet events are likely to increase in the Campos Sulinos region (IPCC 2021), there is a pressing need for further studies to help understand ecosystem-level response to such disturbances.