Figure 1. Theoretical framework indicating how increasing precipitation variability can influence soil moisture regimes as an extension of Knapp et al. (2008), who primarily discussed the effects of increased weather intensity (i.e. larger, more irregular event sizes). Grey bars represent daily precipitation. Day of the year and soil water content are abbreviated by DOY and SWC, respectively. Under climate change, we expect an increase in both intensity and persistence of summer weather in the mid-latitudes (Pfleiderer et al., 2019; Spinoni et al., 2018). In all panels, the precipitation totals are equal to historical weather (a). More intense weather (b) results in less frequent but larger rainfall events intermitted with longer dry spells under higher temperatures, which could exacerbate drought/flooding effects. More persistent weather (c) results in prolonged dry and wet periods, which could also lead to more extreme variation in soil water content. In both cases, the chance increases that critical soil moisture thresholds are exceeded, negatively affecting ecosystem functioning (Ciais et al., 2005).
While research has explored how climate shifts may alter selection of species with specific traits (Siepielski et al., 2017), whether interspecific interactions will change, or how species interactions shape community responses to these regime changes remains unclear (Grant et al., 2014). Direct species interactions, i.e. the direct effect of plant A on plant B and vice versa (Fig. 2), are important determinants of ecosystem functioning (Schmitz et al., 2008). They can be facilitative (positive) or competitive (negative). In grasslands, legumes often facilitate grasses through N-fixation, which increases ecosystem productivity (Ball et al., 2020), in return for improved microclimatic conditions such as wind protection (Walker et al., 2003). Conversely, fast-growing dominant grasses often compete with one another for limited resources, which can result in productivity loss (Corbin & D’Antonio, 2004; Fay et al., 2003). During episodes of extreme abiotic conditions, when water (drought) or oxygen (flooding) become limited, direct interactions between species may become more positive, negative, or not change (Grant et al., 2014; Van den Berge et al., 2014). The net effect of direct interaction changes on ecosystem functioning (e.g., productivity) under such conditions is ecosystem specific and depends on their strength and direction, but also on the affected species. For example, under drought, increased competition between dominant species can exacerbate the productivity decline (Fay et al., 2003; Grant et al., 2014; Suttle et al., 2007).
In diverse communities, indirect interactions can also emerge, through interaction chains of direct interactions (Wootton, 1993) or through higher-order interactions (HOIs) (Billick & Case, 1994). With interaction chains, apparent indirect interaction effects may appear between two distant plants, due to a shared relationship with a third (e.g., species A modifies performance of species B, in turn affecting performance of species C). Unlike interaction chains, HOIs remain poorly understood, although they play an important role in determining net community dynamics and the stability of complex ecosystems by modifying direct interactions (Bairey et al., 2016; Billick & Case, 1994; Brooker et al., 2008; Grilli et al., 2017; Li et al., 2021; Mayfield & Stouffer, 2017; Singh & Baruah, 2021; Xiao et al., 2020). HOIs can either be defined as the combined effect of two or more neighboring plants on a third target plant (Fig. 2), which is determined by the nature of these neighbors’ own pairwise interaction(s) (Levine et al., 2017; Mayfield & Stouffer, 2017), or, as the effect of one species on the interaction between two others (Billick & Case, 1994). Such interaction effects could arise when a Nitrogen (N)-fixing legume C is introduced into a plant community with scarce resources, reducing N competition between two neighboring grasses (Fig. 2). Recent studies, both theoretical and observational, have indicated that HOIs may mitigate intense direct competitive effects between neighbors, facilitating long-term species coexistence in highly diverse plant communities (Grilli et al., 2017; Li et al., 2021; Singh & Baruah, 2021; Xiao et al., 2020). Thus, Grilli et al. (2017) suggest that HOIs could make communities more robust against ecological perturbations. However, their impact on ecosystem functioning and how they are influenced by shifting climate regimes remains unclear (Bimler et al., 2018; Brooker et al., 2008; Xiao et al., 2020). Elucidating their role is important to better understand climate-driven changes in diverse plant communities.