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.