Precipitation anomalies may affect productivity resilience by shifting plant community properties

Abstract

1 INTRODUCTION

Climate change is disrupting historic environmental regimes, including increases in the frequency and severity of extreme climatic events, such as droughts and intense rainfall periods (IPCC, 2021; Smith, 2011). The impacts of droughts on plant communities and their associated ecosystem functions are well appreciated. For example, droughts can alter community composition (Gao et al., 2022; Hoover et al., 2014; Xu et al., 2021) and drive significant reductions in primary productivity (Gao et al., 2019; Liu et al., 2023; Su et al., 2022), and these impacts often persist post-drought (‘drought legacies’; Müller & Bahn, 2022; Vilonen et al., 2022). The consequences of highly wet periods, by contrast, have thus far received less attention, despite heavy rainfall events increasing over the past century throughout the contiguous United States and in many other regions worldwide (IPCC, 2021; Jay et al., 2018). Further, the impacts of extreme wet and dry events are often evaluated independently (although see Isbell et al., 2015; Sala et al., 2012; Wilcox et al., 2017), despite both types of ‘precipitation events’ (see Box 1) increasing in many regions. Therefore, to persist and maintain critical ecosystem functions plant communities must be resilient to both of these contrasting precipitation events.

BOX 1. Key terms and definitions

Precipitation event: Periods when water availability is outside ‘normal’; a drought (SPEI < −1) or wet event (SPEI > 1)

Standardized precipitation–evapotranspiration index (SPEI): Measure of an ecosystem's water availability resulting from the difference between inputs from precipitation and outputs from potential evapotranspiration

Resilience: A multi-dimensional quality that describes an ecosystem's capacity to absorb perturbations and persist in a reference state

Resistance: The degree to which an ecosystem function (e.g. productivity) changes in response to a perturbation

Recovery: The rate at which an ecosystem function returns to pre-perturbation conditions in the year after a perturbation; sometimes called ‘resilience’ (e.g. Pimm, 1984)

Invariability: The degree to which an ecosystem function varies through time. Often used synonymously with ‘stability’

Resilience is a multi-dimensional quality that describes an ecosystem's capacity to absorb perturbations and persist in a reference state (Box 1; Van Meerbeek et al., 2021). While key assumptions of the resilience concept vary among disciplines (i.e. ‘ecological resilience’ sensu Holling, 1973 vs. ‘engineering resilience’ sensu Pimm, 1984), the framework broadly captures both a system's responses to perturbation events and long-term patterns. We can begin to explain variation in resilience across communities and ecosystems by quantifying aspects of resilience—resistance, recovery, and invariability—and linking them to community properties. Resistance is the degree to which an ecosystem function (e.g. productivity) changes in response to a perturbation. Recovery (termed ‘resilience’ by Pimm, 1984) is the rate at which an ecosystem function returns to pre-perturbation conditions. Invariability (often called ‘stability’) expresses how an ecosystem function varies through time. While resilience is often assumed to be beneficial, resilience does not necessarily confer increased ecosystem functioning. For example, wet events can increase productivity (Wilcox et al., 2017), and therefore, resilience in this context would diminish productivity benefits.

Plant community properties, including species richness, evenness, and dominance, can influence resilience to environmental perturbations, including precipitation events. However, the properties that promote aspects of resilience to droughts may differ from those promoting resilience to highly wet conditions. Species richness is widely demonstrated to promote resilience through functional diversity and redundancy (i.e. diversity-stability relationship; Ives & Carpenter, 2007; Tilman et al., 1996). Given that a speciose community should exhibit greater response diversity—the range of species' responses to an environmental change (Elmqvist et al., 2003)—diverse communities on average should have a higher probability of maintaining critical functions under stress (i.e. biological insurance theory; Yachi & Loreau, 1999). Prior work leveraging data from 46 grassland diversity manipulation experiments found species richness increased productivity resistance to precipitation events (both wet and dry events), as well as long-term productivity invariability, but not post-event recovery (Isbell et al., 2015). Other aspects of diversity, such as evenness and dominance can further modulate resilience by affecting functional trait distributions (Hillebrand et al., 2008). Although often thought of as being antithetical to one another, evenness and dominance both likely contribute to determining trait distributions in non-monodominant communities. First, evenness within a community can promote resilience by enhancing trait diversity, functional redundancy, and temporal complementarity among species (Loreau et al., 2021). This is distinct from the effects of richness because even when communities have the same richness, they can differ in evenness. In a low-evenness community, low-abundance species contribute minimally to functional diversity (i.e. low functional evenness). Therefore, not only species counts, but also abundances within the community may be an important determinant of resilience. Second, as dominant species largely determine community-weighted trait values, dominants that are resistant to a given perturbation could confer community-level resilience to that perturbation by sustaining key functions and interactions. Because both species richness and evenness act by increasing trait diversity and temporal complementarity and different species are likely to be more resistant to dry versus wet extremes, these properties might be expected to promote resilience to both wet and dry extreme events. In contrast, whether dominance promotes resilience to wet versus dry events likely depends on the specific dominant species and whether it is resistant to drought, inundation, or both.

The characteristics of constituent species and functional groups additionally impact resilience through differences in physiological tolerances, life history and demographic traits, and responses to environmental alterations (Lloret et al., 2012; McGill et al., 2006; Paniw et al., 2021). For instance, a study across eight European grasslands found graminoids are more drought sensitive than forbs (Mackie et al., 2019), although a study comparing drought responses between a single grass and forb species found the opposite (Hoover et al., 2014). Similarly, under stress, non-natives may be less adapted to resource reductions like drought, resulting in reduced growth relative to native species (Liu et al., 2017; Valliere et al., 2019), although the opposite has also been observed (Meisner et al., 2013). Thus, the relative abundances of certain species and functional groups within a community may further regulate productivity resilience although existing data are still too limited to yield general predictions.

While community properties modulate community resilience to precipitation events, they also respond to precipitation events. As a result, community responses to extreme wet and dry years may impact a community's resilience to future events. Such shifts in potentially relevant community properties, including richness, functional diversity, and forb and grass abundances, have been observed in grassland communities in response to drought (Gao et al., 2022; Hoover et al., 2014; Xu et al., 2021) and elevated precipitation (Collins et al., 2012; Yang et al., 2011). Although precipitation legacies—shifts in community properties and processes driven by drought and wet extremes—are increasingly appreciated (Müller & Bahn, 2022; Sala et al., 2012), their impacts on resilience to subsequent events remain poorly characterized. Further, as wet extremes tend to elevate productivity (Sala et al., 2012; Wilcox et al., 2017), we might assume that we can disregard their impacts on a system's resilience. However, wet events may indirectly impact productivity resilience to future extreme events via their effects on community properties, as described above. Consequently, as precipitation is predicted to become increasingly variable under climate change (IPCC, 2021; Smith, 2011), evaluating the interplay between droughts and wet extremes is critical for accurately capturing the current resilience of plant communities and their functions and for predicting resilience to future precipitation events.

Leveraging a two-decadal (2000–2020) record of primary productivity in a temperate grassland community, we evaluated the community-level factors regulating the resistance and recovery of productivity to contrasting precipitation events (Aim 1). We considered how community properties influenced long-term productivity invariability over the two decades (Aim 2). Finally, we explored how the focal community properties associated with resilience responded to precipitation and if precipitation legacies occur within the community (Aim 3). Together, our research investigates feedbacks between precipitation events, community properties, and resilience to explore if the oscillating precipitation events predicted for the future will have consequences for plant community resilience.