Interactive effects of shading and disturbance on plant invasion in an arid shrubland: Assembly processes and CSR‐strategies

Abstract

Disturbance by small mammals and shrub canopies are ecological factors typical of arid ecosystems that may influence plant invasion through environmental and community changes. Whereas disturbance beneath shrub canopies may promote invasion by removing dominant species, disturbance in open areas may hinder plant invasion by increasing environmental harshness. However, we are unaware of studies explicitly addressing the interactive effects of disturbance by mammals and shading by shrubs on community assembly processes to understand plant invasion. In an arid shrubland, disturbance and shading were caused by the fossorial rodent Spalacopus cyanus and the shrub Flourensia thurifera, respectively. We used functional dispersion data (trait convergence vs. divergence) and Grime´s theory (competitive, stress‐tolerant and ruderal strategies, CSR) to gain insights into the underlying assembly processes. We compared environmental conditions, richness and abundance of native and exotic species, as well as functional dispersion and prevalence of CSR‐strategies across four microsites (60 × 60 cm plots): undisturbed/open, undisturbed/shaded, disturbed/open and disturbed/shaded. We tested for functional differences between native and exotic species. Species richness was similar among microsites for both native and exotic species. Shading ameliorated environmental conditions, promoted trait divergence and increased prevalence of C‐ and R‐strategies. Disturbance increased the abundance of exotic species and removed dominant species in shaded microsites. Exotic and native species were functionally different: exotics showed shorter life span, lower height, thinner stems and smaller leaves than natives. Synthesis. Disturbance, by removing plant biomass, favoured exotic species in shaded microsites, where shrub canopy ameliorated environmental conditions and—by relaxing habitat filtering—promoted niche partitioning processes as well as C‐ and R‐strategies. We illustrate the value of linking trait‐based ecology approaches to micro‐environmental conditions, as it may provide insights into the underlying community processes when studying plant invasions at the local and microsite scales. Disturbed microsites beneath shrub canopies could be ‘hotspots’ where to concentrate efforts to manage plant invasions in arid ecosystems.

Adaptive plasticity to drought of Grime's CSR strategies

“…Conversely, drought consistently decreased LA across populations (Fig. 1c), as was expected from functional considerations and earlier studies (Wright et al 2004, Garnier et al 2015, Escobedo et al 2021). Thus, in plants subjected to drought, leaves showed a change in the ratio between LA and leaf dry weight (LDW), with leaves becoming small in absolute terms but having a greater area relative to mass (SLA).…”

“…CSR theory describes a three‐way tradeoff between three principal strategies (competitor, stress‐tolerator, ruderal) representing trait combinations that jointly confer fitness under conditions of higher competition, lower productivity, or intense (or frequent) biomass removal. CSR theory has been mainly used to classify plants functionally and to understand community structure at local and global scales (Grime and Pierce 2012, Escobedo et al 2021). CSR strategies reflect viable suites of traits that impact plant fitness and survival, yet their adaptive nature has been scarcely addressed (May et al 2017, Vasseur et al 2018, Astuti et al 2019, Estarague et al 2022).…”

“…Second, CSR theory provides ecophysiological basis of plant adaptation to the environment (Grime and Pierce 2012, Guo et al 2018). This allows a quantitative value to be ascribed to functional adjustments of CSR strategies or traits driven by plastic responses (Sultan 1995, Escobedo et al 2021). For instance, when facing a resource‐limitation stress such as drought, plants often decrease their competitiveness (C‐strategy) and favour their stress tolerance (S‐strategy), while no change is expected concerning their ruderality (R‐strategy) (Welles and Funk 2021).…”

“…For instance, when facing a resource‐limitation stress such as drought, plants often decrease their competitiveness (C‐strategy) and favour their stress tolerance (S‐strategy), while no change is expected concerning their ruderality (R‐strategy) (Welles and Funk 2021). Regarding trait plasticity, increased LDMC and decreased SLA and LA are expected responses for plants under drought because of the associated reduction in water loss (Wright et al 2004, Reich 2014, Escobedo et al 2021). However, these traits (SLA, LDMC and LA) cannot individually characterise whole‐plant responses to the environment as CSR strategies do.…”

Microclimatic variation in tropical canopies: A glimpse into the processes of community assembly in epiphytic bryophyte communities

“…In Grime's competitive, stress-tolerant, ruderal (CSR) theory of plant ecological strategies (Grime, 1977), community composition is controlled by selection for traits depending on levels of competition, stress and disturbance. Along a gradient of decreasing habitat filtering, community composition is expected to shift from a dominance of stress-tolerant species to competitive and ruderal species (Escobedo et al, 2021). At larger spatial scales, and hence, as variation in environmental conditions increases, community composition is conversely increasingly driven by environmental filtering (Powell et al, 2015).…”