Resilience and alternative stable states after desert wildfires

Abstract

Improving models of community change is a fundamental goal in ecology and has renewed importance during global change and increasing human disturbance of the biosphere. Using the Mojave Desert (southwestern United States) as a model system, invaded by nonnative plants and subject to wildfire disturbances, we examined models of resilience, alternative stable states, and convergent‐divergent trajectories for 36 yr of plant community change after 31 wildfires in communities dominated by the native shrubs Larrea tridentata or Coleogyne ramosissima. Perennial species richness on average was fully resilient within 23 yr after disturbance in both community types. Perennial cover was fully resilient within 25 yr in the Larrea community, but recovery was projected to require 52 yr in the Coleogyne community. Species composition shifts were persistent, and in the Coleogyne community, the projected compositional recovery time of 550 yr and increasing resembled a deflected trajectory toward potential alternative states. Disturbed sites contained a perennial species composition of predominately short‐statured forbs, subshrubs, and grasses, contrasting with the larger‐statured shrub and tree structure of undisturbed sites. Auxiliary data sets characterizing species recruitment, annual plants including nonnative grasses, biocrust communities, and soils showed persistent differences between disturbed and undisturbed sites consistent with positive feedbacks potentially contributing to alternative stable states. Resprouting produced limited resilience for the large shrubs L. tridentata and Yucca spp. important to population persistence but did not forestall long‐term reduced abundance of the species. The nonnative annual grass Bromus rubens increased on disturbed sites over time, suggesting persistently abundant nonnative plant fuels and reburn potential. Biocrust cover on disturbed sites was half and species richness a third of amounts on undisturbed sites. Soil nitrogen was 30% greater on disturbed sites and no significant trend was evident for it to decline on even the oldest burns. Disturbed desert plant communities simultaneously supported all three models of resilience, alternative stable states, and convergent‐divergent trajectories among community measures (e.g., species richness, composition), timeframes since disturbance, and spatial resolutions. Accommodating expression within ecosystems of multiple models, including those opposing each other, may help broaden theoretical models of ecosystem change.

Post‐disturbance evolution of a prograded foredune barrier during a sustained dynamic restoration project—the role of wind speed, wind direction and vegetation

“…The potential for divergent trajectories of change due to variation in primary forcing factors is well recognized in the broader ecological and geomorphic disturbance literature (i.e. Abella et al, 2021; Gutiérrez‐Cánovas et al, 2019; Laurance et al, 2007; Phillips, 2009) but has seldom been explicitly considered in the context of coastal dune dynamics following disturbance.…”

Long‐term relationships between seed bank communities and wildfire across four North American desert sites

“…Fire is increasing in extent, severity, and frequency across the western United States, and forecasts suggest continued increases in wildfire for these dry ecosystems (Abatzoglou & Williams, 2016; Parks et al, 2016). Many North American desert systems historically have had relatively low fire frequency and long fire return intervals, and a range of studies have assessed the consequences of increasing fire on the aboveground plant communities of these deserts (e.g., Abella et al, 2020; Jurand & Abella, 2013; Miller et al, 2016; Pyke et al, 2010; Shryock et al, 2014). While the importance of the aboveground vegetation trajectories after fire is not to be minimized, terrestrial plants also exist in the soil seed bank and this belowground component of the plant community plays a critical role in determining a site's future plant community.…”

“…Despite our data showing no effect of burn status on densities and richness, we found that seed bank communities were different for both longer and shorter TSFs in our cold desert sites. Studies looking at aboveground vegetation have shown that recovery following fire is variable within and across dryland systems (Abella, 2009; Abella et al, 2020; Morris & Leger, 2016; West & Hassan, 1985). The lack of vegetation recovery after fire is at least in part due to the limited ability of many desert shrubs and cacti to resprout following fire (Abella, 2009; Shryock et al, 2015).…”

“…Studies have even reported a loss of seed over TSF due to a shortage of recruitment events in fire-prone systems (Wright & Clarke, 2009). Short-term loss of seed bank density and diversity immediately following fire that then transitions back to pre-burned values over the longer term (i.e., ~15 and ~30 years) suggests resiliency of systems and illustrates the importance of long-term studies in helping to determine timescales on which seed bank densities may recover, as well as an improved understanding of how this may relate to changes in aboveground vegetation (Abella et al, 2020). Despite our data showing no effect of burn status on densities and richness, we found that seed bank communities were different for both longer and shorter TSFs in our cold desert sites.…”

Extreme drought triggers transition to an alternative soil microbial state

“…This can have important consequences for ecosystem functioning, as the ecosystem stays in the new attraction valley -the alternative state -even if the perturbation that triggered the change has ceased. Numerous studies have demonstrated the existence of abrupt changes to alternative states in aquatic ecosystems 9,11 , terrestrial plant communities 12,13 , and the human gut microbiome in response to perturbations 14,15 . However, abrupt transitions to alterative states in soil microbial communities have so far received little attention 7 , despite their fundamental role in terrestrial ecosystems, driving key processes of organic matter decomposition, nutrient cycling, and carbon and nutrient storage 16 , which regulate ecosystem productivity.…”