Persistence and turnover in desert plant communities during a 37‐yr period of land use and climate change

Abstract

Understanding long‐term changes in ecological communities during global change is a priority for 21st‐century ecology. Deserts, already at climatic extremes, are of unique interest because they are projected to be ecosystems most responsive to global change. Within a 500‐km2 landscape in the Mojave Desert, USA, we measured perennial plant communities at 100 sites three times (1979, 2008, and 2016) during 37 yr to evaluate six hypotheses of community change. These hypotheses encompassed shifts in community measures (e.g., diversity, cover) and species elevational distributions, biotic homogenization, disproportionately large change at the highest elevations, relationships between turnover and species’ responses to disturbance and drought, and that environmental refugia (e.g., moist topographic positions) would receive species during climatic warming and drying. Most community measures changed temporally, such as species density (species/600 m2) increasing 23% and plant cover doubling between 1979 and 2016. There was no increase in nonnative species and minimal evidence for biotic homogenization. High‐elevation communities did not display greater change than low‐elevation communities. Moreover, environmental refugia factored little in species shifts. While species distributional changes were unrelated to affinity for disturbance, the six most persistent species (persisting on >80% of sites) were long‐lived shrubs not associated with disturbance. Overall, seemingly paradoxically, climatic warming and drying was associated with increasing plant abundance. Comparing the 1970s to 2007–2016, precipitation in the study area declined 16% from 17 to 14 cm/yr and average daily minimum temperature rose 13% (1.2°C). The current climate with fewer freezes, together with reduced grazing, could be among the most optimal for desert perennials in the past century, although potential response lags to continuing warming and drying are uncertain. This study of long‐term elevational shifts in communities during global change is among few in deserts, and the average upward elevational shift of 6 m/decade for species in our study is within the range reported for temperate biomes. However, the 41% of species moving downslope is unusually high. We propose that dynamics within desert perennial communities follow a core‐transient species model where a site's species are either highly persistent or transient in approximately equal proportions.

Resilience and alternative stable states after desert wildfires

“…Alternatively, it is possible that climate had minimal influence in comparison to reduced grazing pressure. Livestock grazing and to some extent feral burros ( Equus asinus ) have been reduced around the study area since the 1990s (Abella et al 2019). With minimal evolutionary history of intensive large‐herbivore grazing, native plant cover and diversity could be expected to increase after grazing ceases.…”

Emergent Climate Change Impacts on the Soil C and N Cycles in the Mojave Desert

“…(2014) have estimated a decline in net primary productivity of about 1% per decade in the early twenty‐first century due to changing climate. However, near our study area, continuous plant surveying over the past 30+ years suggests increases in plant density and plant cover, possibly due to decreased freezing and a reduction in grazing intensity (Abella et al., 2019). For the locations examined here, we lack direct information on vegetation changes over time but recognize that such changes may be important contributors to changes discussed previously.…”

Biotic and abiotic treatments as a bet‐hedging approach to restoring plant communities and soil functions

“…The trend for increased abundance of native shrub seedlings in the vertical mulch and microtopography treatments at some sites suggests that at least conditions enabling recruitment opportunities were reinstated. In natural, undisturbed desert ecosystems, pulses of native perennial seedlings can appear relatively frequently among years, but survival of the seedlings across multiple years rarely occurs (Abella et al 2019). Bowers et al (2004), for example, found that on average, only 0.1% of native perennial seedlings lived as long as 4 years in the Sonoran Desert.…”