Loss of biotic resistance and high propagule pressure promote invasive grass‐fire cycles

Abstract

The spread of invasive grasses across Earth are modifying fire cycles resulting in state changes in arid ecosystems. Disturbance, biotic resistance of native biological communities and propagule pressure, are likely the important factors influencing the spread of invasive grasses and their influence on changing fire regimes. Over a 5‐year period (2011–2016), we tested how the potential loss of biotic resistance of native plant and native rodent communities related to fire and rodent exclusion treatments, in concert with increased propagule pressure affected the establishment of Bromus tectorum L. (cheatgrass) and the spread of secondary fires. Our study results suggest that native plant and native rodent communities contribute to biotic resistance against cheatgrass invasion and that fire and high propagule pressure act to diminish biotic resistance by native communities. Five years into the study, cheatgrass establishment was 11‐fold greater in burned plots than in unburned plots (with native plant communities still intact), 2.4‐fold greater in rodent exclusion plots than rodent access plots and 1.8‐fold greater with increased propagule pressure. At the start of the experiment in 2011 cheatgrass was present in the experimental blocks but at very low density (<1 plant/m2). However, by 2016, burned‐rodent excluded plots were fully invaded (1625 stems/m2). High propagule pressure released cheatgrass from biotic resistance of rodent communities in post‐fire conditions but had minimal effects on the biotic resistance of native plant communities in unburned plots. Fire in combination with either rodent exclusion or high cheatgrass propagule pressure produced higher density cheatgrass stands that were positively correlated with the spread of secondary fires that are characteristic of invasive grass‐fire cycles. Synthesis. Loss of native plant cover or reduction in rodent populations due to fire, extreme climatic events or disease outbreaks, which are increasing with human activity, may provide windows of opportunity for invasive grasses to escape biotic resistance and reinforce invasive grass‐fire cycles.

Earlier fall precipitation and low severity fire impacts on cheatgrass and sagebrush establishment

“…and ), the positive response of cheatgrass to early precipitation indicates competition with sagebrush may be more likely to lead to state changes in native perennial shrublands to invasive annual grassland (Fukami and Nakajima , Chambers et al. ) especially if it drives recurrent fires (St. Clair and Bishop ). Cheatgrass' significantly higher densities than sagebrush in response to earlier fall precipitation, suggests that differences in propagule pressure (Figs.…”

“…Our results suggest that nutrient pulses or disruptions of soil crusts in response to fire do not explain positive responses of cheatgrass to fire. The major benefits of fire for cheatgrass may be when initial fires remove native shrubs and reduce competitive effects of native vegetation which result in more severe and recurrent fires (St. Clair and Bishop ). Severe and recurrent fires cause continuous biocrust damage and create stronger nutrient pulses, which may create and promote cheatgrass invasion over time (Johansen , Condon and Pyke ).…”

“…) which when occurring in the field could shorten fire return intervals in deserts (Pilliod et al. , St. Clair and Bishop ).…”

Herbicide effects on the establishment of a native bunchgrass in annual grass invaded areas: Indaziflam versus imazapic

“…Controlling invasive annual grasses proves vital to restoring native plant species, as invasives such as downy brome often outcompete native species after disturbance (St. Clair & Bishop, 2019) and quickly monopolize the seedbank, leaving a reduced opportunity for native species to re‐establish (Humphrey & Schupp, 2001). Pre‐emergent herbicides are commonly used to reduce downy brome abundance (Mangold et al., 2013), but when applied concurrently with seeding, this treatment can negatively impact the establishment of native shrubs (Owen, Sieg, & Gehring, 2011) and perennial grasses (Shinn & Thill, 2004).…”

Grazing disturbance promotes exotic annual grasses by degrading soil biocrust communities

“…, Seipel et al. , St Clair and Bishop ), despite previous evidence for a negative relationship between exotic annual grasses and biocrusts (Peterson , Reisner et al. ).…”