Fuel connectivity, burn severity, and seed bank survivorship drive ecosystem transformation in a semiarid shrubland

Abstract

A key challenge in ecology is understanding how multiple drivers interact to precipitate persistent vegetation state changes. These state changes may be both precipitated and maintained by disturbances, but predicting whether the state change is fleeting or persistent requires an understanding of the mechanisms by which disturbance affects the alternative communities. In the sagebrush shrublands of the western United States, widespread annual grass invasion has increased fuel connectivity, which increases the size and spatial contiguity of fires, leading to post-fire monocultures of introduced annual grasses (IAG). The novel grassland state can be persistent, and more likely to promote large fires than the shrubland it replaced. But the mechanisms by which pre-fire invasion and fire occurrence are linked to higher post-fire flammability are not fully understood. A natural experiment to explore these interactions presented itself when we arrived in northern Nevada immediately after a 50,000 ha wildfire was extinguished.We hypothesized that the novel grassland state is maintained via a reinforcing feedback where higher fuel connectivity increases burn severity, which subsequently increases post-fire IAG dispersal, seed survivorship, and fuel connectivity. We used a Bayesian joint species distribution model and structural equation model framework to assess the strength of the support for each element in this feedback pathway. We found that pre-fire fuel connectivity increased burn severity and that higher burn severity had mostly positive effects on the occurrence of IAG and another non-native species, and mostly negative or neutral relationships with all other species. Finally, we found that the abundance of IAG seeds in the seedbank immediately post-fire had a positive effect on the fuel connectivity 3 years after fire, completing a positive feedback promoting IAG. These results demonstrate that the strength of the positive feedback is controlled by measurable characteristics of ecosystem structure, composition and disturbance. Further, each node in the loop is affected independently by multiple global change drivers. It is possible that these characteristics can be modeled to predict threshold behavior and inform management actions to mitigate or slow the establishment of the grass-fire cycle, perhaps via targeted restoration applications or pre-fire fuel treatments.

Structural heterogeneity predicts ecological resistance and resilience to wildfire in arid shrublands

“…The presence of a single optimal scale is consistent with previous evidence from forest ecosystems: pulse disturbances like wildfire may impose functionally similar effects on vegetation structure despite differences in site conditions, time since fire, and wildfire severity (Atkins et al 2020). Wildfires in the Great Basin may equally remove large and small vegetation from the landscape (Miller et al 2013;Requena-Mullor et al 2019;Mahood et al 2023), inherently erasing or modifying structural patterns across multiple scales. Postdisturbance heterogeneity across scales is unequal and depends on vegetation properties.…”

“…Mapping structural heterogeneity enables insights into ecosystem functions that determine ecological resistance and resilience (Koontz et al 2020;Mahood et al 2023). We found strong evidence for scale-dependent effects of structural heterogeneity on resistance and resilience to wildfires in a semiarid ecosystem.…”

Local factors mediate climate and land use effects on global fire regimes (2003-2020)

“…Additionally, interactions between warming and anthropogenic influences are highly circumstantial. Humans directly influence fire regimes via the introduction of ignitions (Balch et al, 2017;Cattau et al, 2020), and indirectly through the introduction and spread of invasive plants, which can either increase (Fusco et al, 2019(Fusco et al, , 2021 or decrease (Collins et al, 2021) fire activity depending upon their effect on flammability, fuel load, and fuel continuity (Keeley and Pausas, 2019;Mahood, Koontz and Balch, 2023). The connection between global patterns of fire emissions and land use is also related to the state of development along a continuum from biomass burning to fossil fuel combustion (Balch et al, 2016).…”