Fuel connectivity, burn severity, and seedbank survivorship drive the grass fire cycle in a semi-arid shrubland.

Introduced grasses can initiate novel grass-fire cycles that alter ecosystem structure and function, and threaten biodiversity. In sagebrush communities in the western United States, annual grass invasion increases the connectivity of fine fuels, which increases the size and spatial contiguity of fires. This increase in fire size and contiguity results in post-fire plant communities that are dominated by introduced annual grasses (IAG), which are themselves more likely to promote large fires and initiate a novel grass-fire cycle. But the mechanisms by which pre-fire invasion and fire occurrence are linked to higher post-fire flammability are not fully understood. Here, we investigate the successive mechanisms in a potential positive feedback that maintains the novel annual grass-fire cycle. We used total vegetation cover (TVC) as a proxy for fuel connectivity and found that pre-fire TVC increased burn severity. We then used a Bayesian joint species distribution model to examine how burn severity affected the proportion of IAG in the seed bank, and found that higher burn severity had mostly positive or neutral effects on the occurrence of IAG and other non-native species, and mostly negative or neutral relationships with native species. 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, thus completing a positive feedback promoting IAG. These results suggest that measurable characteristics of ecosystem structure (e.g. TVC) and fire (dNBR) may be used to inform management actions to mitigate the negative effects of the grass-fire cycle, perhaps via targeted restoration applications or pre-fire fuel treatments.