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

Authors: Mahood, Adam; KOONTZ, M - UNIVERSITY OF COLORADO; BALCH, J - UNIVERSITY OF COLORADO

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2022
Publication Date: 12/26/2022
Citation: Mahood, A.L., Koontz, M.J., Balch, J.K. 2022. Fuel connectivity, burn severity, and seedbank survivorship drive ecosystem transformation in a semiarid shrubland. Ecology. 4(3). Article e3968. https://doi.org/10.1002/ecy.3968.
DOI: https://doi.org/10.1002/ecy.3968

Interpretive Summary: It is well-known that annual grass invasion can lead to a grass-fire cycle. Yet, the underlying mechanisms are unclear. We hypothesized that seed dispersal is the key pathway through which fire operates to initiate the grass-fire cycle. We found evidence for a positive feedback loop. We found that pre-fire fuel connectivity brought on by annual grass invasion increased burn severity. Burn severity favored annual grasses in the seed bank. Annual grass abundance in the seed bank then then led to higher post-fire fuel connectivity. This work provides a more detailed understanding of how the annual grass fire cycle works. This may allow land managers to prioritize restoration and fire risk mitigation actions. This may also assist fire managers in improving fire risk assessments.

Technical Abstract: 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.