Seed bank composition altered by burn severity to favor annual grass abundance in invaded sagebrush shrublands

Background/Question/Methods

Alternative stable states are an important topic in the context of global change in which internal positive feedbacks are required to maintain two qualitatively different types of plant communities under the same climatic conditions. In Artemisia tridentata communities in the western United States, annual grass invasion changes the fuel connectivity, which increases the size and spatial contiguity of fires. This results in post-fire systems that are dominated by introduced annual grasses. But it is unclear exactly how fire directly benefits the introduced annual grasses. There are many possible mechanisms by which altered fuel properties influence fire properties, and how fire itself influences the post-fire biotic community and abiotic conditions. One mechanism by which fire can influence plant communities that is rarely directly studied is the alteration of the species composition of the seed bank. Here, we collected seed bank samples at 14 locations across a range of burn severities immediately after a large fire was extinguished, and examined the how burn severity affected the species composition. We focused on a positive feedback loop where fuel connectivity increases burn severity, and burn severity alters the seed bank composition such that it has a higher proportion of introduced annual plants. First, we used total vegetation cover as a proxy for fuel connectivity (TVC) and examined how it affected burn severity (H1), and then used a joint species distribution model to examine how burn severity affected the proportion of introduced annual plants in the seed bank (H2). We then tested if post-fire seed bank composition affected post-fire TVC (H3).

Results/Conclusions

We found that pre-fire fuel connectivity and burn severity were positively related (H1). For H2, higher burn severity had mostly positive or neutral effects on the occurrence of non-native species, and mostly negative or neutral relationships with native species. For H3, we found that the abundance of annual grass seeds in the seedbank immediately post-fire had a positive effect on the fuel connectivity 3 years later. This is the first study that we are aware of in the Artemisia tridentata ssp. wyomingensis ecosystem that shows how burn severity influences seed bank species composition, as well as examining the full feedback loop with fuel connectivity.