Laying the groundwork: post-fire complex early-seral forest community trajectories in western Cascadia

Abstract

Climate warming and associated increases in fire activity elevate the importance of understanding post-fire complex early-seral forest (CESF) plant communities – assemblages that dominate post-fire until tree canopy closure. CESFs support high levels of biodiversity and ecosystem function, influencing successional trajectories of forest development; yet relatively little is known about CESFs compared to tree regeneration post-fire. This is especially true in forests characterized by infrequent high-severity wildfires, where opportunities to track post-fire CESFs are rare. Here, we characterize CESF plant communities after wildfire in western Cascadia (temperate maritime conifer forests in western Washington and northwestern Oregon, USA), and ask: (Q1) What is the flora of CESF communities compared to analogous unburned forests? How do community composition (Q2), and alpha diversity (species richness and evenness within stands) and beta diversity (compositional change among stands) (Q3), vary with pre-fire stand age and burn severity? We established 86 1-ha plots (stands) across four wildfires where we recorded percent cover for graminoids, bryophytes, and vascular plants to species, across strata combinations of three levels of pre-fire stand age (young, mid-seral, late-seral) and burn severity (unburned, low, high). Across strata, we compared relative abundance of species and life forms in post-fire and unburned stands (Q1), and differences in community composition (Q2) and alpha and beta diversity (Q3) across pre-fire stand age and burn severity using a range of univariate and multivariate analyses. We observed 206 species overall, with 91 species common to both burned and unburned stands. Burned stands had 83 unique species and were primarily dominated by native herbs. In contrast, unburned stands had 32 unique species and were largely shrub-dominated. Differences in CESF communities among stands were strongly and consistently driven by differences in burn severity and secondarily by differences in pre-fire stand age; composition varied widely across stands within and among strata, leading to high beta diversity. Within-stand alpha diversity was high (species richness: 22.3-31.8) across strata, with the exception of mid-seral unburned stands (species richness: 11.9); evenness did not differ among strata. Our findings have important implications for understanding and managing forests pre- and post-fire in infrequent and stand-replacing fire regimes. Specifically, our results show that an abundant, though relatively species poor, forest condition in western Cascadia (mid-seral forests) may experience the greatest increase in biodiversity and compositional heterogeneity following fire. Finally, different combinations of pre-fire conditions and fire severity on stands provide unique community assemblages, which can support a diverse array of ecosystem processes and provisions.