Effects of post-fire management on vegetation and fuels following successive wildfires
Figure 1. Conceptual model of potential pathways (hypotheses) for post-fire vegetation and fuel dynamics following initial fires and short interval (5-15 year) reburns. Pathways are coded for the type of ecological feedback that we expected would occur in response to different fire severity levels and the effects these changes would have on subsequent fires
In the face of changing climatic regimes and increases in extreme fire events, many western forests are poised to burn, not only once but multiple times, sometimes in short succession. As such, land managers have a limited opportunity to effectively alter post-fire vegetation and fuels to make them more resilient to future disturbances like fire. Our study took advantage of a unique opportunity to examine vegetation and fuels after successive fires, and in doing so identified several management-relevant pathways by which post-fire vegetation and fuel development influence the severity and ecological outcomes of a subsequent wildfire.
Key Results/Findings
In our study fires, resistance to high severity reburn was contingent on a combination of factors, including topography, fire weather, vegetation structure, and woody fuels. Areas that were situated in more variable and mesic terrain were less likely to reburn at high severity. In stands that burned at lower initial severities, high relative humidity and low wind speeds during the reburn also lowered the probability of high-severity fire effects. In areas that initially burned at high severity, high densities of snags and down woody fuels were associated with high severity effects in the second fire. Variability and density of vegetation also played an important role in moderating reburn severity. In early seral habitats (i.e. those burned initially at high severity), areas with relatively sparse understory and variable sub-canopy were most likely to avoid repeat high-severity fire. Forests that burned initially at low to moderate severity were most likely to resist high-severity fire if they supported sparse understories and relatively dense but heterogeneous vegetation in upper height strata (> 2 m).
Following the reburn, areas impacted by successive high severity fires had little to no live conifer overstory, retained high cover of shrubs in the understory, and had low conifer regeneration. In contrast, successive low to moderate severity fires significantly reduced tree density, increased tree regeneration, and in some severity combinations (e.g. low followed by moderate severity) promoted colonization by shrubs. In our study, multiple low to moderate fires did not shift forest composition toward dominance of more fire tolerant or shade intolerant species.
Applications to Management
Taken together, the results of our study suggest that post-fire vegetation structure and woody fuels play an important role in subsequent fire severity patterns and ultimately, influence the resilience of post-fire landscapes to future fire. In areas where high severity reburn is undesirable, managers should consider treatments that reduce the density and continuity of vegetation, standing snags, and large woody surface fuels. In areas where proactive reforestation is necessary (e.g. to facilitate the transition from early-seral vegetation to conifer forest) planting in areas that are in rough or mesic terrain may reduce the likelihood of high severity reburn. The results of our study also suggest that active post-fire management may be necessary in areas that have burned at low to moderate severity in order to maintain or promote the restorative benefits of an initial fire or to restore the dominance of fire resilient tree species.
