Early vegetation response to repeated high-severity fire in the Klamath-Siskiyou Mountains, Oregon, USA

Reburns have historically been a fire regime component in certain forest types of Western North America, and may become more prevalent under current climate trajectories, but scant information exists on forest vegetation response to reburning.  We measured regenerating vegetation two years after a 15-year burn-reburn event (1987 Silver Fire – 2002 Biscuit Fire) in the Klamath-Siskiyou region of Southwestern Oregon, USA, and compared it to vegetation in once-burned and unburned (mature/old-growth) stands on similar sites.  A core assemblage that included most species found in unburned stands was consistently identified in both burn types, indicating high community resistance through one and even two high-severity fires.  The reburn was further augmented by several open/disturbance-adapted forbs and subshrubs co-existing with the core assemblage, resulting in higher total plant cover and species richness.  The augmentations were likely associated with the development of a propagule bank for early seral species during the period between fires. Hardwood shrub reproductive modes (seeding vs. sprouting) and resprouting vigor were similar in the reburn, indicating that resprouting potential was largely undiminished by multiple fires.  Initial conifer establishment occurred at similarly high densities in both burn types (range 298-6086 ha^-1 in the reburn; 406-2349 ha^-1 in the single burn), which reflected similar availability of seed source and germination substrates. The timeframe over which the initial compositional differences persist or influence stand development remains to be determined and will distinguish whether the reburn footprint is an ephemeral landscape feature or an important successional/floristic agent over time.  Reburning appeared not to be deleterious to native flora, and the co-occurrence of a unique early seral assemblage with key successional species (e.g., conifers) indicated that reburns can contribute to the landscape heterogeneity inherent to mixed-severity fire regimes while still retaining the potential to attain late successional conditions.