Impacts of Wildfire on the Composition and Structure of Riparian Forests in Southern California

Abstract

In southern California, wildfire is a ubiquitous agent shaping plant communities. Although fire impacts have been widely studied in chaparral-covered uplands, few data are available regarding fire and riparian vegetation. This study provides an example of the impact of a severe fire on riparian habitat. Plant species found in southern California gallery forests are typically adapted to maintaining populations following flood disturbances; we seek to determine whether structural and compositional changes following fire here demonstrate a similar quasi-equilibrium response. We sampled 65 quadrats on 11 transects along two streams in the Los Padres National Forest to characterize tree species size–class distributions before and after the 2002 Wolf Fire. We tested whether species exhibited differential patterns of survivorship and regeneration following the fire, and also tested for spatial variability in mortality within the floodplain. Alnus rhombifolia dominated the pre-fire forest, but experienced severe mortality in the fire and showed very limited resprouting after 3 years. Other prominent taxa (Populus, Salix, Quercus spp.) also lost considerable standing basal area, but had substantially greater rates of resprouting, resulting in a dramatically altered post-fire vegetation composition and structure. Fire impacts did not vary with landform position, leading to a distinctive homogenizing disturbance that contrasts with the spatially zoned and relatively stabilizing compositional influence that flood events have in this same riparian setting.