Disturbance and riparian tree establishment in the Sespe Wilderness, California, USA

The riparian forests of southern California are subject to disturbance by both fire and flood. These agents are capable of causing pulses of mortality and recruitment, but it remains unclear how they interact to determine patterns of stand development. We use dendrochronology to identify establishment dates for stems of major riparian tree species in the Sespe Creek watershed, in order to examine their relationship to regional flooding and fire history. Our 11 study sites were burned by major fires in 1932 and 2002, with a smaller 1975 fire affecting only two sites; major floods were concentrated within the second half of the 1933-2009 streamflow record, with the largest floods occurring in 1969, 1978, and 1983. Three periods of stand development are evident: (1) the oldest alder (Alnus), cottonwood (Populus), and oak (Quercus) stems became established soon after the 1932 Matilija Fire, (2) minimal stem establishment between the 1940s and mid-1960s, and (3) continued, although irregular, recruitment of alder and cottonwood since the late 1960s. These patterns show episodes both of regeneration following a catastrophic site-clearing event (Matilija Fire) and of more localized stem replacement during the recent period of increased flood magnitude, with implications for changes in the composition of these forests.