Monday, August 6, 2007

PS 18-178: Persistent effects of type-conversion on post-fire chaparral recovery

Mark E. Parlow, Marcia G. Narog, and Jan L. Beyers. Riverside Forest Fire Laboratory

San Dimas Experimental Forest (SDEF), Los Angeles County, CA was the site of post-fire chaparral type-conversion experiments in the 1960s.  Several watersheds were seeded after fire with annual and perennial grasses and treated with herbicide to suppress chaparral regeneration.  In subsequent years converted watersheds were colonized by a few chaparral species but had predominantly grass and subshrub cover.  In 2002 most of SDEF burned in the Williams fire, including the old type-conversions and some “black line” areas burned in 1999 in anticipation of a prescribed fire.  We investigated vegetation recovery in six small watersheds: three type-converted (TC) and three untreated (U), including the black-line area (BL) of two of the watersheds.  Prefire vegetation in U watersheds consisted of mixed chaparral dominated by chamise (Adenostoma fasciculatum) and species of Ceanothus.  We hypothesized that seed rain from adjacent U watersheds could provide a source for chaparral recolonization of TC watersheds after stimulation by fire.  First year regrowth was prolific in all plots due to well-spaced growing season precipitation.  Typical chaparral shrub seedling density and diversity were greatest in the U plots (average 3.0 versus 0.3 and 0 seedlings/m2 in TC and BL plots, respectively), while TC plots had more subshrub seedlings.  By four years postfire, seedling density had decreased substantially in all plots.  Shrub cover and total species richness were higher in U plots, while herbaceous and subshrub covers were greater in TC plots.  No obligate-seeding Ceanothus seedlings were found in BL plots.  Despite some shrub colonization, type-converted watersheds are remaining essentially type-converted.  This suggests that loss of chaparral may not easily be reversed without management intervention.

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