Plant community diversity after fire along chaparral fuel breaks

Background/Question/Methods A system of fuel breaks was constructed during the 1950s in the chaparral-covered North Mountain Experimental Area (NMEA), Riverside County, California.  The network has been sporadically maintained by chemical and/or mechanical means, and most of the fuel breaks were colonized by herbaceous species and sub-shrubs in the last few decades.  Many of the herbaceous species are non-native grasses and forbs, which have the potential to invade chaparral after disturbance such as frequent fire.  A great deal of the NMEA burned in October 2006 in the 40,000 acre (16,100 ha) Esperanza fire, which was only the most recent of many wildland fires to affect the area.  We hypothesized that invasive grasses and forbs would be most likely to spread from the fuel breaks into adjacent chaparral where vegetation was youngest before the most recent fire.  To test this, we measured plant species diversity and cover within fuel breaks and immediately adjacent chaparral, both within the 2006 burn area and at sites that escaped the fire.  Sampling transects, placed along 2 miles of 3 stretches of dirt roads, were 40 m long with seven 1 m² plots located at 5 m to 20 m intervals.  Sampling was done during June 2007 to get immediate postfire vegetation composition and will be repeated in June 2008 to assess spread of non-natives from within the fuel break to surrounding recovering chaparral.   

Results/Conclusions There was up to a 20 percent difference in species diversity among transects located on the 3 roads, which differed in surrounding chaparral age.  Higher species diversity occurred on the more disturbed plots at the fuel break edge compared to plots 40 m into the chaparral.  We identified 68 species in 2007: 1 tree, 8 shrubs and sub-shrubs, and the rest forbs and grasses.  Plots ranged from 100 percent bare in freshly burned areas to 100 percent plant cover on unburned plots.  Even though plant diversity was lower, higher numbers of native woody plants were found at transect ends within surviving chaparral.  Non-native species tended to predominate on plots within fuel breaks.  Additional data from 2008 will further elucidate how repeated burns affected species occurrence.