Fire severity and vegetation age class effects on post-fire chaparral seed banks

Background/Question/Methods The Santa Ana wind-driven Esperanza fire of 2006 burned through the North Mountain Experimental Area (NMEA) and vicinity, including all or parts of 10 previous fires.  Multiple images of the fire’s progression were taken using Riverside Fire Lab’s airborne FireMapper thermal-imaging system, which can be used to estimate fire front temperature and intensity.  These images, combined with historic aerial photos and fire occurrence maps, existing fuels and resource data, and post-fire imagery, provided an opportunity to investigate relationships between chaparral age class, fire severity, and vegetation response.  Points within the fire perimeter were selected in a stratified-random manner across image fire intensity classes and prefire fuel age classes.  Fire severity was categorized at each point based on fuel consumption and stump basal diameter, and stem density (and species identity where possible) was recorded in a 7.5 m diameter circle at each point.  Soil seed bank samples were collected at 5 locations from around each point at two depths (0-2 and 2-5 cm) to compare with aerially-estimated fire intensity and to characterize plant community regeneration potential.  Samples were spread on sterile potting medium in flats and germinated in an unheated greenhouse during winter-spring 2008. 

Results/Conclusions Prefire vegetation age ranged from 3 to 98 years since the last fire, with most samples from the burned area collected from sites that burned 30 years or more previously.  A total of 59 species were identified as of May 2008, mostly native species typically found after fire in chaparral ecosystems.  Seedling density ranged from 15 to several hundred produced from each 78 cm² area sample.  Non-native grass seeds (mainly red brome, Bromus rubens ssp. madritensis, and cheatgrass, B. tectorum) were present most often in younger samples, but a few were found in the oldest samples as well.  Samples from high intensity burn points were dominated by fire followers such as Phacelia brachyloba and Camissonia species.  Despite the presence within the NMEA of long-term fuel breaks, which were dominated by non-native grasses and burned lightly if at all during the Esperanza fire, very few seed bank samples contained substantial numbers of non-native seeds.  Adequate regeneration of chaparral seems assured throughout the burn area.