OOS 5-4
Shrub recovery after fire in chaparral of different ages – implications for type-conversion risk
Chaparral is the shrub-dominated vegetation type characteristic of middle elevations throughout California. Chaparral burns in stand-replacement crown fires with a natural return interval of 20 to over 80 years, depending on species composition and topography. Shrub species regenerate through sprouting from basal burls or from soil-stored seed. Close-interval fire was used historically in rangelands to reduce populations of obligate-seeding shrubs and increase grass abundance for grazing; today anthropogenic ignitions in close succession can result in regeneration failure and vegetative type-conversion. The abundance of non-native (European) annual grasses throughout California means that most chaparral stands are colonized by those grasses from the second year post-fire until shrub cover is re-established (5 to 10 years). We examined chaparral vegetation recovery after a large inland southern California wildfire that burned through stands ranging from 1 to over 100 years in age since previous fire to determine how stand age affects shrub seedling abundance. Vegetation cover was determined 2 and 3 years postfire at 65 points in the burn area, and shrub seedling density was quantified. We also obtained shrub seedling density and vegetation cover values for comparison from earlier studies conducted for other reasons (rainfall and temperature patterns post-fire varied among sites).
Results/Conclusions
Cover of dominant chaparral shrubs was inversely related to non-native annual grass cover in the sampled stands within the large burn. However, virtually all plots had substantial cover of non-native grasses by three years post-fire, and some younger plots (<10 years) burned incompletely in the fire and thus had high sprouting shrub cover at our second sample period. Shrub seedling density trended lower in the younger plots but varied greatly depending on pre-fire shrub composition: species of Ceanothus were the most abundant post-fire seedlings. The coastal sites had higher Ceanothus seedling density, and seedlings grew faster than at the inland site. We found that shrub seedling density was much lower at a coastal site that had burned 5 years earlier compared to older stands that burned. Part of the large study site reburned last summer, 7 years after the study fire. We will present seedling density data gathered this spring from those sites as well.