Evaluating Response of Southern California Chaparral Landscapes to
Short-interval Fire and Drought (1984-2018)
Abstract
Regrowth after fire is critical to long-term persistence of chaparral
shrub communities in southern California. This region is subject to
frequent fire, habitat fragmentation, and protracted droughts linked to
climatic change. Short-interval fire (SIF) is considered an inhibitor of
recovery and cause of “type conversion” in chaparral, based on field
studies of small extents and limited time periods. Sub-regional scale
investigations based on remotely sensed data, however, suggest that SIF
may explain little variance in postfire chaparral recovery. Drought may
contribute to poor recovery or worsen the impact of repeated,
short-interval fires. Previous studies have not shown whether drought
reduces chaparral recovery significantly across the region, while
variations in response among community types and climate zones are not
well resolved. This research evaluates a regional pattern of chaparral
recovery, based on series of Normalized Difference Vegetation Index
(NDVI) from annual, June-solstice Landsat images (1984–2018). High
resolution aerial images were used in validation and calibration. The
main objectives were (1) to assess effects of fire-return interval and
number of burns on chaparral recovery using 0.25 km2 sample plots (n =
528) which were paired and stratified for experimental control, and (2)
to explain recovery variations across the region based on geospatial
climate, vegetation, soil, terrain, and temporal drought metric data
(seasonal precipitation, climatic water deficit (CWD), and Palmer
Drought Severity Index (PDSI)) from 982 locations. Results suggest that
SIF is most impactful in sites that burned three times within 25 years.
More substantial effects were observed due to drought. In particular,
ecotonal chaparral bounding the Colorado Desert is most subject to
drought impact. We also highlight utility in landscape-scale predictors
of drought impact on recovering chaparral, including Very
Atmospherically Resistant Index (VARI).