Grid-based fire-scar dendrochronology and vegetation sampling in the mixed-conifer forests of the San Bernardino and San Jacinto Mountains of Southern California

Little is known of the fire regimes in southern California mixed-conifer forests prior to the start of suppression in 1900. Past studies using fire scar dendrochronology have sampled the most accessible trees, or trees with multiple scars, over small areas. Sample bias and small sample size has led to ambiguous findings on FRI estimates and landscape-scale fire intervals. Fire scars represent a spectrum ranging from single-tree events to mass burning, leading to ambiguous estimations between point-based and landscape-scale FRIs. A record of fires was developed for the mixed-conifer forests at the Black Mountain (BM) and Big Pine Flat (BPF) areas of the San Bernardino National Forest using a systematic grid sampling technique designed to reduce point-based fire-scar sampling bias. This record includes both pre-suppression and suppression periods. The average all-sample-mean-fire-intervals (AASM) is the best estimation of fire return intervals (FRIs) on the landscape. The AASM mean was 33 years for “westside” mixed-conifer forests on BM and 50 years for “eastside” mixed-conifer forests at BPF. Non-spatially based years between all fires sampled at BM and BPF ranged from one fire every year to 71 years, and median FRIs were 5 years. Fire sizes within the grid varied from numerous single-tree events, to fires in excess of 24 ha within the sample area. Fires greater than 24 ha on the grid were part of much larger fires on the landscape. Fires generally occurred the mid- to late growing season from late-July until mid- to late-September. The vegetation exhibited discrete cohorts of overstory trees by size and age class from that established prior to suppression, however there was no discrete cohort development during suppression. There has been an increase in stand density since 1932 due to suppression. No change in overstory composition as a function of time-since-fire was found.