Disturbance is important to forest ecosystem function and overall health, but climate change is likely to increase the frequency and intensity of forest disturbances in the future. Ecosystems with increasingly frequent and intense disturbances are more likely to experience overlapping (compounding) disturbance effects. Compounding disturbances exert unpredicted, non-additive stresses on ecosystems, leading to novel conditions exceeding these ecosystems’ capacity for recovery. We further hypothesize that compounding disturbances could limit recovery by altering physical and chemical growing conditions in forest soils and affecting forest structure and regeneration for years after disturbance events. A better understanding of these remnant effects is important in coast redwood (Sequoia sempervirens) forests, which are projected to see increased risk of fire under future climate scenarios. Our objectives in this study were to: 1) Assess the effects of burn history (single vs. twice-burned) and time since fire on stand structure of coast redwood ecosystems; 2) Model coast redwood regeneration as a function of time since fire and abiotic site variables. Abiotic variables included percent available light, soil ammonium- and nitrate-N, C, P, pH, CEC, bulk density, and percent moisture across four burn years spanning 1985 - 2013 in Big Sur, California.
Results/Conclusions
Significant differences in coast redwood stand structure between the once-burned and twice-burned areas were identified using Wilcoxon and unequal variance t-test comparisons with a Bonferroni correction. Differences were significant for both seedling (p < 0.0001) and sapling (p < 0.01) composition and structure. Twice-burned plots were skewed toward a younger cohort and dominated by coast redwood individuals. Coast redwood regeneration was also evaluated using a generalized linear mixed model fitted with a Poisson error distribution and log link function to model coast redwood regeneration counts at 99 randomly placed 0.01-ha subplots. Our data were best fit by a model using percent soil moisture, year of last burn, and a random effect for geographic plot location as predictors of coast redwood regeneration. Both fixed effects were highly significant (p < 0.0001), and the random effect showed additional unexplained variance related to plot location. Our results indicate that compounding fire disturbances drove a shift in this ecosystem by favoring regeneration of coast redwood. Compounding disturbances may create novel communities through the loss of ecologically significant species and their replacement by better adapted species.