Influence of compounding fires on coast redwood regeneration and stand structure

Abstract

Disturbance is fundamental to forest ecosystem function, but climate change will continue to increase both disturbance frequency and intensity in the future. Forests subject to increasingly frequent and intense disturbances are more likely to experience overlapping (compounding) disturbance effects. Compounding disturbances may exert unpredicted, non-additive stresses on ecosystems, leading to novel conditions that may exceed the capacity for local species to survive and regenerate. We further hypothesize that compounding disturbances could alter physical and chemical growing conditions in forest soils in ways that disadvantage tree species poorly adapted to the impacts of novel disturbance regimes. Forest composition, structure, and function could shift following increased pressure on one or more species. A better understanding of these remnant effects will be essential to managing and conserving coast redwood (Sequoia sempervirens (D. Don) Endl.) forests, which are projected to see increased frequency of fire under future climate scenarios. Our objectives in this study were to quantify the effects of time-since-fire and single vs. compounding disturbances on coast redwood forest structure, composition, and regeneration dynamics, and to evaluate the effects of abiotic soil qualities on post-fire regeneration. We stem mapped and field sampled coast redwood forests burned in 1985, both 1985 and 1999, 2008, and 2013 and modeled observed regeneration counts as a function of single vs. compounding fires, understory light, and post-fire nutrient levels. Forest structure, composition, and regeneration following compounding disturbance were most similar to the redwood-dominated forest of the recent 2013 burn. There were no unique effects of compounding disturbance on soil nutrient levels, although calcium peaked following the most recent fire. None of the predictors in our hypothesized model were significant, showing that soil legacies and nutrients may not be highly influential in regeneration processes following fire and that the number of fires in an area may have a complex influence on regeneration dynamics. However, this study underlines the need for further research into additional compounding fire disturbances in coast redwood forests to confirm whether the observed homogenized forest conditions were the result of compounding fire disturbance. Such outcomes would have negative implications for ecosystem services and overall function if compounding disturbances are more frequent in the future.

Parent Publication

• Coast redwood science symposium—2016: Past successes and future direction

Citation

Brousil, Matthew R.; Bisbing, Sarah. 2017. Influence of compounding fires on coast redwood regeneration and stand structure. In: Standiford, Richard B.; Valachovic, Yana, tech cords. Coast redwood science symposium—2016: Past successes and future direction. Proceedings of a workshop. Gen. Tech. Rep. PSW-GTR-258. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 87-98.