Tree regeneration, understory development, and biomass dynamics following wildfire in a mountain hemlock (Tsuga mertensiana) forest

Abstract

Processes initiated by wildfire largely determine ecological characteristics of forested landscapes in subsequent decades, including vegetation composition, habitat quality, carbon balance, and probability of fire recurrence. Post-fire biomass dynamics have rarely been observed directly for high-elevation forests of the Pacific Northwest. We examined changes in total biomass and its components (attrition of coarse woody debris (CWD), growth of shrubs and herbaceous plants, and tree recruitment) over the first 15years following wildfire in a mountain hemlock (Tsuga mertensiana) forest in Oregon, using permanent plots representing random samples of four levels of fire-severity, from unburned to >90% tree mortality. Understory vegetation was transformed by fire, inasmuch as only shrubs were detected in unburned plots, while burned plots also had significant amounts of graminoids and forbs. Conifer recruitment was sparse in plots with high fire-severity two years after fire, but was abundant after 15years. Recruitment was predominantly mountain hemlock and most were seedlings <1m tall. The breakage of snags and resulting increase in logs created microsites (north sides of logs) conducive to tree regeneration. Mass of understory vegetation and conifer regeneration was far outweighed by CWD. Toppling, fragmentation, and advancing decay of snags resulted in a decrease in snag mass and an increase in log mass. The loss of snag mass exceeded the increase in log mass, resulting in an overall decline. For some portions of the burned area, particularly areas of high fire-severity, it may be more than a century before the growth of live trees balances the loss of mass of CWD.