Complex Effects of High Severity Fire on a Serotinous Conifer

Background Baker cypress ( Hesperocyparis bakeri (Jeps.) Bartel) is a serotinous conifer associated with high severity fire regimes. Until recently, observations of wildfire effects to Baker cypress populations were rare. There was little direct evidence of fire effects on germination immediately after fires or on population dynamics over longer time frames. We sampled four Baker cypress populations that burned in wildfires between 2006 and 2017 to evaluate postfire germination. We continued data collection at two sites and established a third experimental site to identify factors that contribute to the survival, growth, and health of Baker cypress seedlings over time. Results Although Baker cypress is associated with high severity fire regimes, we found that the effects of fire severity were complex and included both positive and negative, as well as short and long-term effects. Germination rates were positively related to crown scorch, but negatively affected by char bole height. These findings suggest that while Baker cypress requires heat to open cones, there are limits to the temperature or duration of heat that seeds can tolerate. Although high severity fire is thought to be necessary to create environmental conditions necessary for germination, we found that ground cover variables were not significant predictors of postfire germination rates at our study sites. Over longer time frames (9–10 years), Baker cypress density, survival, growth, and health were significantly impacted by fire severity and its effect on environmental variables, including overstory canopy closure, rock cover and shrub cover, demonstrating that fire severity has persistent legacy effects on Baker cypress populations. Conclusions In an era of altered fire regimes, optimizing fire effects to achieve sufficient crown scorch while minimizing fire behavior that causes extensive bole char may help ensure the persistence of Baker cypress populations into the future. This kind of nuanced understanding of the effect of fire on Baker cypress would not have been possible without immediate postfire assessments of fire severity that were not possible until recently.