Complex effects of fire severity on a serotinous conifer

Abstract Background Baker cypress ( Hesperocyparis bakeri (Jeps.) Bartel) is a serotinous conifer adapted to high severity fire regimes. Until recently, observations of wildfire effects on Baker cypress populations were rare and there was little direct evidence of how fire influences seedling density immediately after fires or population dynamics over longer time frames. We sampled four Baker cypress populations that burned in wildfires between 2006 and 2017 to evaluate first-year postfire seedling density. 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 First-year postfire seedling density was positively related to scorch height, but negatively affected by bole char 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. However, ground cover characteristics, prefire canopy seed storage, and other severity metrics such as tree mortality were not related to first-year postfire seedling density. Over longer time frames (9–10 years), higher shrub cover predicted lower seedling heights and higher survival rates, while lower canopy closure predicted greater seedling height and survival. Baker cypress seedling density, survival, growth, and health were all significantly higher in high severity plots after 9–10 years, demonstrating that fire severity has persistent positive effects on Baker cypress populations. Conclusions We found that the effects of fire severity on Baker cypress were complex and included both positive and negative, as well as short- and long-term effects. Optimizing fire effects to achieve sufficient crown scorch while minimizing fire behavior that causes extensive bole char may promote seedling density. Fire effects that reduce overstory canopy closure can promote Baker cypress survival over time. This understanding of the complex effects of fire on Baker cypress would not have been possible without immediate postfire assessments of fire severity, as well as longer-term studies that evaluate the effect of fire severity over time.