Compositional legacies of fire suppression maintained after fire re-entry

Abstract Background Legacies of fire suppression include changes in forest composition and structure that tend to increase fire severity in historically frequent-fire forests. Despite the expectation that fire re-entry should favor fire-tolerant species, indirect effects of fire include shrub proliferation that may hinder recovery of pre-suppression forest compositions. It therefore remains unclear under what conditions the legacies of fire suppression are maintained or reversed after fire reintroduction. We investigated how interactions between pre-fire forest structure and composition, topography, fire severity, and post-fire shrub growth affected conifer regeneration 10 years after fire re-entry in Yosemite National Park, CA, USA. We measured post-fire shrub demography, tree seedling density, and environmental factors along 38 transects at a permanent study site, the Yosemite Forest Dynamics Plot. Results Overall, interactions between forest density, fire severity, and post-fire shrub growth together perpetuated suppression-era forest composition characterized by fire-sensitive tree dominance. Direct effects of fire included higher tree seedling regeneration in high-severity burned areas, where severity was driven by higher tree density. However, fire also increased fire-responsive shrub growth, leading to negative indirect effects of fire on conifer regeneration. Importantly, shrubs inhibited fire-tolerant Pinus seedlings more than fire-sensitive Abies , leading to greater total Abies regeneration across different landscape positions. Conclusions Together, findings show that suppression-era forest compositions dominated by fire-sensitive species were maintained after fire re-entry, as post-fire shrub production enabled fire-sensitive tree regeneration more than fire-tolerant tree regeneration. This study offers the perspective that single fire reintroduction was insufficient to reverse the compositional legacies of fire suppression, though the effect of reburns on forest composition in an era of change remains uncertain.