Fire legacies, heterogeneity, and the importance of mixed-severity fire in ponderosa pine savannas

Globally, savanna ecosystems are shifting outside of "safe operating spaces" due to removal of their primary self-reinforcing feedback-fire-and subsequent erosion of disturbance legacies. Restoring savannas will require reinstating fire feedbacks. But knowledge gaps in the nature of historic fire regimes and how mechanisms such as time-since-fire and fire severity interact to produce disturbance legacies hinders development of ecologically relevant restoration targets. A theory-based approach for determining restoration targets is to compare structures produced by time-since-fire/fire severity interactions to structures that fostered animal communities that historically inhabited savannas. Here, we use a space-for-time substitution to quantify interactive effects of timesince-fire and fire severity on vegetation structures related to known animal community habitat preferences by surveying sites in 10-year-old and 27-year-old mixed-severity fires that occurred in an eastern ponderosa pine (Pinus ponderosa) savanna where fire was excluded since European settlement. Our specific objectives are to 1), quantify the relative strengths and interactive effects of time-since-fire and a full fire severity gradient on multivariate vegetation structure across landscape patches and 2), assess relationships between multivariate vegetation structures and time-since-fire/fire severity classes across landscape patches. We used a stratified random design to distribute 112 sampling plots by fire severity (unburned, low, moderate, and high) and time-since-fire (10 years and 27 years) and measured stand structure, tree cavity characteristics, and coarse woody debris. The interaction of time-since-fire and fire severity drove structure complexity for 27 years post-fire, but fire severity explained > 14 times the amount of variation in structure than time-since-fire alone. A full fire severity gradient (low-, moderate-, and high-severities) and structural changes within patches that experienced different fire severities generated sufficient patch-level heterogeneity to foster animal communities historically native to eastern ponderosa pine savannas. Structures generated by low fire severity alone, which is a common goal of fire management in easternmost ponderosa pine savannas, did not reflect sufficient structural complexity to support the diversity of endemic animal species. This indicates information legacies (i.e., represented by the distribution of species traits in a community) were shaped by mixed severity fire regimes, which provides further support for the scientific premise that management goals seeking to minimize variation in fire regimes (e.g., low intensity and low-severity fires only) is less able to support a full array of biodiversity. Rather, mixed-severity fire is an important driver of structural heterogeneity, fosters diverse information legacies, and enhances ponderosa pine savanna resilience to extreme fire.