Fire regime alteration and regeneration dynamics in Pinus ponderosa stands of the Southwestern United States

Abstract

The wildfire regime of dry conifer forests dominated or co-dominated by ponderosa pine (Pinus ponderosa Douglas ex C. Lawson) in the Southwestern United States has been increasingly altered in the last decades. These changes, caused by the ongoing climate change and by the fuel build-up due to several decades of fire exclusion, resulting in a denser and more homogeneous forest structure, are leading to uncharacteristically large and severe wildfires, uncommon for these stands, adapted to fire regimes characterized by frequent low-severity surface fires. Large openings resulting from high-severity fires, together with post-fire drought and competition from herbaceous and shrubby vegetation, may hinder ponderosa pine regeneration, leading to shifts in forest composition or transition towards shrubland or grassland. The lack of pine regeneration caused by the higher severity of a first fire event can be reinforced by subsequent reburning, whose frequency is also increasing in the area. This study investigates how an altered fire regime can interrupt successional pathways in ponderosa pine stands in the Southwestern United States, focusing on the ecological and management implications associated with repeated fire occurrences after an initial high-severity wildfire. We analyzed the spatial and temporal patterns of post-fire regeneration after reburns through a combination of field surveys, remote sensing, and historical fire records, considering fire severity, topography, distance to seed trees, and time between the reburn events. The study aims to enhance our understanding of post-fire regeneration dynamics in the context of an altered fire regime and the ecological consequences and management strategies associated with this phenomenon.