Transitions of semi-arid forests and woodlands in the face of severe wildfire and climate change

Abstract

Increased wildfire activity and climate change are altering semi-arid forests and woodlands in the southwestern U.S. Large, severe wildfires, driven by increased fuel density from centuries of fire suppression and spread of invasive species, combined with increasing drought conditions and temperatures, raise concern for the ability of forests to regenerate within their historical ranges. We studied piñon-juniper woodlands (PJ) and ponderosa pine forests (ponderosa), which exist at the hot and dry climatic limit of forested ecosystems in North America, and are at risk of conversion to non-forested ecosystems or shifts to more suitable climates. PJ woodlands often border ponderosa pine forests at the ecotone boundary, providing an indicator for potential range shifts. We utilized data from 201 sites across 24 high-severity burns in southern and western Colorado and northern New Mexico on a gradient of PJ woodland, PJ-ponderosa ecotone, and ponderosa pine forest to observe tree regeneration 17+ years post-fire. We examined seedling regeneration against changes in 30-year means of climate variables from pre-fire to post-fire, vegetation cover, geographic variables, and surface fuel load. We found that seedling densities of juniper, piñon pine, and ponderosa pine significantly decreased from their pre-fire densities. Piñon pine and juniper seedling regeneration were highest in sites with less increase in vapor pressure deficit (VPD) and longer time since fire (TSF); ponderosa pine seedling regeneration was highest in sites with less decrease in winter precipitation; and seedling regeneration of each species was highest closer to seed sources and when associated with a nurse object. Substantial proportions of PJ woodland (41%), ecotone (37%), and ponderosa pine forest (20%) sites transitioned to grassland or shrubland communities, while mean elevations of PJ woodlands and ponderosa pine forests trended upwards post-fire. In sites that regenerated as PJ woodland or ecotone, piñon pine seedlings were either non-existent or found at much lower densities than juniper. Transitions of woodlands and forests to non-forested communities and higher elevations are expected to be exacerbated under an increase 2°C in the future climate, however, natural shifts to suitable climates are unlikely given the rate of climate change and increased large, homogenous severe wildfires. Our findings suggest that PJ woodlands and ponderosa pine forests are contracting in the southwestern US, and management will likely be required to assist regeneration in suitable climates if persistence of these ecosystems is desired.