Land cover change in the Southwest: Wildfire risk, drought-induced tree mortality, and the convergence of climate, land management, and disturbance trends in regional forests and woodlands
Extensive high-severity wildfires and drought-induced tree mortality have intensified over the last 2 decades in southwestern US forests and woodlands, on a scale unseen regionally since at least pre-1900. Abundant and diverse paleo-ecological and historical sources indicate substantial variability in Southwest fire regimes and forest patterns over the past ~10,000 years, providing context for recent fire and vegetation trends. In particular, over the past ~150 years regional forest landscapes and fire regimes have responded sensitively, strongly, and in understandable ways to changes in human land management, as well as to interactions with climate variability and trends. Widespread, high-frequency surface fire activity ceased on most Southwest landscapes in the late 1800s due to changed land use patterns. As a result, woody plant establishment exploded into the 1900s, fostered by several favorable wet climate windows for tree regeneration and growth, and fire suppression. By the early 1990s many Southwest forests likely were near their maximum potential levels of tree density and biomass storage, and had reached unsustainable conditions. New research derives a forest drought-stress index (FDSI) for the Southwest using a comprehensive tree-ring growth data set representing AD 1000–2007, driven by warm season temperature and cold-season precipitation.
Strong correspondence exists between FDSI and forest productivity, tree mortality, bark-beetle outbreaks, and wildfire in the Southwest, illustrating the interactions among climate, land use history, and disturbance processes in this region. Recent increases in severe wildfires and overall tree mortality in response to early 21st century warmth and drought conditions suggest that a transition of southwestern forest landscapes toward more open and drought tolerant ecosystems may already be underway. If regional temperatures increase as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Multiple lines of evidence now foreshadow 21st century changes in forest structures and compositions to novel distributions in the Southwest. Cascading ecological effects and drivers of these interactive landscape changes are presented, along with adaptation strategies to enhance ecosystem resilience in the context of ongoing and projected climate trends.