Forest Ecosystem Reorganization Underway in the Southwestern United States: A Preview of Widespread Forest Changes in the Anthropocene?

Abstract

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 vegetation 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, grading into increasingly vigorous active fire suppression after 1910. This allowed woody plant establishment to explode in much of the 1900s, fostered by several wet climate windows favorable for tree regeneration and growth, and fire suppression, including ca. 1905-1922 and late 1970s to mid-1990s. By the early 1990s many Southwest forests likely had fluffed up to near their maximum potential levels of tree density, leaf area, and biomass (and carbon) storage, and had reached unsustainable conditions. Naturally episodic drought returned to the region in the late 1990s, with sustained severe drought affecting the Southwest since 2000 through the present (August 2013). 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. Substantial warming over the past 20 years is significantly amplifying regional forest drought stress, likely by increasing atmospheric vapor pressure deficits during the growing season months. 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. A pulse of recent research on physiological responses of diverse tree species to climate variables is providing important insights into the powerful roles of drought and heat stress in driving forest Draft – Do not cite, reference, or distribute productivity and health, physiological thresholds of tree mortality, and forest disturbance processes (add new refs here). Recent large increases in severe wildfire activity and overall tree mortality in response to early 21st century warmth and drought conditions, along with documented changes in the elevational distributions of many plant species, illustrate that southwestern forest landscapes already are transitioning toward more open and drought-tolerant ecosystems. 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 indicate ongoing changes in forest structures and compositions in the Southwest, pointing toward increasingly novel distributions emerging over the course of the 21st century. Some cascading ecological effects and drivers of these interactive landscape changes are presented, along with adaptation strategies to enhance forest ecosystem resilience in the context of ongoing and projected climate trends. Forests globally exhibit great diversity in environmental drivers, histories, dominant ecological patterns and processes, biodiversity, etc. – which are expected to produce diverse responses (and levels of resilience) to projected global changes in climate and human uses this century. Even given this global diversity of forests and expected global change responses, the observed convergence of climate, human land use patterns and histories (including livestock grazing, forest management, fire suppression, human settlement/WUI, and ignitions), and disturbance trends in the southwestern US may presage widespread forest ecosystem changes more broadly in North America, and globally.