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

Abstract

Con t en tsC h a p t e r t h r e e

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Substantial warming over the past 25 years has significantly amplified regional forest drought stress, likely to a large degree by increasing atmospheric vapor pressure deficits during the growing season months. Strong correspondence exists between FDSI and forest productivity, tree mortality, bark-beetle infestations, and wildfire in the Southwest , illustrating the powerful interactions among climate, land use history (livestock grazing, fire suppression), disturbance processes, and forest dynamics in this region (Allen 2015). After nearly a century of fire suppression, a wet period highly favorable to tree growth occurred from ca.…”

“…As projected hotter droughts intensify in coming decades, post-mortality forest recovery could be limited, delayed for long time periods, or impossible because hotter droughts in association with more frequent and severe disturbances will create less favorable conditions for regeneration of many currently dominant species (Suarez and Kitzberger 2010, AndersonTeixeira et al 2013, Tarancón et al 2014, Fensham et al 2015. Additionally, hotter droughts drive shorter lifespans for surviving and establishing plants (Adams et al 2009) and can be expected to favor: smaller-statured trees; transition of resprouting tree species toward more shrub-like morphologies; and non-tree life forms such as resprouting shrubs and grasses in many areas (Zeppel et al 2014, McDowell andAllen 2015). Drought mortality can disproportionately affect the most dominant tree species (Fensham et al 2015) and the biggest old-growth trees (Phillips et al 2010, McDowell andAllen 2015) that contain large biomass carbon pools (Stephenson et al 2014, Fauset et al 2015 and old-growth characteristics that are irreplaceable for centuries (Lindenmayer et al 2012).…”

“…Additionally, hotter droughts drive shorter lifespans for surviving and establishing plants (Adams et al 2009) and can be expected to favor: smaller-statured trees; transition of resprouting tree species toward more shrub-like morphologies; and non-tree life forms such as resprouting shrubs and grasses in many areas (Zeppel et al 2014, McDowell andAllen 2015). Drought mortality can disproportionately affect the most dominant tree species (Fensham et al 2015) and the biggest old-growth trees (Phillips et al 2010, McDowell andAllen 2015) that contain large biomass carbon pools (Stephenson et al 2014, Fauset et al 2015 and old-growth characteristics that are irreplaceable for centuries (Lindenmayer et al 2012). Relative to rapid mortality processes, tree establishment and growth are slow due to natural regeneration v www.esajournals.org processes, dispersal limitations, or lags in migration or regeneration (Feeley et al 2012, Frank et al 2015b)-thus old sequestered carbon lost to amplified forest die-off may take significant time to fully recover, even in productive environments with rapid biomass regrowth, and particularly in dry or cold environments where growth is slower (Janisch and Harmon 2002; Fig.…”

“…Since ca. 2000 the Southwest has been subject to large increases in tree mortality in response to the combination of protracted drought and early 21st century warmth (Breshears et al 2005, Williams et al 2010, Allen 2015, as part of a broader sub-continental pattern of hotter drought that has been driving historically unprecedented insect outbreaks (Raffa et al 2008) and wildfire (Westerling et al 2014) in western North America. While past severe droughts are documented to have caused substantial tree mortality in the Southwest during the 1950s and likely even as far back as the 1580s ''megadrought'' (Swetnam and Betancourt 1998), the recent hotter ''global-change-type'' drought (Breshears et al 2005) has caused particularly pronounced tree mortality.…”

“…After nearly a century of fire suppression, a wet period highly favorable to tree growth occurred from ca. 1978-1995 that supported maximal development of dense high-biomass forests in the Southwest, followed by the onset of warmer drought conditions that have driven historically extreme levels of drought-induced tree mortality, bark beetle outbreaks, and high-severity fires (Williams et al 2010, 2014, Allen 2015. Tree rings document 2002 as the worst year for regional tree growth since at least AD 1000, due to hotter drought conditions .…”