Forest responses to increasing aridity and warmth in the southwestern United States

Abstract

In recent decades, intense droughts, insect outbreaks, and wildfires have led to decreasing tree growth and increasing mortality in many temperate forests. We compared annual tree-ring width data from 1,097 populations in the coterminous United States to climate data and evaluated site-specific tree responses to climate variations throughout the 20th century. For each population, we developed a climate-driven growth equation by using climate records to predict annual ring widths. Forests within the southwestern United States appear particularly sensitive to drought and warmth. We input 21st century climate projections to the equations to predict growth responses. Our results suggest that if temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century. As tree growth declines, mortality rates may increase at many sites. Increases in wildfires and bark-beetle outbreaks in the most recent decade are likely related to extreme drought and high temperatures during this period. Using satellite imagery and aerial survey data, we conservatively calculate that ≈2.7% of southwestern forest and woodland area experienced substantial mortality due to wildfires from 1984 to 2006, and ≈7.6% experienced mortality associated with bark beetles from 1997 to 2008. We estimate that up to ≈18% of southwestern forest area (excluding woodlands) experienced mortality due to bark beetles or wildfire during this period. Expected climatic changes will alter future forest productivity, disturbance regimes, and species ranges throughout the Southwest. Emerging knowledge of these impending transitions informs efforts to adaptively manage southwestern forests.forest mortality | climate change | drought | fire | tree rings I ntense droughts in recent decades have been linked with extensive tree mortality in many temperate regions globally (1). Attribution of the causes of such mortality within forest ecosystems is challenging because wildfire, insect pests and diseases, invasive species, and land-use effects (e.g., grazing, fire suppression) all interact with climatic effects, confounding interpretation of mortality drivers. Forest vegetation is particularly sensitive to changes in moisture availability in semiarid landscapes (2). Within the southwestern (SW) United States (US), rapid and extensive changes in forests and woodlands recently have been associated with drought-related tree mortality, including attacks by cambium-feeding beetles and severe wildfires (1,(3)(4)(5). Landuse effects, however, probably also have contributed to changing wildfire regimes, particularly in SW ponderosa pine forests (6). High severity wildfires in SW forests can result in a multidecadal (or longer) failure of some forest stands to regenerate, converting large patches to grassland or shrubland (7). If warming continues and further amplifies severities and extent of climate-related disturbances in the SW US, the ability of forests to recover to previous species composition may be limit...

Projections of aridity and its regional variability over China in the mid‐21st century

“…Enhancement ET o may have negative implications on ecosystem and agriculture, like crop growth in the Haihe River Basin which is an important grain production base in eastern China (Wang et al, 2011). Moreover, increased aridity has the potential impact on grassland composition and productivity (Clark et al, 2002), and the forest growth would decline and mortality rates may increase substantially response to rising aridity (Williams et al, 2010). Therefore, ecosystem across the eastern China would be affected adversely by future warming and increasing aridity.…”

“…Increased aridity is linked to changing biogeochemical cycles, and could negatively affect key ecosystem functions and services (Delgado-Baquerizo et al, 2013). For example, if aridity continues to increase in the future, trees will experience substantially reduced growth (Williams et al, 2010). Moreover, changing aridity may affect the global water balance; have a significant influence on runoff (Arora, 2002); and cause anomalies in the intensity, spatial extent, and frequency of droughts (Nastos et al, 2013).…”

Predictability of biomass burning in response to climate changes

“…If this is indeed the case, a key issue is whether there is further potential for landscape fragmentation to offset climateinduced increases in fire in the future [see, e.g., Kloster et al, 2012] given that over 75% of the land area is considered to already be impacted by human activities [Ellis and Ramankutty, 2008]. The large number of regional studies documenting increases in fires in the last two decades [e.g., Barlow and Peres, 2004;Cary, 2002;Gillett et al, 2004;Groisman et al, 2007;Kajii et al, 2002;Meyn et al, 2007;Le Page et al, 2007;Pausas et al, 2008;Soja et al, 2007;Stocks et al, 2003;Westerling et al, 2006;Williams et al, 2010] suggest that we may already have reached the point at which landscape fragmentation is not an effective means of fire suppression, and indeed these recent increases have been explicitly linked to global warming [Running, 2006;Soja et al, 2007] Indeed, projections of future fire activity by Pechony and Shindell [2010] show an increase in global fire activity with warming that is not offset by human influences on ignitions or land use.…”

United States Forest Disturbance Trends Observed Using Landsat Time Series

“…The changes in fire dynamics follow the widely recognized pattern of drier and warmer conditions, more frequent fires, and more intense insect outbreaks over the last two decades in the western US and Canada (Williams et al 2010;Westerling et al, 2006;.…”