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Headed uphill? Aridity limits lower treelines in much of the western U.S.

Date: August 25, 2020

Lower elevation forests in much of the western U.S. are limited by aridity and likely to retreat uphill in response to climate change.


Pinyon-juniper lower treeline in the Toiyabe Mountains, NV.
Pinyon-juniper lower treeline in the Toiyabe Mountains, NV. Photo by Alexandra Urza.
Many forests in dry mountain regions are characterized by a lower elevational treeline, below which trees are unable to grow. This lower treeline is often the edge beyond which conditions are too dry for trees, and as such is likely to shift in response to global climate changes that result in increased aridity.

Extensive drought-induced tree mortality has been observed at dry forest range edges in the Intermountain West, facilitating the invasion of annual grasses and altering fire regimes. Lower treelines that are currently at their climate limit are expected to be more sensitive to changing climate. However, physical barriers, land use, and fire may also constrain the distribution of trees, preventing lower treelines from reaching what would otherwise be their climate-driven potential. Lower treelines constrained by non-climatic factors are less likely to respond directly to climate change but may be sensitive to other processes, such as changing disturbance regimes or land use practices. Understanding the controls on the position of lower treeline can help managers anticipate forest shifts in response to climate change and prioritize vegetation treatments.

In this study, we used existing vegetation classifications to map lower treelines for a 1.7 million km2 region in the U.S. Intermountain West. We randomly generated 7,000 sampling points within a 1-km buffer distance of the lower treeline boundary for each of three major forest types. We modeled the topoclimatic drivers of lower treeline position for each forest type to identify topoclimatically limited treelines. We then used spatial data of soil properties, recent fire, and land use to identify lower treelines potentially constrained above their ecophysiological limits by non-climatic processes.

Key Findings

Illustration of lower treeline delineation and distribution of random sampling points.
Illustration of lower treeline delineation and distribution of random sampling points.

  • Three forest types accounted for 65.2% of lower treeline in the Intermountain West: Colorado Plateau pinyon–juniper woodlands, Great Basin pinyon–juniper woodlands, and Northern Rocky Mountain dry mixed-conifer forests.
  • In pinyon–juniper woodlands of the Colorado Plateau and the Great Basin, 46% of the lower treeline was found to be topoclimatically-limited, occurring in locations that were climatically unsuitable (too dry) for trees. This suggests that increasing temperature and associated droughts may result in widespread treeline shifts to higher elevations or more mesic locations.
  • In dry mixed-conifer lower treelines in the northern portion of the study area, only 10% of lower treelines were found to be limited by climate. These forests were frequently located adjacent to perennial and seasonal wetlands, recently burned areas, and human land uses, indicating that lower treelines are largely constrained above their climate potential and will therefore be less responsive to climate change.
  • Maps of predicted topoclimatic limits to lower treeline (Fig. 3) can be used by managers at broad scales to identify portions of the landscape at risk of climate-driven woodland contraction and to prioritize vegetation treatments.

 

 Predicted topoclimatic limits on lower treeline for three forest types of the Intermountain West. Lower treelines are topoclimatically limited where the probability of forest cover is <0.5 (red lines). Gray lines are lower treelines of other forest types
Predicted topoclimatic limits on lower treeline for three forest types of the Intermountain West. Lower treelines are topoclimatically limited where the probability of forest cover is <0.5 (red lines). Gray lines are lower treelines of other forest types

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Principal Investigators: 
Principal Investigators - External: 
Peter Weisberg - Department of Natural Resources and Enviromental Science-University of Nevada Reno
External Partners: 
Tom Dilts, University of Nevada-Reno, Department of Natural Resources and Environmental Science
Research Location: 
Intermountain West, USA