Dryland Forest Sustainability

Science Center Objects

Forests in the semiarid southwestern U.S. are expected to be highly vulnerable to increasing aridity anticipated with climate change.  In particular, low elevation forests and the processes of tree regeneration and mortality are likely to be highly susceptible to climate change.  This work seeks to characterize how, where and when forest ecosystems will change and identify management strategies to promote resilience.

Background & Importance

Climate change, as well as elevated climatic extremes, represents a major challenge facing forest managers, policy makers and forest scientists today. In particular, severe drought events cause widespread tree mortality and decreased growth in forest habitats across the globe, including areas with cool and mesic climates where drought impacts are not widely recognized.  As the reality of climate change and increasing drought frequency and severity become clear, forest managers seek strategies to increase adaptation capacity, specifically by promoting forest resistance (minimizing negative impacts during the drought) and resilience (maximizing recovery rates following drought).   Ecological theory suggests that forests with greater complexity and biodiversity should have greater capacity to resist change and or recover ecosystem function in the face of drought.  However, the applicability of this theory to practical habitat conservation and forest management are unclear. 

Dead trees killed by wildfire 20 years ago lying on the ground. The area is now dominated by grasses

A wildfire burned this area in northern Arizona more than 20 years before the picture was taken, but regeneration is extremely limited.  Our research is working to understand this episodic regeneration and forecast the consequences of climate change for dryland forests. (Credit: John Bradford, USGS. Public domain.)

General Methods

Forest managers and policy makers need actionable results about management practices that enhance forest adaptation to increasing aridity.  Approaches to filling these interacting knowledge gaps are largely undeveloped, especially approaches for identifying and developing management strategies.  SBSC researchers, in cooperation with researchers at universities and the US Forest Service, are working to characterize how climate change will impact forest regeneration, growth and mortality over broad climatic gradients, and to quantify the efficacy of forest management strategies designed to promote resistance and resilience to drought.  This work includes several projects supported by funding from the U.S. Forest Service, the Western Wildland Environmental Threat Center and the National Science Foundation. 

Important Results

Tree regeneration is highly sensitive to soil moisture conditions and may already be reflecting tree migration

Our regional-scale study indicated that regeneration of many tree species is already shifting upslope and to wetting conditions, setting the stage for future tree distribution changes.

However, many species have nowhere to go

Mountainous terrain has very limited migration options for higher-elevation species.  Our work quantified that subalpine fir and Engelmann spruce, in particular, have very limited opportunity for migration to maintain climatic suitability, while ponderosa pine has more potentially suitable future locations.

Forest management can promote drought resistance and minimize future tree mortality

Our research indicated that reducing forest density, an easily manipulated forest structural condition may allow remaining individual trees to more effectively survive harsh drought conditions. Results indicate that thinning can decrease drought-induced growth declines and tree mortality, providing an opportunity for forest managers to guide transitions to future forests.

Future Directions

Identifying forest management strategies that maximize climate resilience

Long dead ponderosa pine lying on the ground in area now dominated by annual grasses

Ponderosa pine stand in northern Arizona destroyed by fire about 20 years past. There has been very little regeneration by ponderosa pine since the fire. (Credit: John Bradford, USGS. Public domain.)

Building upon several long-term forest management experiments, this project is identifying characteristics that can help trees sustain growth and avoid mortality despite changing climate and increasing climatic variability.

Describing west-wide patterns and controls over tree mortality

This project is teasing apart the individual and combined effects of climate, weather, disease and competition on aspen mortality. 

Lower forest ecotones: the key to forest sustainability  

This project will assess how lower forest boundaries in dryland ecosystems respond to changing climate and disturbance and assess forest vulnerability and sustainability.  Results will provide valuable information for land managers about the future distribution and abundance of forests in the southwest U.S.

Quantifying future changes in ecosystem carbon stocks 

This work will quantify how shifts in the distribution of forests in the western U.S. will impact carbon cycling and storage, potentially providing a feedback to increasing atmospheric CO2.

Incorporating drought extremes into tree niche modeling

This work will enhance niche models by integrating the influence of extremes in ecological drought – both unusually wet conditions that facilitate episodic regeneration and unusually dry conditions that cause tree mortality.