Drought and Western Forests

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USGS WERC's Dr. Phil van Mantgem and his collaborators are using plot-based methods to describe change and vulnerability to drought in the forests of the western United States. A focus of this work is the installation and maintenance of large (1 ha) monitoring plots. Many other vegetation monitoring strategies are based on small (0.1 ha) plots, which may not be sufficient to detect changes in forest tree populations and communities. Earlier work has established the climatic sensitivity of temperate forests, but many questions remain as to where and how forests in the western United States will respond to drought. 

Long-term monitoring of forest resources in the western U.S. is an essential part of understanding biological responses to drought. Using the current severe drought in the southwestern U.S. as a natural experiment, USGS scientists and partners are combining direct measurements of tree mortality with tree-ring indices of tree health—providing the first large-scale assessment of prescribed fire as a management tool to increase forest resistance to severe drought.

WERC dead tree in Whiskeytown National Recreational Area
(Credit: Phil van Mantgem, USGS WERC. Public domain.)

The goal of this research program is to understand and predict the effects of drought on forests of the western U.S. The greatest limitation to understanding and predicting the effects of future global changes is the lack of a precise mechanistic understanding of how contemporary forest structure and function are controlled by the interactions among the physical environment and biotic processes. Our research program therefore places landscape patterns within the context of the physical template (abiotic factors such as climate and soils) and biotic processes (demography, growth, and competition). Our program focuses on developing a mechanistic understanding of these interactions as it applies to forests of the Klamath region in particular, but also for the montane forests of western North America in general.

Forests in western North America are responding to changes such as: declining fraction of precipitation falling as snow, declining snowpack water content, earlier spring snowmelt and runoff, and the lengthening of summer drought. Resulting forest responses include: increasing background mortality rates, increasing frequency of die-backs of entire stands, and greater susceptibility to insects and pathogens.

Mature trees are large organisms, and consequently large plots are required to monitor forest tree populations and communities. The terrestrial vegetation protocols of many of our DOI client agencies (e.g., National Park Service, NPS) rely on small (0.1 ha) plots that are adequate to describe herbaceous vegetation, but will not contain a sufficient number of trees to accurately measure basic forest tree demographic rates (recruitment, death and growth) or forest community structure (e.g., stem density, basal area, species composition). Small plots are therefore considered inadequate for describing temporal changes in forests.

For these reasons we have installed and continue to maintain multiple large (1 ha) forest plots that compliment DOI client agency vegetation monitoring efforts. This work also builds upon earlier studies documenting changes occurring in forests across the western United States. These plot data allow us to document changes in important measures of forest structure and dynamics, and how those changes vary across different forest communities, soil types, and climates. These data serve as a backbone to further data collection when needed to address specific questions in particular locations. We also combine our data with other ongoing plot-based forest monitoring efforts, such as the NPS Fire Effects monitoring program.

WERC map of large plots for forest monitoring in Klamath region
16 large plots at several protected areas in the Klamath region including, Crater Lake NP, Cascade-Siskiyou NM, Lassen Volcanic NP, Oregon Caves, NM,  Lava Beds NM and Whiskeytown NRA and Redwood NP (Public domain.)

Taking the pulse of the forest: Klamath Network old-growth forest plots

Old-growth forests may appear timeless, but there are many recent examples showing how drought may already be impacting these forests.  How will old-growth forests and their associated biota respond to drought across the Klamath region?  A joint venture between NPS, BLM and USGS to measure changes in old-growth forest is attempting to answer this question. 

Our goal is to understand and predict the effects of drought on forests of the Klamath region. The greatest limitation to understanding and predicting the effects of drought to these globally unique and diverse forests is the lack of a precise mechanistic understanding of how contemporary forest structure and function are controlled by the physical environment and biotic processes.  This project seeks to build on the current Klamath Inventory and Monitoring Network (KLMN) terrestrial vegetation monitoring program by installing large (1 ha) forest plots to gather detailed information that will place forest patterns and dynamics within the context of the physical template (abiotic factors such as climate and soils) and biotic processes (demography, growth, and carbon storage).

Increasing Forest Resistance to Drought using Prescribed Fire - Fighting Drought with Fire

Prescribed fire is a primary tool used by resource managers to restore southwestern U.S. forests, following more than a century of fire exclusion. Prescribed fire reduces fire risk partly by removing small trees, shrubs, and surface litter. It is also assumed that following fires, there is less competition for water, nutrients and light, so that these remaining trees are more resistant—more likely to survive—in the face of additional stressors, such as drought. Yet this idea remains untested.

In the face of ongoing climatic changes, is a dollar best spent on increasing forest resistance using prescribed fire, or is it best spent on other climate change adaptation needs?  The severe and on-going drought across much of the southwestern U.S. provides a remarkable natural experiment to test whether prescribed fire helps trees survive stressful conditions.

If current practices for modifying forest conditions through fire are found to increase drought resistance, this project could help forest managers apply these methods broadly across western U.S. forests. The results of this study may show that current practices for forest restoration may also be a useful global change adaptation strategy.

We also propose to measure the effects of prescribed fire only. Forest managers frequently treat long-unburned stands with mechanical thinning prior to the application of prescribed fire, and while it would be of great interest to include surveys of thinning and burning treatments in our proposed work, we find it unrealistic to thoroughly measure these treatments across the Southwest during this initial study. However, this research will pave the way for future research to consider the combined effects of thinning and burning treatments.

Our specific questions are then as follows:

  • Does prescribed fire increase forest resistance to drought, and if so, by how much?
  • If so, does the strength of that resistance depend on time since fire? For example, is drought resistance initially low following a burn, but increasing in subsequent years and decades?
  • Do the benefits of prescribed fire, with regard to resistance, vary among tree species, size classes and with residual stand density? In particular, is the resistance of large trees—which sequester the most carbon and are critical to wildlife—enhanced by prescribed burns?