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How do tree density reduction treatments influence soil moisture conditions?
Background
Pinyon-juniper (PJ) woodlands are widespread across the western U.S. and and provide many valuable ecosystem services. Historically, management by federal and state agencies was largely focused on tree removal for conversion to other vegetation types or other resource values, rather than promoting tree health. In recent decades interest in PJ management has increased substantially, primarily driven by three objectives. First, wildfire mitigation objectives are motivated by increasing wildfire risk and activity, which is expected to continue as temperatures rise. Second, objectives focused on sustaining habitat quality are motivated by observations of declining wildlife populations in species associated with PJ woodlands and adjacent systems. Third, and less well recognized, is the potential objective of enhancing drought resilience of pinyon and juniper trees via silvicultural thinning, which has been demonstrated in other forest systems but this has not been well-studied in PJ ecosystems.
Objectives & Approach
In this literature review, we assess the state of knowledge about soil moisture response to treatment in PJ woodlands. To conduct our synthesis, we assembled information from publications over the past 60 years that examined experiments of soil moisture response to treatments in PJ woodlands. We identified common questions that those publications addressed, and synthesized those findings to assess the state of knowledge about how treatments influence soil moisture, including across soil depth, among seasons, treatment type, time since treatment, and climate setting. We tallied the support for positive, negative, and neutral responses under each question, and asked how soil moisture response to treatment varies across PJ geography, and if moisture responses are driven by broad climate patterns that are known to shape PJ ecosystems, namely, aridity and monsoonality (proportion of precipitation falling in summer months).
Outcomes
Our literature search identified 28 publications from 15 experiments from the past 60 years of research. These experiments represent a large span of geography, from the far northwestern to southeastern corners of the broad overall PJ distribution, yet there are still several large areas and climate conditions that support PJ woodlands without representative experiments (e.g., central and southern California, southern Arizona, New Mexico and Nevada, eastern Utah).
From these experiments we tally support for several aspects of soil moisture responses to treatment, such as: which season the moisture response was observed in, what soil depth, if moisture response changed over time, with treatment type and precipitation relative to normal. We explore these results further in the manuscript, but in brief, moisture response to treatment was highly variable across many aspects of the experiments. Moisture gains from treatment were more consistently observed at deeper soil depths, while moisture responses were more variable in summer months than in spring months.
How can these findings inform management decisions?
How can these findings inform management decisions?
Management in PJ woodlands frequently entails tree density reduction treatments to mitigate fire risk and restore habitat, yet treatments may also have the potential to enhance drought and climate resilience for remaining trees and understory vegetation by increasing soil moisture. Our findings suggest that treatments do not reliably increase soil moisture in PJ woodlands, and that soil moisture response to treatment varied with soil depth, season, treatment type, and time since treatment, which may further interact with site environmental conditions such as monsoonality and aridity. While there were many experiments that measured soil moisture response to treatments, a majority of these used full tree removal treatments to study non-PJ management objectives and thus our results may be biased towards drastic site moisture responses that may not represent conditions resulting from thinning treatments. Thus, nonuniform moisture responses may be unsurprising as tree density reductions simultaneously impact conditions that increase loss of soil moisture, (e.g., evaporation), and conditions that increase retention soil moisture, and how the net outcome of impacts on these competing processes falls out may differ in climatically distinct subsets of PJ. We summarize some important processes that drive moisture response to treatment and conditions that may modify moisture responses in this concept figure. This work will be published soon.
Collaborators
Ian Barrett - Wildland Fire Service
Chris Domschke - BLM Oregon / Washington State Office
Madelon F. Case - USGS Forest and Rangeland Ecosystem Science Center
Alexandra K. Urza - Whitebark Institute
Miranda D. Redmond - Department of Environmental Science, University of California-Berkeley
Ellis Q. Margolis - USGS Fort Collins Science Center New Mexico Landscapes Field Station
Edward M. Hill - Department of Forest and Rangeland Stewardship, Colorado State University
Support
USGS Climate Adaptation Science Centers
Bureau of Land Management
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