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This article is part of the Spring 2022 issue of the Earth Science Matters Newsletter.

In the intermountain western U.S., expansion of Pinyon (Pinus edulis) and Juniper (Juniperus spp.) woodlands into grasslands and shrublands is a pervasive phenomenon, and an example of the global trend towards enhanced woody growth in drylands. Mechanical and chemical Pinyon and Juniper reduction treatments have been a long-standing practice in the region due to concerns that these changes may be impacting ecosystems, including biodiversity, hydrology, soil stability, wildfire, and livestock forage. However, conducting robust analysis on the effectiveness of these treatments has always been a challenge. It is practically impossible to do randomized, controlled experiments at the scale of these efforts to really test their effectiveness, and extreme climate variations such as drought in the western U.S. confounds simple before-after comparisons. 

This need to uncover robust and detailed understanding of causal factors from large natural experiments is not unique to land and species management. Indeed, the 2021 Nobel Prize in Economics was awarded to David Card (UC Berkley), Joshua Angrist (MIT), and Guido Imbens (Stanford) for their work in developing new techniques to draw more definitive conclusions from ‘natural experiments’. To better understand the effectiveness of land treatments in the western U.S., scientists from the USGS-Southwest Biological Science Center (SBSC) adopted the same techniques pioneered by the Noble Prize winners to analyze the effectiveness of brush management treatments in the upper Colorado River Basin. The study included hundreds of locations in the four corners region that had been treated for perceived tree encroachment into grasslands and shrublands over decades. 

tree removal in Arizona
Tree removal at a USFWS funded habitat improvement project in Arizona. Photo courtesy of Steve Cassady, Arizona Game and Fish Department.

The SBSC scientists were able to evaluate land treatment outcomes using Landsat remote sensing data and comparison of those observed outcomes to a modelled ‘control’ scenario that served as a baseline. From 1,569 distinct treatment areas, the scientists found evidence that treatments successfully reduced tree cover and largely increased shrub and perennial herbaceous cover for 10 or more years. The analysis also found that site-specific soil, vegetation, and climate information was critical to accurately predicting how effective a management practice was, with some settings consistently returning to pre-treatment conditions (primarily return of trees) within 10-15 years and others exhibiting more persistent changes in plant communities. Additionally, variation in management practices were also important for determining outcomes, particularly combining tree removal with seeding, which increased herbaceous and decreased tree cover. An important implication of this study is that outcomes of land treatments are highly context specific and necessitate site-specific, actionable information or decision support tools. 

This work demonstrates a new approach to a problem that has long challenged resource agencies that fund and implement land treatments; how to evaluate the relative effectiveness of treatments without relying too heavily on scant monitoring data or simple before-and-after assessments. The approach here used Landsat remote sensing data to measure cover of trees, shrubs, and herbaceous species and while these data are tremendous, the authors caution that many important indicators for achieving habitat or soil conservation objectives are not detectable from Landsat satellites alone and it is important to continue expanding current on-the-ground treatment outcome monitoring efforts. However, the approach demonstrated in this study can be considered a quantitative and scalable analytical approach for assessing land treatments. Understanding plant, soil, and habitat outcomes from past land treatments is critical to improving the effectiveness of future management activities, as well as increasing accountability.   

The work was supported by the USGS Ecosystem Mission area and a grant from the USDA-Natural Resources Conservation Service, Soil and Plant Sciences Division. The paper, “What determines the effectiveness of Pinyon-Juniper clearing treatments? Evidence from the remote sensing archive and counter-factual scenarios” was recently published in the journal Forest Ecology and Management.

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