Investigating Shrub-Grass Interactions in Big Sagebrush Ecosystems

The following article by Rachel R. Renne, Doctoral Candidate at the Yale School of the Environment, is based on research that was partially supported by funds from the NPSO’s field research grants program. Special thanks to Rachel for taking the extra effort to provide this perspective that illustrates the kinds of research that our society supports via your membership and donations. Her work looks at how the relationships between soil texture and precipitation influence the dominance of grasses versus shrubs in big sagebrush ecosystems.– Dan Luoma

In May 2021, I walked 750 miles across the cracked and rumpled basaltic plains of central and southeastern Oregon on the Oregon Desert Trail. Like the subtly shifting tapestry of plants that textured the landscapes I passed through, my days were defined by finding, storing, and utilizing water. Although animals like me can travel relatively large distances to seek out water sources, plants must make do with what is available where they grow. Each species extracts water from the soil at different depths, depending on how deep its roots are and how easily it can extract water from soils with different water contents. Together, the rooting depth and water-extraction ability of each species defines its ecohydrological niche and determines where different species are able to grow.

In general, shrubs—including big sagebrush (Artemisia tridentata Nutt.), my patient companion through eastern Oregon—have long, woody roots that can reach water deep in the soil, below the roots of herbaceous species like grasses. Consequently, the depth distribution of water in the soil (soil water) can influence the relative importance of shrubs and grasses in water-limited (dryland) ecosystems. Precipitation amount and season (winter or summer), along with soil texture, are the main determinants of the depth distribution of soil water and predict vegetation type (shrubland versus grassland) relatively well (Figure 1).

This relationship is broadly observable across the drylands of the western United States. We find grassland vegetation in the Great Plains, where most precipitation arrives in the summer, and shrubland vegetation in the Intermountain West (including eastern Oregon), where a substantial amount of precipitation arrives during the cold season.

Yet, precipitation patterns are highly variable across the big sagebrush region, and most of these ecosystems support a mix of shrubs and grasses. In fall of 2021, I entered the doctoral program at the Yale School of the Environment to further explore the relationships between the relative importance of shrubs and grasses and patterns of soil water. Although shrubs and grasses are thought to coexist by using water in different parts of the soil to avoid competition, their roots overlap in shallow soils, and it is likely that they do interact in some way. Previous research suggests that the nature of interactions between plants (positive, neutral, or negative) may depend on the overall availability of resources, including soil water. My goal was to explore how soil water amount and depth distribution influence the relative importance and interactions between shrubs and grasses in sagebrush ecosystems.

To answer these questions, I identified fifty-one relatively undisturbed sites in potential big sagebrush habitat across eight states (Figure 2), including six sites in Oregon.

In 2022, supported by a NPSO Field Research Grant, I visited these sites and measured the cover of shrubs and grasses to determine which growth form was dominant (Figure 2). I also recorded the size, height, and number of perennial grasses and other herbaceous plants growing under shrubs and in open spaces between shrubs (interspaces). I used an ecosystem water balance model to estimate average patterns of soil water using historic weather data (1991-2020). Most of my previous experience in sagebrush ecosystems has been in Wyoming and eastern Idaho, and I was impressed by the subtle and dramatic differences I found in the sagebrush plant communities in Oregon. Sagebrush tends to be much larger in Oregon than Wyoming, and I finally got to meet Thurber’s needlegrass (Achnatherum thurberianum (Piper) Barkworth) in the field- this species is widespread in Oregon, but uncommon in Wyoming.

My preliminary results suggest that shrub-grass dominance and interactions depend more on the depth distribution of soil water than total precipitation. The sites in Oregon all favored shrubs over grasses (Figure 2), and interactions ranged from strongly positive (shrubs facilitated grasses) to moderately negative (shrubs were competitive with grasses; Figure 3).

Figure 3: Measuring shrub size and cover at two field sites in Oregon with contrasting shrub-grass interactions. The site on the left is located between the Warner Highway and the Nevada border, and big sagebrush is moderately competitive with perennial grasses. The site on the right is located between the Ion Highway and the Owyhee River, and big sagebrush is strongly facilitative to perennial grasses. Photo by Rachel R. Renne.

Across all fifty-one sites, I found that shrub-grass interactions tended to be more positive when soil water conditions favored shrubs (more water in deeper soils) and more negative when soil water conditions favored grasses (more water in shallower soils). I also found that shrub-grass interactions tended to be more neutral or negative at wetter sites compared to drier sites, regardless of the depth distribution of soil water. These results suggests that the relative importance of shallow and deep soil water may control the ability of shrubs and grasses to effectively partition soil water resources.

Support from the NPSO in 2022 allowed me to visit more sites in Oregon than I had initially proposed—including the wettest site in my study, located on the Winter Rim above Summer Lake. I was generously granted funds above the amount I requested, which was especially helpful as fuel prices were unexpectedly high that summer. I presented this project at the Ecological Society of America meeting in Portland in August 2023. This research will contribute to the first chapter of my dissertation and eventually to a publication in a peer-reviewed journal. I look forward to my next foray into Oregon’s drylands! – Rachel R. Renne ra**********@ya**.edu