Key Findings

• The rate, pattern, and impact of woody plant expansion vary significantly across different regions in the western U.S. Understanding these regional differences is essential for developing effective, localized management strategies.
• Woody plant expansion generally leads to a reduction in forage productivity, which directly affects the economic viability of range-based beef production. These ecological changes undermine the sustainability of rangeland ecosystems and lead to substantial economic losses for livestock producers.
  
• Preventing the initial establishment of woody plants is crucial, as is the need for sustained management efforts to control and reverse encroachment. However, management must be continuous and adaptive to respond to ongoing changes in vegetation dynamics and environmental conditions to support sustainable beef production.

background

This project addresses the environmental pillar of sustainability by attempting to understand the potential impacts of conifer expansion to range-based beef production industry such as loss of forage productivity, habitat and diversity values, and watershed function and soil stability. The expansion and infilling of woody plant species is one of the major contemporary issues facing rangelands across the western United States, and understanding the impact to natural resources is a key focus of sustainability research. Conifer expansion jeopardizes the long-term sustainability of the western range beef cattle industry as well as the landscapes and the social and economic infrastructure that are dependent upon it. Tree encroachment into grass and shrub-dominated rangelands threatens the integrity of these agroecosystems and limits the resiliency of the rural communities that rely on these diminishing resources as an economic engine primarily associated with range beef cattle production. Given the lack of consistency across bioclimatic regions in the ecology of conifer expansion, there is a need to synthesize existing regionally specific knowledge to provide best available science to land managers and to identify knowledge gaps where research is needed to provide regionally relevant land and resource management direction. The objective of this project was to review and synthesize relevant peer-reviewed literature for research-based information related to woody plant expansion to compare and contrast mechanisms of invasion, rates of change, ecological impacts of conifer increase, and management opportunities and constraints within four bioclimatic regions in the western U.S. 

methodology

The research team reviewed and synthesized relevant peer-reviewed literature for research-based information related to woody plant expansion to compare and contract mechanisms of invasion, rates of change, ecological impacts of conifer increases, and management opportunities and constraints within four bioclimatic regions in the western U.S., where sustainable range beef production is a significant contributor to agriculture industry. The regional evaluation included the Southwestern U.S., the southern Great Plains, the northern Great Plains, and the Northwest.

results and discussion

Over the last 30 years, increases in woody plant cover have been significant across much of the western and midwestern U.S. The increase in tree cover is associated with several expansion patterns: woody plants encroaching into previously intact grasslands and shrublands, forest boundaries widening into adjacent areas, and woody plants filling areas already forested. Losses of large areas of what was historically rangeland serves to heighten the concern related to the impacts of woody plant expansion to sustainable rangeland management and use. Many of the ecosystem impacts associated with conifer expansion can lead to a positive feedback loop where the presence of woody species further degrades understory herbaceous and shrub species, and this loss in understory species fuel more invasion of the woody species. From 1990 to 2019, tree cover in the west has increased by 50% and more than 25% of rangelands have been affected by woody plant encroachment. Forage production losses in that time totaled 302 +/- 30 teragrams (3.33 x 10⁸ metric tons) of dry biomass, which equates to $4.1 – 5.6 billion of lost revenue for livestock producers. Region-specific findings are listed below.   

Southwestern U.S.

The expansion of creosote bush and mesquite, primarily through seed dispersal and vegetative reproduction, has significantly impacted range-based beef production. These species thrive on erodible soils, a situation worsened by historic heavy grazing. The establishment of woody plants increases soil erosion and alters nutrient distribution, which favors further woody expansion over herbaceous species. This leads to a decline in grasslands, reducing forage availability and diminishing livestock production. Additionally, decreased fire frequency promotes woody dominance and creates microclimatic conditions unfavorable to herbaceous species. Management strategies include using fire to control seedlings, though it is less effective against mature plants. Mechanical treatments such as root plowing, chaining, and grubbing can be effective, but are disruptive and require follow-up seeding. Herbicides are also useful but may increase bare ground and erosion if not managed properly.   

Southern Great Plains

The rapid encroachment of eastern redcedar is transforming grasslands into woodlands, significantly altering ecosystem dynamics. This encroachment often begins near propagule sources, such as shelterbelts and riparian zones, and spreads outward. The loss of forage biomass is substantial, estimated at 22.4 million tons annually, resulting in significant economic losses for producers. Increased soil erosion, reduced grassland biodiversity, and degraded watershed function are also common. Dense canopies of eastern redcedar outcompete understory plants, reducing overall productivity. Management strategies must vary with the stage of encroachment, from fire and mechanical treatments in early stages to intensive mechanical removal in advanced stages. Maintaining core intact grasslands and expanding treatments outward is recommended. Herbicides are effective for smaller trees, but larger trees require mechanical removal.   

Northern Great Plains 

The rapid expansion of woody species is driven by reduced fire frequency and intensity, with high productivity sites being more susceptible to faster rates of woody plant dominance. This expansion leads to a significant reduction in herbaceous biomass and increased soil erosion, altering hydrologic functions and increasing competition for water resources by deeper-rooted woody plants. Positive feedback loops, where woody species further degrade the understory, exacerbate their own expansion. Early-stage management includes prescribed burns and herbicides for smaller trees, while mechanical treatments become necessary in later stages, coupled with reseeding and soil stabilization measures post-treatment. Strategies must account for local environmental conditions and site-specific challenges.  

Northwest

Pinyon-juniper expansion occurs primarily through the infilling of existing woodlands rather than the expansion into new areas. Historical fire suppression has allowed these species to increase in density. This leads to a decline in herbaceous species richness and cover, increased soil erosion and runoff, and elevated fire risks due to increased fuel loads from dense tree cover. The reduction in forage availability negatively impacts livestock production potential. Management strategies involve using prescribed fire in early stages of encroachment to improve forage production, while mechanical thinning and removal are necessary in more advanced stages. These measures are often coupled with reseeding to prevent exotic species invasion. Long-term strategies should focus on maintaining open woodlands and preventing further infilling to sustain range-based beef production.

industry Implications

Although woody plant expansion is a global phenomenon with wide ranging ecological and economic impacts, regional differences exist in the kinds of woody plants, rates, patterns, and mechanisms of expansion, and associated management opportunities. Woody plant expansion is having a significant impact on rangeland ecosystems across the western U.S., negatively impacting the sustainability of the range-based beef cattle industry. The industry is reliant on high-quality forage availability, which is declining precipitously in response to increasing woody plants. The effect of woody plant expansion and the opportunities for management vary across regions (i.e. Great Plains vs. Great Basin) and over time, and requires a refined, site-specific understanding of woody plant expansion to develop appropriate management strategies to ensure long-term sustainability of range-based beef production.