Ecological Impacts of Woodland Encroachment

Healthy sagebrush shrublands support numerous ecosystem services and values important to people and wildlife in the American West, including over 350 plant and animal species of conservation concern. Unfortunately, these habitats are being lost and degraded due to a wide variety of threats, such as invasive species, unnatural wildfire, development, and woodland encroachment. Woodland encroachment alters shrubland vegetation and processes, undermining ecosystem resilience to fire, drought, and other disturbances.

Forage

Healthy shrublands provide critical forage for livestock and wildlife. Perennial grasses and forbs make for productive grazing lands that support the livelihood of rural communities.

Sagebrush Habitat

Many animal species need sagebrush rangeland for habitat. Some wildlife, such as the greater sage-grouse are completely dependent on the shrubs and herbaceous plants in these ecosystems for food and cover.

High Diversity

A high diversity of native shrubs, grasses, and forbs grow in sagebrush ecosystems. Greater plant diversity can result in an ecosystem more resilient to pests and disturbances, including the invasion of non-native species.

Belowground Carbon

Healthy populations of shrubs, grasses, and forbs lead to expansive root systems. This belowground network sequesters carbon that is protected from loss by disturbances such as fire.

Tree Cover

Under encroachment of pinyon-juniper woodlands, trees begin to grow in sagebrush shrublands. Increasing tree cover alters habitat and food sources for animals which results in a decline of many wildlife populations.

Low Diversity

Trees outcompete shrubs, grasses, and forbs for water, sunlight, and nutrients. This competition leads to lower diversity of shrubs, grasses, and forbs and decreased resilience to disturbances.

Invasive grasses

A decrease in cover of native shrubs, grasses, and forbs from pinyon-juniper encroachment reduces resistance to invasion of non-native annual grasses. With fewer native species to compete with invasive species, there can be an abundance of non-native grasses, such as cheatgrass.

Soil Erosion

Greater tree cover reduces understory vegetation and leads to larger patches of bare ground. Increases in bare ground connectivity cause higher water runoff and soil erosion.

Aboveground Carbon

With a conversion from shrubland to woodland there is a greater ratio of carbon sequestered above ground than below. These large aboveground carbon pools are susceptible to loss by fire, releasing sequestered carbon into the atmosphere.

Learn More About Ecosystem Impacts

Carbon & Nutrient Cycling

Pinyon-juniper encroachment alters nutrient cycling and availability by relocating nutrients to accumulate around trees, depleting other areas, and through increasing aboveground nutrient pools.

Invasive Species

The combination of invasive grasses and encroaching pinyon-juniper forest is a threat for sagebrush ecosystems as both outcompete native vegetation and increase the potential of high-severity fire.

Wildlife

Pinyon-juniper encroachment affects wildlife by altering and fragmenting habitat. Woodland expansion reduces shrubs, forbs, and grasses that wildlife use for food and cover, and also decreases the amount of water available for wildlife in these arid landscapes.

Hydrology

The encroachment of pinyon-juniper woodlands into sagebrush ecosystems affects hydrologic processes by 1) reducing water availability and retention, and 2) increasing runoff and soil erosion.

Wildfire

Pinyon-juniper encroachment into sagebrush ecosystems causes changes in fuel load, structure, and continuity that lead to altered fire regimes where larger and more severe fires impact nutrient pools and plant community dynamics.