The Ecology of Woodland Encroachment 

In parts of the sagebrush biome, pinyon-juniper and other conifer woodlands have increased by six-fold over the last two hundred years. Roughly 90% of woodland encroachment has occurred in adjacent sagebrush shrublands. Sometimes referred to as “the green glacier,” woodlands are slowly encroaching upon previously treeless shrublands changing the structure, function, and resilience of these ecosystems.

Why is woodland encroachment happening?

Pinyon-juniper woodlands have always been part of the fabric of western landscapes. However, human land use, reduced fire occurrence, and climatic conditions have contributed to widespread changes. Historically, woodlands were restricted primarily to areas where grassy fuels were limited. Periodic fire, and occasionally severe drought, killed trees and maintained a balance of grasslands, shrublands, and woodlands. Altered fire regimes have allowed more trees to establish, grow, and spread by seeds that are dispersed by birds and small mammals. Encroachment describes the process of tree expanding into previously treeless shrublands, and infill describes the process of woodlands getting thicker.

Sagebrush threats illustration

Credits: © The Nature Conservancy | Illustrator: Kelly Finan | Adapted From: Western Association of Fish and Wildlife Agencies Sagebrush Conservation Strategy— Challenges to Sagebrush Conservation

When did woodland encroachment happen?

Unprecedented increases in the rate of woodland expansion and infill have been documented between the mid-1800’s and mid-1900’s according to tree ring data, packrat middens, and pollen records. Historical photos provide further evidence of woodland change. More recently, the satellite-derived data shows woodlands continue to expand and get thicker with shrublands transitioning to woodlands at a rate of about 0.5% per year.

Mojave desert in the sun
1953
Contemporary
Current
Tree cover increase map

Percent tree cover increase on rangelands across the sagebrush biome in the western U.S. (1990-2019). Data: Rangeland Analysis Platform, Cartography: Eric Jensen, USDA-NRCS Working Lands for Wildlife

Where is woodland encroachment happening?

Encroachment of woodlands into grasslands and shrublands is a global phenomenon. In the sagebrush biome of the western U.S., pinyon pine, juniper, and other conifer trees are expanding primarily along the ecotone between shrublands and woodlands or forests. Previously thought of as a localized issue, satellite technology has revealed the widespread scope of this issue.

Learn about the ecological impacts of woodland encroachment