Western Juniper Woodland Expansion: Natural or Unnatural?

by Michael M. Borman, OSU Extension Service
from the Western Juniper Newsletter, Vol. 1, No. 1, January 1996. 
 

Previous Expansions Compared to Current Status

Western juniper (Juniperus occidentalis) woodlands have expanded and contracted a number of times over the last 7000 years. Current juniper expansion (1850s to present) is quite different, though, than previous expansions. Is this expansion natural or abnormal? How this question is answered will affect landowner and land manager decisions about treatment options.

There is evidence of western juniper in northern California and eastern Oregon dating back 7000 years. Based on pollen records, it appears a major juniper expansion occurred during a wetter period between 4000 and 2000 years ago. Grasses also expanded, which helped create conditions for fire return intervals of probably 10-30 years. Frequent fire left a patchwork of scattered woodlands, rather than the extensive stands seen today.

Drier conditions from 2000 years ago through approximately 1850 resulted in contraction of both juniper and grasses. From about 1850 through the 1910s though, milder and wetter winter conditions contributed to another juniper expansion. This period also coincided with expansion of cultivation, prompted in part by abnormally good crop growing conditions, and extensive and continuous overgrazing by livestock. As a result, grass production did not increase along with juniper expansion, and fire was unable to reduce juniper dominance as it had during the last major expansion.

Trees established during the early 1900s are just now beginning to produce significant amounts of seed. This vast and growing seed bank will contribute to juniper expansion and increasing densities for the foreseeable future. Other factors affecting expansion and increasing densities include drier climatic conditions, longer fire-return intervals, decrease in proportion of grasses, and presence of newly-introduced noxious weeds.

Today there are about four million acres of western juniper woodlands (defined as having at least 10% canopy cover) in the Pacific Northwest. A large percentage of that area, probably 90% for example in Oregon, is covered with young, even-aged stands resulting from the latest expansion.

Effects of Western Juniper Expansion

Western juniper is adapted to a variety of sites. The majority of old growth (greater than 120 years old) stands and trees are located on rocky rimrock and shallow-soil, low sagebrush sites. Juniper does not appear to impact these sites much.

Increasing juniper densities, however, can have a significant impact on sites occupied by the latest expansion. These sites, with moderately deep to deep soils, can support mountain big sagebrush and other diverse vegetative communities. Dramatic declines in understory vegetation and diversity are observed when canopy co ver reaches 30-35%, especially when there is a hardpan 12 to 24 inches below the surface.

In the absence of fire or other disturbance, mountain big sagebrush/ grassland communities tend to move through a successional process: Grasslands (created by the last fire) to sagebrush-grasslands to tree-brush-grasslands to tree-dominated woodlands. This successional process appears to cross a series of thresholds, each one further limits management options.

• As juniper woodlands become more closed, fine fuels are reduced which limit the ability of fire to return the site to grassland dominance and begin succession again;
• Lack of soil protection eventually results in increased erosion, which reduces site productivity even more.
• Excessive soil loss eventually means that the site can no longer produce the kinds or quality of vegetation that it could previous to conversion to a juniper woodland.

If your goal is to maintain healthy ecosystems, it is inherently more economical and productive to treat juniper woodlands earlier in the succession process than later.

For more information, contact Michael M. Borman, Rangeland Resources Extension Specialist, Oregon State University, (541) 737-1614.