Western Juniper (Juniperus occidentalis), a native semi-arid
forest tree, is moving off its traditional “natural” old growth sites and
displacing adjacent sagebrush grassland, mountain mahogany, and even aspen
stands in the western United States. Although old growth stands are often over
200-400 years old (the oldest juniper tree found in Central Oregon is believed to
be over 1600 years old!), this new reproduction has largely occurred in several
waves since 1860, probably as a result of fire suppression and heavy grazing
that reduced fine fuel loads present to carry fire interacting with weather
cycles that favor juniper establishment.
Juniper trees initially grow slowly for a decade or so and may be fairly
small and unobtrusive when young . They are easily killed by fire at this
stage. Once well established, they are fierce competitors for soil water and
nutrients and often begin to noticeably impacts biotic community
structure and function when 35-45 years old. Juniper captured sites desertify in
a very predictable sequence. Over a
period of 30-150 years, forb canopy
cover is reduced, followed by shrub canopy, and on drier sites, grass canopy
may also be severely reduced or eliminated, leaving a landscape of juniper
trees standing over bare unprotected soil. When it rains, water runs off
quickly rather than entering the soil, often carrying surface soil with it,
eroding the site and reducing stored water available for plant growth. The earliest waves of invasion are now
reaching this obviously degraded state. However, several million acres of invading
juniper are still in early to intermediate stages of degradation and may be
spared this fate if surviving understory vegetation is relieved from current or
potential juniper competition. Reestablishment of fire through controlled
burning and mechanical removal of larger trees are popular approaches, but all
actions need to be tailored to the particular characteristics of the specific
site.