- In the Northwest Great Basin, aspen (Populus tremuloides) communities
uniquely contribute to the biodiversity of a semi-arid, sagebrush-dominated
landscape. In this same region, western juniper (Juniperus occidentalis) is
encroaching into aspen stands. This study determined the timing, extent, and some
of the effects of this expansion.
Aspen stands below 2,133 m elevation were sampled in northwest Nevada,
northeast California, and southeast Oregon for density, canopy cover, age, stand
structure, and recruitment of western juniper and aspen. Soils and tree litter from
both species were collected to analyze the effects of western juniper in areas
previously influenced by aspen. Additionally, two large aspen complexes in
southeast Oregon were intensively aged to determine disturbance (fire) frequencies.
Western juniper encroachment into aspen stands peaked from 1920 to 1939
with 77% of all juniper trees sampled establishing during this period. Five percent
were greater than 100 years and none exceeded 145 years. Three-fourths of aspen
stands sampled have established populations of western juniper. Twenty-three
percent have a dominant canopy of western juniper. Twelve percent of aspen stands
sampled were completely replaced by western juniper. Average density of western
juniper was 1,573 trees per hectare of aspen. Seventy percent of aspen stands
sampled had zero recruitment of new aspen. Within the study area aspen stands
averaged 98 years old. Forty-eight percent of stands were greater than 100 years old.
There was an inverse correlation between aspen canopy cover and western juniper
canopy cover (r²=.80, p=.0001).
Soils influenced by western juniper had a higher C:N ratio and pH; higher
amounts of salts, lime, and sulfate; and lower amounts of magnesium, iron, copper,
and manganese (p<.05). Aspen litter had a lower C:N ratio than western juniper
Prior to 1870, the two major aspen complexes sampled had mean fire return
intervals of 10 and 11 years. However, the most recent disturbance in either
complex was 80 to 90 years ago. This lack of disturbance (fire) coupled with aspen
stand decadence and low recruitment levels leaves aspen communities in the
Northwest Great Basin vulnerable to western juniper encroachment and replacement.