- I studied the relationship of environmental variables
to six plant associations on McDonald and Paul M. Dunn
Forests. Indirect gradient analysis was used to determine
relationships. The plant associations were developed in a
companion study by C.J. Hubbard (1991).
McDonald-Dunn Forests are located north and northwest
of Corvallis, Oregon. The Forests comprise approximately
11,000 acres. These lands are owned by Oregon State
University and managed by a staff within the College of
Forestry. This study, and the companion study by Hubbard
(1991), was funded and supported by the Forest management
team in order to provide information that would facilitate
a more integrated-resource management process.
Data for this study was collected on one hundred and
fifteen plots stratified to represent the diversity of
physiography, soils series, and timber types found within
the Forest. Plots were located within as mature a stand
condition as could be found, considering past disturbance.
Data collected on each 5,382 square foot plot (500 square
meters) included physiography (e.g., elevation, slope, and
aspect) ; vegetation cover (e.g., grass, forbs, shrubs, and
trees); and soils (e.g., soils series, texture, and
depth). Other variables used in this study were taken
from the Forest's 1985 timber inventory. Inventory variables
included total live basal area for conifers, periodic
annual increment, and height and diameter for conifers.
Environmental conditions vary within McDonald and
Dunn Forests. Elevations increase generally toward the
northwestern portion of McDonald Forest, especially toward
McCulloch peak. Elevations decrease toward the southern
portion of McDonald Forest. The Dunn Forest has lower
elevations than McDonald Forest. Elevations decrease to
the north within the Dunn Forest from Forest Peak to the
lower Soap Creek drainage. Slope, rock percent, baregroud
percent, and gravel percent also increase as elevations
There are relationships between environmental variables
and plant associations within McDonald-Dunn Forest. The structure and composition of the vegetation occur in
response to environmental conditions.
Elevation, direction and angle of slope, quality of
soil, and especially direct solar radiation share in explaining
the variability of vegetative structive and
composition within the McDonald-Dunn Forests. The underlying
environmental gradient for McDonald-Dunn Forests is
a light-influenced gradient. Elevation, slope, aspect,
and amount of solar radiation have all interacted for eons
to influence the soil development and vegetation gradient
as it exists today.