| Summary: | Forest stands were studied to determine if old-growth
forest structure could be mimicked in younger stands via
overstory manipulation. Cover and species composition of
understory plants were systematically sampled in sixteen
thinned second-growth stands and sixteen adjacent unthinned
second-growth Douglas fir (Pseudotsuga menziesii (Mirabel)
Franco.) stands. The stands were thinned twenty-four to
thirteen years ago. These were compared to seven nearby old-growth
stands. Thinned and unthinned stands had matching
elevations, aspect, and soils, yet differed primarily in
management treatment. Leaf area indices were determined for
these stands.
Thinned stands differed from the old-growth and
unthinned stands in having significantly higher cover values
and species numbers, apparently resulting from increased
light to the forest floor and a greater variety of
microhabitats created by thinning. Young unthinned and old-growth
stands were comparable in terms of cover and
richness, but differed in species composition. Diversity
indices showed no difference in species diversity between
the three types of stands.
Ordination of the species/sample data using Detrended
Correspondence Analysis showed that understory species
composition of the young unthinned and thinned stands was
nearly identical. Species composition of old-growth stands
differed from thinned and unthinned stands. The ordination
indicated that age of the stands, structure of the canopy
layers and climate were major determining factors in the
species composition of the understory plant communities.
Management manipulation of the second growth stands did
not yield stands with understory vegetation communities that
mimicked those of old-growth stands. The conclusions of this
study were: 1) Shrub cover increased with thinning as
compared to unthinned and old-growth stands. 2) Thinning
increased the species richness of the stands, without
increasing the number of exotics. 3) Diversity was not
altered by thinning. Old-growth, thinned and unthinned
stands did not differ in diversity values. 4) Patterns of
community composition in thinned stands were more similar to
unthinned equivalent stands than to nearby old-growth. === Graduation date: 1996
|
|---|
