5th International Conference on
Forest Vegetation Management
COMPENSATORY EFFECTS OF OVERSTORY AND UNDERSTORY COMPETITION ATTENUATE
GROWTH RESPONSES OF CONIFER SEEDLINGS IN 40- TO 70-YEAR-OLD DOUGLAS-FIR STANDS
Timothy Harrington1
At three forest sites near Olympia, Washington, USA, seedlings of Douglas-fir (Pseudotsuga menziesii),
western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata) were planted in early 2001
within clearcut-harvested, shelterwood, or thinned stands of 40- to 70-year-old Douglas-fir. The objectives
of the study were to identify competitive interactions between overstory and understory vegetation and to
quantify how they influence resource availability and conifer seedling growth. From 2002 to 2004, shrub
and herbaceous vegetation was left non-treated or it was removed via herbicides within either 50% or
100% of the growing space of 486 randomly-selected seedlings (i.e., areas of 2.1 m x 2.1 m or 3 m x 3 m,
respectively) growing within the three stand types. Overstory cover, soil water content, and seedling
growth were monitored for three growing seasons. Orthogonal polynomials in analysis of variance were
used to identify the type of resource and seedling responses to decreasing understory competition and
whether they differed among overstory densities (i.e., clearcut, shelterwood, or thinned stands). From
2002 to 2004, overstory cover averaged 0%, 26 to 31%, and 58 to 61% in clearcut, shelterwood, and
thinned stands, respectively. Total cover of understory vegetation decreased at a diminishing rate
(quadratic response) with area of understory vegetation control (AVC), and it ranged from 5 to 96% cover
in 2004. In mid-summer of each year, soil water content increased linearly with increasing AVC. However,
responses of both understory cover and soil water to AVC did not differ among overstory densities. In
2004, seedling stem volume responses to treatment differed among species: the interaction of overstory
and understory competition was significant for Douglas-fir, the separate effects of each factor were
significant for western hemlock, and only the effects of understory competition were significant for
western redcedar. Although Douglas-fir growth increased with decreasing overstory density, it increased
linearly with increasing AVC only in clearcut stands. Western hemlock growth increased at a diminishing
rate (quadratic response) with increasing AVC, and the steepness of this relationship increased with
decreasing overstory density. Western redcedar growth peaked at 50% AVC (quadratic response), and
this response did not differ among overstory densities. Growth responses of fir and hemlock to AVC were
attenuated by presence of overstory competition, while those of cedar were unaffected. Results of this
study can be used to identify opportunities for increasing conifer seedling growth through appropriate
manipulations of overstory and understory competition.
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PNW Research Station, USDA Forest Service - 3625 93rd Ave SW, Olympia, WA 98501, USA
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Abstracts Fifth International Conference on Forest Vegetation Management: Usable Science, Practical Outcomes & Future Needs Corvallis, Oregon, USA June 20-24, 200