5th International Conference on
Forest Vegetation Management
FIVE-YEAR DEVELOPMENT OF PLANT COMMUNITIES AND PLANTED DOUGLAS-FIR WITH AND
WITHOUT ANNUAL HERBICIDE APPLICATIONS
David Peter1 and Constance Harrington1
The Fall River Long-Term Site Productivity Study was established to evaluate the effects of several
current or potential forest management treatments on growth of Douglas-fir, development of plant
communities, and changes in plant and soil characteristics. The highly productive study area is located 35
km west of Chehalis, WA (USA) on a western hemlock–swordfern-oxalis plant association with deep soils
and high annual precipitation (2260 mm). The previous stand was clearcut in 1999 and planted with 1+1
Douglas-fir seedlings in spring 2000. This report characterizes 5-year tree growth and development of
plant communities in two of the study treatments – bole-only harvest of the previous second-growth stand
with and without subsequent annual application of herbicides to control competing vegetation. Each
treatment was assessed annually in eight 15-m diameter plots. Without herbicide applications, there was
rapid development of cover of herbaceous vegetation and a much slower but consistent development of
shrub species. At age 5, cover averaged 100% for herbs, 8% for shrubs and 53% for planted trees.
Twenty-three percent of the species present and 65% of the cover in year 5 were intolerant exotic
species. Plant community composition did not change markedly over the 5-yr period but relative
coverages by species did. Of the species present in year one, 79% were present in year 5. The most
common non-tree species in year 5 in terms of cover and frequency are Hypocharis radicata, Holcus
lanatus, Epilobium angustifolium and Oxalis oregana. With repeated application of herbicides, combined
non-tree cover averaged less than 8%. There were 29 species present in year 1 and in year 5, but only
62% of the year 1 species survived to year 5. The species with the greatest coverages in year 5 exhibited
different survival strategies. For example, Oxalis oregana is a native rhizomatous, shade-tolerant species
able to exploit areas with lower levels of herbicide such as under tree branches, accumulations of slash,
or edges of logs while Hypocharis radicata is an exotic species that apparently survives by rapid initial
growth combined with prolific production of wind-disseminated seeds. Tree survival was excellent in both
treatments (94% by year 5). Tree growth and crown cover were consistently greater in years 2-5 in the
vegetation-control treatment. Crown cover and volume index in 2003 (diameter squared x height) were
strongly correlated. Douglas-fir cover and volume index were negatively correlated with biomass and
herbaceous cover. Douglas-fir cover increased by 21% when non-tree cover was reduced 92%. The
relationship of Douglas-fir crown area to non-tree cover was linear. The relationship of volume index to
herbaceous cover or biomass was logarithmic. The response to vegetation control was greater than many
would have predicted on this very productive site where the competing species were primarily
herbaceous and did not overtop the seedlings. Although annual application of herbicides is unlikely to be
cost effective or ecologically desirable, such studies are useful to understand the factors limiting growth
and the mechanisms of response to vegetation control.
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1
USDA Forest Service, Pacific Northwest Research Station - 3625-93rd Ave SW, Olympia, WA 98512,
USA
36
Abstracts Fifth International Conference on Forest Vegetation Management: Usable Science, Practical Outcomes & Future Needs Corvallis, Oregon, USA June 20-24, 200