Graduate Thesis Or Dissertation
 

Establishment and growth of natural and planted conifers ten years after overstory thinning and vegetation control in 50-year-old Douglas-fir stands

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  • Forest managers of public lands in western Oregon and Washington have become increasingly interested in creating additional conifer cohorts in young, even-aged, second-growth Douglas-fir stands. The purpose of our research was to assess the establishment, survival, and growth of naturally-regenerated and underplanted conifers 10-13 years after overstory thinning and understory vegetation control in 50-year-old Douglas-fir stands. Two sites within the Oregon Coast Range were used in our study. One site was relatively dry (~130 cm of annual precipitation) and contained a mostly pure Douglas-fir overstory. The second site was moister (~175 cm of annual precipitation) and contained a Douglas-fir/western hemlock mixed-species overstory. At each site, stands were thinned to basal areas ranging from approximately 18 to 32 m2/ha using either a uniform or gappy thinning pattern. In the gappy treatment, 20% of the total area was comprised of 0.06- and 0.10-ha gaps. Understory vegetation was controlled across a portion of each thinning plot using a broadcast application of herbicides prior to thinning. After thinning, the following species were underplanted: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), and grand fir (Abies grandis, dry site only). Establishment of natural regeneration and survival of underplanted conifers was greater for all species under lower overstory densities at the drier site, but on the moister site, where western hemlock regenerated prolifically in the understory, no differences were detected. At both sites, underplanted and dominant naturally-regenerated conifers growing under lower overstory retention levels were generally taller and had larger diameters after ten years. Thinning pattern had no effect on the establishment, survival, or growth of understory conifers at the overall stand level, but the tallest individuals were located within gaps. Controlling competing vegetation increased the rate of establishment for naturally-regenerated Douglas-fir and the rate of survival for planted Douglas-fir and western hemlock. Vegetation control generally resulted in larger underplanted seedlings of all species after ten years, but the size of dominant naturally-regenerated conifers was not affected. Shade-tolerant species generally outperformed Douglas-fir in understories except where western redcedar suffered heavy browsing damage. Natural regeneration of western hemlock may contribute to the development of an understory conifer layer when a seed source is present in the overstory, but in mostly pure Douglas-fir stands, underplanting would likely be required to supplement sporadic and slow-growing natural regeneration. In either case, future thinnings of rapidly closing overstory canopies would be required to maintain the long-term development of an understory conifer layer.
  • Keywords: western hemlock, stand structure, stand density, understory, vegetation management, Douglas-fir, thinning, underplanting, western redcedar, natural regeneration, seedling growth
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