Graduate Thesis Or Dissertation
 

The structure and development pattern of mixed-species forest stands in the interior cedar-hemlock zone : moist cold subzone of northwestern British Columbia

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  • A field study was established to explore stand structure and development patterns of mature, mixed-species forests in the Interior Cedar-Hemlock (ICH) zone: moist cold subzone of northwestern British Columbia. The species of interest in the study area were: western hemlock (Tsuga heterophylla (Raf.) Sarg.), western redcedar (Thuja plicata Donn), lodgepole pine (Pinus contorta var. latifolia Engelm.), hybrid spruce (Picea glauca (Moench) Voss x sitchensis (Bong.) Carr.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), and paper birch (Betula papyrifera Marsh.). Eighty stand structure plots, 150 to 600 m² in size, were used to assess the species, size and age variation within the study area. Three representative plots, 1,000 m² in size, were then selected for destructive sampling. Stand reconstruction techniques were used to: 1) characterize the existing species, age and height / diameter structure, 2) describe and quantify the historical height development pattern, 3) describe the development stage and spatial pattern of the stand and it's different components and 4) determine the influence of the above characteristics on the current stand composition and structure. The study stand is 135 years old and originated after a stand destroying wild fire. The resulting mixed-species stand had a very long recruitment period and is currently near the end of the stem exclusion stage of development. Because of the variety of growth patterns exhibited by the different species, neither individual tree height nor diameter were good indicators of stand age structure. The age structure and stage of development were much better defined by direct measures of age, stand density, the diameter distribution / mortality relationship or the overall spatial pattern. Although partially influenced by age structure and spatial arrangement, the developmental pattern of the mixed-species stand was most strongly related to individual height growth characteristics and inter-tree competition. While lodgepole pine, hybrid spruce, western redcedar and paper birch developed in height along characteristically predictable lines, western hemlock exhibited a number of different growth patterns. Subalpine fir followed an atypical, aggressive height growth pattern, developing more like a pioneer species. The variety of height growth patterns frequently allowed younger trees to overtop older individuals, both within and between species. This resulted in numerous shifts in height dominance by different species during the first 80 years of stand development. Forest structure is intimately related to disturbance type and intensity, site quality and the availability of seed or propagules for regeneration. This study examined the structure and development pattern of one type of mixture of tree species common to the ICH zone of northwestern British Columbia. Any difference in the factors listed above would likely produce a different species mixture and therefore a different structure and development pattern. Because most silvicultural operations are tied to the different structures at various stages of stand development, a more complete understanding of these patterns will allow managers to better predict the growth impacts of a variety of treatments and work with, rather than against the natural system.
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