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Site-forest productivity relationships and their management implications in coastal lowland ecosystems of East Graham Island, Queen Charlotte Islands

University of British Columbia

Relationships between soil, physiographic, floristic, and stand properties were examined in second-growth stands on a range of imperfectly to poorly drained ecosystems on east Graham Island, Queen Charlotte Islands. The major objective was to describe ecological factors associated with variation in tree growth, as expressed by site index of western redcedar. Cedar site index was found to be strongly correlated with soil nutrient content, particularly total N and exchangeable Mg, expressed on a kg/ha basis. Decreasing site index was associated with decreasing rooting depth, due to slowly permeable horizons, and poor soil aeration, reflected by high volumetric moisture content. A simple model using total N content and volumetric moisture content summarized the relationship between cedar site index and soil properties, and accounted for 78% of the site index variation. Three site index classes used as sampling strata (redcedar site index ≤ 15, 16-20, and >20 m/50 yrs. b.h. age) could be successfully differentiated by soil properties using discriminant analysis. Natural structure in the soils data revealed through principal components analysis and cluster analysis also reflected, with minor overlap, these three site index classes. Understory vegetation could be used to differentiate the poorest site index class, however the remaining two classes could not be floristically differentiated. A simple model relating cedar site index to reciprocal averaging scores derived from vegation data, and the frequency of the nutrient-medium indicator species group only explained 44% and 31% respectively of site index variation. Site index of Sitka spruce, western hemlock, and lodgepole pine were all highly correlated with redcedar site index. Hemlock and pine showed similar height growth patterns to cedar, while spruce was markedly different, showing slower height growth than cedar on poorer sites and greater height growth on better quality sites. The sensitivity of spruce to limiting site conditions was also reflected in its increasing stand volume composition with improving site quality. The sampled sites generally supported high stand volumes given the site limitations, with MAI averaging 5 m³ha⁻¹yr⁻¹ on the poorest sites (site index class 1) to 13 m³ha⁻¹yr⁻¹ on the best sites (site index class 3). Management strategies were recommended based on relationships observed in the study. Preferred tree species to manage are: for site index class 1 - cedar, pine, and hemlock; for site index class 2 - cedar, hemlock, pine, (with minor spruce); for site index class 3 - hemlock, spruce, cedar. Site productivity of these ecosystem may be improved by increasing the volume of aerated soil exploitable by tree roots. This could be achieved by site preparation which created relatively low, but wide mounds.

Text

2010-08-16

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http://hdl.handle.net/2429/27462

Forestry

University of British Columbia

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