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The growth of Abies amabilis (Dougl. ex Forbes) in relation to climate and soils in southwestern British Columbia Splechtna, Bernhard E.

Abstract

The coastal high-elevation forest has been increasingly used as a timber resource, despite uncertainties about the growth performance of second-growth stands. To provide comprehensive and coherent information on productivity and on the factors that constrain growth of mature trees, I studied the growth of amabilis fir (Abies amabilis) across the native range of the species in southern coastal British Columbia in five relatively independent studies. I developed conventional polymorphic and climate-specific height growth and site index models for amabilis fir from stem analysis data obtained from 67 study plots, which were deliberately chosen to represent the largest variation in climate and soil conditions across the study area. When tested against an independent data set consisting of 31 plots, the climatespecific models improved height and site index predictions compared to the conventional polymorphic models. The previously available model for amabilis fir was biased. Thus, the models developed in this study are recommended for height and site index estimation of amabilis fir stands within a range of breast-height age from 15 to 160 years in southern coastal British Columbia. To quantify the gradient of plant available nutrients on high-elevation sites, samples were collected of forest floor and mineral soil (0 to 30 cm) from montane and subalpine sites across a continentality gradient. Using discriminant analysis 70% agreement was found between field-identified SNRs and the classification based on measured (chemical) soil properties. Nitrogen-related variables (total N , mineralizable-N, and C :N ratio) and the sum of extractable Ca, Mg, and K were the properties most strongly related to the field-identified SNRs. These findings agree with the results of previous studies suggesting that mineralizable-N of the mineral soil is the best property for a quantitative characterization of a soil nutrient gradient. A dendroecological approach was used to identify climatic factors that limit radial growth of amabilis fir at high-elevation sites in the study area. I developed 11 residual ringwidth chronologies and 3 residual maximum density chronologies from stands on intermediate sites located along a continentality gradient. According to principal components, correlation, and pointer-year analyses, three types of growth responses to year-to-year variations in climate were associated with a strong regional ring-width pattern: i) a negative response to April 1st snow depth, ii) a positive response to July temperature of the current year, and iii) a negative response to summer temperature of the previous year. A strong positive radial growth response to winter temperature was observed only in submaritime and subcontinental strata but not in the maritime strata, indicating that harsh winter conditions in a subcontinental climate appear to limit radial growth of amabilis fir near the easternmost limits of its distribution. To determine the influence of elevation, continentality of climate, soil, and seasonal weather patterns on 9 tree-ring properties, I utilized dbh-discs from three dominant trees sampled at 62 stem analysis plots (Studies 1 and 5). I correlated (i) 30-year plot-averages of tree-ring properties with site factors and (ii) chronologies for 9 tree-ring properties for each of 8 site groups (combination of two continentality strata, two orographic strata, and two soil strata) with series of monthly climate data. The widths of earlywood and latewood as well as total ring width decreased significantly with increasing elevation. Width of earlywood and total ring width showed a significant decrease on nutrient-deficient sites, while percent latewood and ring density increased. Ring width, latewood width, and maximum density were reduced by a delayed start of the growing season and by low summer temperature in montane and subalpine. Moreover, the response to these two climatic factors was stronger on subalpine sites. Due to the differential responses, width of earlywood and latewood, and maximum density are recommended for studying tree growth responses to weather patterns. To provide quantitative information about the influence of the environment on amabilis fir growth, I examined relationships between continentality strata, orographic strata, elevation, soil moisture regime, and soil nutrient regime with site index and the diameter at breast height (1.3 m) at 30 years (D30). Stem analysis data provided site index and D30 from 98 stands across the study area (see Study 1). The study area was delineated into 3 continentality strata - maritime windward, maritime leeward, submaritime/subcontinental. On zonal sites, site index decreased by 1.9 m every 100m increase in elevation in the maritime windward stratum, by 2.7 m in the maritime leeward stratum, and by 0.8 m in the submaritime/subcontinental stratum. The influence of soil moisture regime and soil nutrient regime was significant for site index and D30. Two predictive models for site index were presented, one climate model (R² = 0.75) and one model using elevation, continentality, and the variable SOIL (R² = 0.83). Both models were unbiased when tested against independent data. Precision was relatively low for both models restricting their application to the forest level rather than to the stand level.

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