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Relationships between stand and site factors in naturally established fire-origin lodgepole pine stands in the upper foothills of Alberta Brisco, David James

Abstract

60 sample plots within unmanaged, fire-origin, immature lodgepole pine dominated stands, were established in the Upper Foothills natural subregion of western Alberta. The plots were deliberately located to represent four forested ecosites: b (bearberry/lichen), d (Labrador tea - mesic), e (tall bilberry/arnica), and h (Labrador tea-subhygric); and were used to study the relationships between ecological measures of site quality, and soil and stand attributes. Soil nutrient regimes (SNRs) and soil moisture regimes (SMRs) were quantitatively characterized for the study area. Significant differences in soil chemical properties were found between SNRs, in particular with those properties linked to soil fertility. Direct measures of soil nutrients agreed with field-identified SNRs, justifying their use. Following climatic and water balance analysis, relative SMRs were related to actual SMRs. The 'mesic' relative SMR is equivalent to the fresh actual SMR, with no significant water deficits or surpluses during the growing season. Relationships between foliar and soil nutrients were examined and quantified. Strong positive relationships between the two groups of variables demonstrate that increases in the availability of soil nutrients are reflected in increased foliar nutrient concentrations and improved stand nutrient status. This relationship has strong implications for operational fertilization. Of all categorical measures of site quality, SMR accounted for the greatest amount of variation (63%) in stand productivity. Stand density was explained relatively equally (-30%) by all three (SNR, SMR and ecosite) categorical measures of site quality. As a combination of both soil moisture and nutrient conditions, the use of ecosites as a management tool is justified, although its relationships with various measures of forest productivity and density are not as strong as would be expected. Along with additional stand measures such as density and age, ecosites can be used as a viable framework for developing site-specific silvicultural management strategies. All stands, regardless of ecosite, undergo severe reductions in lodgepole pine site index as density increases. The degree of height repression due to increasing stand densities is influenced by site quality. Stand productivity on SMR fresh sites demonstrated a significantly different relationship with increasing stand densities than observed on either moisture deficient or surplus sites, having a steeper regression slope and exhibiting much greater site index values at low densities. Within the range of sites conditions investigated during this research, differences in available soil moisture appeared to explain the majority of observed differences in lodgepole pine productivity and development. At densities greater than 30,000 stems per hectare, the characteristics of 40-year-old stands remain relatively uniform with increasing density. This may qualify as a definition of lodgepole pine 'stagnation'.

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