Abstract
The cold and dry boreal forests of the Southwest Yukon are dominated by white spruce (Picea glauca), trembling aspen (Populus tremuloides) and balsam poplar (Populus balsamifera), and the variability in structure and composition of stands depends on the favourability of disturbance, climate and site conditions for stimulating regeneration. In this study, we investigated relationships between stand structure and ecological, climatic and disturbance factors in the southwest Yukon. We found that white spruce dominates mature forests across the landscape, but it is regenerating proportionately less than trembling aspen. Nevertheless, regeneration of all the three species was abundant following any type or severity of disturbance. Height and diameter of both species varied with several environmental variables, particularly site physiography. Mixed stands of aspen and white spruce were more productive than pure stands of aspen or spruce. However, overall productivity in mixed stand decreased when density of aspen was more than 1000 stems/ ha. These results suggested that mixed stands of deciduous and coniferous species where appropriate should be promoted maintaining aspen density below 1000 stems/ha as the productivity declined beyond this threshold. Similarly, we suggest carrying out selection harvesting of co-dominant trees and regular thinning of intermediate trees to promote the height and diameter growth of the remaining trees.
Highlights
Stand structure is an important attribute of forest ecosystems, with implications for forest health, biodiversity conservation and forest management (Kimmins, 1997; Kuuluvainen, 2002; Brassard and Chen, 2006) and is generally affected by multiple biogeoclimatic factors and the temporal and spatial scales of disturbances (Bonan and Shugart, 1989)
White spruce dominates the landscape in terms of its density, basal area, average height and DBH, its regeneration was proportionately less than that of trembling aspen, especially in disturbed plots, suggesting broadleaved species may prevail if disturbance persists in the region
This research found that mixed stands of aspen and white spruce had higher basal areas per hectare than pure stands of trembling aspen or white spruce and that average stand diameter was positively correlated with tree density up to 1000 trees per hectare, beyond which the average diameter decreased
Summary
Stand structure is an important attribute of forest ecosystems, with implications for forest health, biodiversity conservation and forest management (Kimmins, 1997; Kuuluvainen, 2002; Brassard and Chen, 2006) and is generally affected by multiple biogeoclimatic factors and the temporal and spatial scales of disturbances (Bonan and Shugart, 1989). 340,000 hectares of forest were affected by the beetle outbreak (Northern Climate Exchange 2006; Government of Yukon, 2010), which was the second largest infestation recorded after the infestation in south central Alaska, USA where the SBB killed approximately 1.2 million ha of white spruce stand (Berg et al, 2006). The outbreak is regarded as one of the most dramatic disturbance events to have affected the regions predominant mature white spruce (Picea glauca) forests (Berg et al, 2006)
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