Abstract
We analyzed productivity, in terms of periodic annual increment (PAI) in volume, of pure and mixed unmanaged naturally regenerated boreal stands mainly comprised of Populus tremuloides Michx. (trembling aspen) and Picea glauca (Moench) Voss (white spruce) ranging from 25–260 years old, in Alberta, Canada. Measures of density, site occupancy (Reineke’s stand density index-SDI), height, and site quality were evaluated in non-linear regression models aiming to explain the variation in PAI for the separate component species and for the entire stand. Analyses also included examination of the influence of stand composition. Results indicated a positive effect of both density of deciduous trees and of the percentage of deciduous tree stand basal area on trembling aspen volume PAI. White spruce PAI was positively influenced by spruce and aspen basal area, although effects of aspen were small. Height of both aspen and spruce were the most consistent independent variables in the models tested. Maximum PAI was achieved in mixed stands when site occupancy of both species was at its maximum, indicating the ecological combining ability of these two species. Variation in maximum stocking densities (i.e., stockability) has a strong influence on increment in these forests and is a major factor leading to increased productivity of mixed compared to pure stands.
Highlights
Stand density has important effects on growth rates
Combinations of selected independent variables explained up to 80% of the variation in trembling aspen periodic annual increment in volume
Average periodic annual increment for trembling aspen in boreal mixedwoods, over the 48–105 years of age at breast height, ranges from 9.2 m3 ̈ha1 ̈year1 to roughly 11.5 m3 ̈ha1 ̈year1 [25]; for white spruce growing in mixed stands and with ages ranging from 4 to 64 years at breast height, reported average biomass periodic annual increment values are around 1.5 Mgha1 [25]
Summary
Stand density has important effects on growth rates. Understocked stands experience reduced growth rates essentially because trees are not fully occupying the site and are unable to fully utilize the site’s resources. Well stocked stands will achieve their maximum growth potential. As density increases above that required to completely utilize a site, growth is allocated to more and more stems, and results in smaller individual trees. At maximum attainable densities (i.e., maximum stockability), substantial tree mortality induced by competition is experienced. This imposes a constraint beyond which stand growth may not occur [1]. Stand density representing full stocking, in terms of number of trees per unit area, will vary with tree size ( crown size), and, with age
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