Influence of overstory composition on understory vegetation and resource environment in the boreal forests of Canada

The forest canopy is a major determinant of microclimate within the forest ecosystem, as it intercepts, absorbs and modifies light environment in the understory. The availability of resources in the understory has implications on the growth and establishment of understory plants. The objectives of this thesis were to: (1) determine by means of literature synthesis whether understory plant species diversity is driven by resource quantity and/or resource heterogeneity, and (2) examine the influence of overstory composition on understory vegetation, and on the understory light resource environment in mature boreal forest stands of central and eastern Canada. A database of studies that investigated the effects of resources on understory plant diversity was compiled and analyzed using log-linear models. Whether resource quantity or resource heterogeneity is the determinant of understory plant diversity in individual studies was dependent on stand successional stage(s), presence or absence of intermediate disturbance, and forest biome within which the studies were conducted. Resource quantity was found to govern species diversity in both young and mature stands, whereas resource heterogeneity dominated in old-growth stands. Resource quantity remained the important driver in both disturbed and undisturbed forests, but resource heterogeneity played an important role in disturbed forests. The results suggested that neither resource quantity nor heterogeneity alone structures species diversity in forest ecosystems, but rather their influence on understory plant diversity vary with stand development and disturbances in forest ecosystems.;Patterns of species richness, abundance, and composition of three understory vegetation layers (shrub, herbaceous, and bryoid layers) were examined along an overstory broadleaf compositional gradient in the boreal mixedwood forests of central Canada. Shrub cover, as well as herbaceous species richness and cover, increased with increasing proportions of broadleaves in the overstory, whereas bryoid richness and cover declined. Among vegetation layers, herbaceous richness increased with increasing shrub richness, and bryoid cover decreased with increasing shrub and herbaceous cover. Species richness, cover, and composition of the three vegetation layers studied showed different patterns of association with measured environmental variables. Shrub and herbaceous richness and cover peaked along the broadleaf compositional gradient, while bryoid richness and cover peaked in conifer and less often in mixedwood ranges. On the basis of resource homogeneity and heterogeneity in pure and mixed species stands, these results suggest that resource quantity may be the important driver of herbaceous richness, shrub cover, and bryoid cover, whereas both resource quantity and resource heterogeneity may drive bryoid richness and herbaceous cover. Therefore, maintaining a diverse overstory in the boreal forests will ensure diversity of various life forms, because each overstory type, through modification of resources, may favor the establishment of different understory plant communities.;Instantaneous measurements of photosynthetic photon flux density (PPFD) were taken at 1.3 m above the forest floor in the understories of pure and mixed, closed-canopy boreal stands, dominated by Populus tremuloides (trembling aspen), Pinus banksiana (jack pine), and Picea mariana (black spruce), in eastern and central regions of the boreal mixedwood forests. Light quantity, calculated as percent PPFD, was higher under trembling aspen canopy (14.9 +/- 0.8%) than mixedwood (10.4 +/- 0.7%), and black spruce (12.9 +/- 0.6%) stands in the eastern boreal region, but least under aspen canopy (7.6 +/- 1.3%), and highest in mixedwood stands (12.6 +/- 0.6%) in the central boreal region. Light heterogeneity, calculated as spatial variation of%OPPFD within a stand, was higher in mixedwoods than single species stands. Light transmission was generally higher under overcast than under clear sky conditions. Overstory tree composition, as well as tall shrubs affects understory light quantity and heterogeneity. In closed-canopy stands, light measurements taken on both clear and overcast sky conditions are required to adequately quantify total light resource quantity and heterogeneity, available to understory plants over a growing season.;In summary, overstory composition influences resource availability in the understory and consequently on the richness, cover and composition of understory plants. Understory resource conditions may be fairly homogeneous in broadleaf and conifer stands, and heterogeneous in mixed stands. Patterns of species diversity in the resource limited understory may be governed by both the average supply (resource quantity) and spatial variability (resource heterogeneity) of available resources.;Keywords: Understory vegetation; resource availability; resource heterogeneity; plant diversity; boreal forest; photosynthetic photon flux density; broadleaf forests; bryoid.