study areas : thunder bay region, northwestern ontario ; northeastern quebec.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 ofunderstory plants. the objectives ofthis 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.