Species diversity and aboveground productivity relationships in forest ecosystems

Empirical and theoretical studies have attributed the observed positive diversity-productivity relationship (DPR) across various ecosystems to a complementarity effect through niche differentiation and/or facilitation among constituent species in more diverse communities, but such biological mechanisms have rarely been demonstrated in the published DPR studies. Moreover, a mechanistic understanding of a general framework on the multifaceted relationships between diversity, productivity, species coexistence, and their interactions with environmental gradient, has not been established, and calls for a novel multivariate approach. In forest ecosystems, studies focused on the above general framework are rare because of the complex temporal and spatial dynamics. The objectives of this thesis were to: (1) determine the relative influences of plant life-history traits, species diversity, biome, and stand origin on productivity, and (2) examine the multiple causal relationships between standing biomass and species diversity, variation in DBH within stand, stand age, and soil nutrient regime in boreal forests.;Data from 448 sampling sites were used to examine the multivariate hypotheses based on theories and empirical studies by applying structural equation modeling (SEM). Bivariate relationships between variables were examined to facilitate the interpretation of the SEM results. The SEM model provided a strong fit to the data (chi2 = 5.314, df = 6, P = 0.504, CFI = 1.000, RMSEA < 0.001) and the set of hypotheses were supported by the data. Our results showed that tree size variation among individuals was the central mechanism linking resources availability, standing biomass, and species diversity. Standing biomass and Shannon's species index were both positively correlated with DBH variation among individuals within stands.;Keywords Boosted regression trees, effect size, life-history variation, meta-analysis, productivity, shade tolerance, Shannon's index, structural equation modeling, standing biomass, DBH variation;In a meta-analysis containing 53 studies in forest ecosystems, we used a trait-based approach to show the effects of species diversity, life-history variation, biomes, and stand origin (naturally established versus plantation) on productivity, which was calculated as effect size. Boosted regression tree analysis indicated that Shannon's index had a 41.1% relative influence on effect size, life-history variation had a total of 41.3% relative influence with 27.2% for shade tolerance, 5.4% for leaf habit, 4.4% for growth habit, and 4.3% for nitrogen-fixing, whereas biome and stand origin had 16.2% and 1.38% relative influence, respectively. The effect size increased with Shannon's index and plateaued when diversity was high, and it was positive with presence of interspecific life-history variation. We conclude that a positive DPR occurs regardless of biome and stand origin when interspecific life-history variation is present among constituent species in polycultures, whereas an insignificant or negative DPR appears to dominate when interspecific life-history variation is absent.