Patterns and mechanisms of tree species diversity effects on fine root processes associated with stand development in boreal forests

Abstract

Over the last two decades, one major advance in ecology has been the demonstration that biodiversity has positive effects on a broad range of ecosystem functions. However, diversity-ecosystem functioning studies for belowground are underrepresented, due to methodological limitations and the relative inaccessibility to root systems. This lack of understanding of belowground processes has cast doubt on the predictability of various ecosystem models; the forecasting of which serve as the basis for numerous global policies. The objective of this dissertation, therefore, is to improve the understanding of patterns and mechanisms of tree species diversity effects on fine root processes associated with stand development in natural forest ecosystems. To achieve this goal, I initially conducted a global meta-analysis on the effects of species diversity on fine root productivity in diverse ecosystems by synthesizing the results of 48 published studies. This meta-analysis demonstrated a positive mixture effects on fine root biomass and production, and showed that the mixture effects increased with species richness across all ecosystem types. More importantly, the meta-analysis also revealed shifts in diversity effects over time in both forests and grasslands. Inspired by the results of the meta-analysis, I conducted an empirical diversity experiment in the central region of the North American natural boreal forest, to examine the temporal (seasonal and developmental) changes in fine root production, and their underlying mechanisms associated with tree species diversity. I found that annual fine root production was higher in mixtures than the mean of single species dominated stands in all age classes, with a significantly higher magnitude of effects in mature than young stands. My results also indicated that the increased positive diversity effects with stand development was the result of multiple mechanisms, including higher horizontal soil volume filling, a thicker forest floor layer for rooting, a higher magnitude of complementarity in deep nutrient-poor soil layers, and stronger nutrient foraging toward soil layers with high nutrient concentrations in older than younger stands. Whether the results obtained on productivity can be generalized to other ecosystem processes remains patchy. I therefore examined species mixture effects on fine root turnover and mortality along stand development. I found that like biomass production, fine root turnover and mortality were also higher in mixtures than the mean of single-species-dominated stands in all age classes, with a higher mixture effects in mature than young stands. Moreover, my results suggested that increased mixture effects with stand development resulted from a higher competition intensity that was induced by the overyielding of fine root biomass production in mixtures. Moreover, most published diversity and productivity relationship (DPR) studies focus on one component of ecosystem production. Species diversity could alter production allocation, at least, in part, contributing to divergent DPR relationships. By synthesizing the production data of all individual components (i.e., aboveground trees, litterfall, understory vegetation, coarse roots, and fine roots) of boreal forest stands, collected from the same study sites, I examined how species mixtures affected the production of the entire ecosystem, and production partitioning among individual components along stand development. I found that the overyielding of the entire ecosystem production occurred in young stands, but not in older stands, despite the fact that fine root production was higher in species mixtures than single-species dominated stands in all ages. Species mixtures led to more production allocated to belowground than expected from single species-dominant stands. These studies offer a new and important understanding of DPR by showing the temporal changes of mixture effects on fine root dynamics (i.e., production, turnover, and mortality), production allocation, and their underlying mechanisms. The results have relevance for calculating the energy allocation, as well as the carbon storage of terrestrial ecosystems, and may provide a broad guide for management practices with the aim of increasing belowground productivity, element cycling, and carbon sequestration.