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.