Data from: Effects of species diversity on fine root productivity increase with stand development and associated mechanisms in a boreal forest

There is a growing interest in understanding the relationship between
diversity and below-ground productivity due to the critical contribution
of below-ground systems to overall terrestrial productivity. Yet, the
temporal (seasonal and developmental) changes in diversity effects on
below-ground productivity and their underlying mechanisms remain unclear.
We hypothesized that (i) diversity effects on fine root productivity
increase with stand development, and (ii) increased diversity effects
associated with stand development result from augmented horizontal soil
space utilization, increased forest floor depth for rooting, enhanced
effects in nutrient-poor soil layers and/or foraging towards high nutrient
availability. We investigated the effects of tree species diversity on
fine root productivity by sampling 18 stands dominated by single species
and their mixtures in post-fire boreal forests of two stand ages (8 and 34
years following stand-replacing fire). Species evenness was significantly
higher in species mixtures than in single-species-dominated stands at both
age classes, while species richness did not differ across stand types and
age classes. We found that the annual fine root production was higher in
mixtures than the mean of single-species-dominated stands in both stand
ages, with a significantly higher magnitude of effects in the 34-year-old
than 8-year-old stands. Mixtures had higher horizontal soil volume filling
than single-species-dominated stands with a more pronounced increase in
the 34-year-old than 8-year-old stands. Compared with the 8-year-old
stands, the 34-year-old stands had increased forest floor depth and
greater overyielding with soil depth, and their fine root productivity was
more responsive to the vertical variation in soil phosphorus
concentrations among soil layers. Synthesis. Our results provide evidence
for increasing positive diversity effects on fine root productivity with
stand development in heterogeneous natural forests. Moreover, our results
indicate 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 nutrient-poor deep soil layers
and stronger nutrient foraging towards soil layers with high nutrient
concentrations in older than younger stands.