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

Abstract

Summary 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.