Evergreenness influences fine root growth more than tree diversity in a common garden experiment

Recent studies have reported positive net diversity effects on aboveground tree growth. However, whether similar effects occur belowground through root investment, and whether such effects are related to evergreenness of tree communities, is less clear. Here we studied vertical distribution of standing fine root biomass of twelve North American temperate tree species planted in a common garden tree diversity experiment of varying species richness and evergreenness to test whether belowground niche complementarity of trees could explain positive diversity effects reported aboveground. We tested two alternative hypotheses: trees in mixtures increase uptake of soil resources (1) by increasing vertical root stratification and/or producing a greater fine root density (mg cm^-3) or (2) by producing similar or fewer fine roots that are potentially more efficient. Additionally, we hypothesized that proportional allocation to belowground biomass increases with evergreenness of tree communities. Fine roots were sampled in six layers of 5-10 cm, from 0 to 40 cm depth in single-, two- and four-species mixtures. We did not observe an effect of species richness on rooting depth or root density, refuting the hypothesis that aboveground overyielding in tree mixtures is linked to fine root overyielding. Rather, we observed a significant negative diversity effect (- 7.6%) on total fine root density, suggesting overall less investment to fine roots with increasing diversity. The strong positive effect of evergreeness on proportional allocation to fine roots over aboveground parts suggests that deciduous tree roots may be generally more efficient at absorbing soil resources, at least in the early years after tree establishment.