Focusing on individual plants to understand community scale biodiversity effects: the case of root distribution in grasslands

Spatial resource partitioning between species via differences in rooting
depth is one of the main explanations for the positive
biodiversity-productivity relationship. However, evidence for the
importance of this mechanism is limited. This may be due to the community
scale at which these interactions are often investigated. Community
measures represent net outcomes of species interactions and may obscure
the mechanisms underlying belowground interactions. Here, we assess the
performance of ~1700 individual plants and their heterospecific neighbours
over three growing seasons in experimental grassland plots containing one,
four or 16 different plant species and tested whether their performance in
mixtures compared to monocultures was related to their own rooting depth
vs. the rooting depth of their heterospecific neighbours. Overall,
individuals of deep-rooting species performed better in mixtures and this
effect significantly increased when surrounded by more shallow-rooting
species. This effect was not apparent for the shallow rooting species.
Together, including both deep and shallow rooting species increased
mixture performance. Our results show that taking the perspective of the
individual rather than the community can elucidate the interactions
between species that contribute to positive biodiversity effects,
emphasizing the need for studies at different scales to disentangle the
myriad interactions that take place in diverse communities.