How do leaf functional traits and age influence the maximum rooting depth of trees?

Maximum rooting depth is a key functional trait to increase the fitness of trees and also influences terrestrial ecosystem processes. Despite its importance, the drivers of the interspecific variation of maximum rooting depth or its relation to other plant traits and plant age are not well understood. In this study, we aimed to clarify the drivers of the interspecific variation of maximum rooting depth with special reference to its relation to plant leaf traits and age. We analyzed how maximum rooting depth of single individuals of 227 tree species planted in the same common garden in the temperate region of central Japan is correlated to commonly measured leaf functional traits (specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen (N) concentration) extracted from the TRY database. We did this by employing the phylogenetic comparable method and included the age of all target trees. When excluding the effect of phylogenetic signals from the relationships between rooting depth and leaf traits, SLA was negatively correlated with maximum rooting depth in deciduous, but not evergreen species. Further, rooting depth and leaf N concentration were negatively correlated in evergreen trees, a pattern driven by young trees. These results implicate that the relationship between maximum rooting depth and leaf traits differed depending on the leaf habits and age of the tree species.