The role of fine-root mass, specific root length and life span in tree performance: A whole-tree exploration

The root economics spectrum (RES) hypothesis predicts that fast-growing tree species have short-lived roots with high specific root length (SRL) to allow rapid resource uptake, and opposite trait expressions for slow-growing species. Yet, the mixed support for this hypothesis suggests that trees can adopt alternative strategies to increase resource uptake, besides an increase in SRL. We combined a novel mechanistic whole-tree model and empirical fine-root data of 10 tree species to test the effects of one of these alternative strategies, notably increasing fine-root mass, on the tree's net C gain (used here as a proxy for tree performance), and to assess how fine-root life span influences the relative importance of SRL and fine-root mass for the C balance of trees. Our results indicate that accounting for the short life span of high-SRL roots has important implications for explaining tree performance and the role of roots herein. Without considering their faster turnover, high-SRL roots and low fine-root mass resulted in the highest performance as predicted from the RES. Yet, when their higher turnover rates were accounted for, a high fine-root mass and low SRL lead to the highest performance. Both our model outcomes and field data further show a negative relationship between SRL and fine-root mass through which species aim to realize a similar root length density. This trade-off further indicates how high a SRL and low fine-root mass as well as opposite trait values can both lead to a positive C balance in a similar environment. Our study may explain why high-SRL roots do not necessarily lead to the fastest tree growth as often hypothesized and demonstrates the importance of fine-root mass in combination with fine-root life span for explaining interspecific differences in tree performance. More generally, our work demonstrates the value of identifying and investigating different below-ground strategies across species from a whole-plant modelling perspective, and identifies the relationship between SRL, fine-root biomass and life span as an important functional dimension to variation in species' performance. A free plain language summary can be found within the Supporting Information of this article.