The plant economics spectrum (PES) provides a framework for examining plant functional strategies as shaped by their evolutionary history. The PES as originally described, mainly focused on the functional tradeoffs of aboveground organs, mostly stems and leaves. Recent work with roots has extended the PES to show that the functional trade-offs in roots are multidimensional with relatively little integration along a single axis of functional variation. Additionally, above and belowground traits appear to be orthogonal to each other, contradicting the expectation of whole-plant trait integration of the PES. In contrast with the PES predictions, root trait syndromes seem better explained by relatedness than in alignment to leaf traits, which partially explains the lack of tight trait correlation in broad interspecific comparisons, but there is still little information how root trait integration may vary within closely related species. We hypothesized that trait tradeoffs should be more apparent among closely related species, and trait integration across tree organs would be more apparent than those previously observed in far related species.
To test our hypothesis we investigate functional traits for >90 species of trees in the genus Ficus, from three botanical collections in Miami, Florida. We measured six root morphological variables associated with uptake capacity and compared them to SLA, Leaf N and branch density. Then, we expected H1) to find a single axis of variation describing trait integration in root systems and H2) traits belowground should be associated with aboveground traits as expected from the PES hypothesis.
Results/Conclusions
Our results supported a strong integration among root traits. For example, root diameter root tissue density correlates positively with diameter but negatively with SRL and root tip abundance. A multidimensional analysis showed that one axis of variation explained 60% of the whole plant average trait variation, confirming the fast-acquisition vs resource-conservation trade-off in plant strategy. However, we found little evidence of integration between root and leaf or stem traits (H2). In fact, a few trait relationships opposed the PES, such as root branchiness being negatively correlated with leaf N content (r = -0.35, p = 0.003).
We conclude that evidence for root acquisition trade-offs are easier to detect among closely related species, likely related to niche adaptations during large diversification episodes. Our results support the idea that selection for trait strategies is independent among plant organs, creating higher morphological and physiological space for selection in the case of hyper-diverse genera.