Biogeographic constraints on the worldwide leaf economic spectrum

Background/Question/Methods

The worldwide leaf economic spectrum (LES) describes tight coordination of leaf traits across global floras, reported to date as being largely independent of phylogeny and biogeography.  However, an alternative, historical perspective predicts that biogeography places significant constraints on global trait evolution, leading to detectable deviations in leaf trait relationships between isolated floras that were historically influenced by different magnitudes of genetic constraint and selection.  The large variation among floristic regions in size, age, history, and diversity could limit the degree to which convergence of leaf trait relationships is realized, even within isolated regions of the same biome. We use a large trait database (GLOPNET) and species’ native distribution data to test for variation in leaf trait relationships modulated by floristic region.  Standardized major axis (SMA) analyses were used to evaluate these potential effects on bivariate relationships between LES traits, including relationships of photosynthetic capacity and dark respiration rate (A_mass-R_d-mass), leaf lifespan and mass per area ratio (LL-LMA), and photosynthetic capacity and nitrogen content (A_mass-N_mass).  We tested three biogeographically meaningful regional contrasts: 1) Northern vs. Southern Hemisphere floras 2) East Asian vs. Eastern U.S. temperate floras and 3) Hawaiian Islands vs. mainland tropical floras. 

Results/Conclusions

Independent of climate or biome effects, floras of distinct evolutionary histories exhibited different leaf trait allometries.  Floras of the Northern Hemisphere exhibited a more efficient carbon economy than those of the Southern Hemisphere, which was likely due to the vicariant history of Southern Hemisphere floras.  The more phylogenetically diverse temperate East Asian flora exhibited a generally more efficient carbon economy than that of Eastern U.S., with the exception of the scaling of A_mass-N­_mass.  In contrast to our hypothesis, plants of the floristically isolated Hawaiian Islands exhibited a similar A_mass-N_mass relationship to those of mainland tropical regions, likely a reflection of the younger, volcanic soils of Hawaii.  Overall, we suggest that significant differences in leaf trait allometries among global floristic regions provides evidence for an explicitly historical perspective in understanding leaf trait relationships and emphasizes the roles of competitive pressures and phylogenetic constraints in plant trait evolution.