Evaluating correlated leaf trait evolution: Evidence from Helianthus

Background/Question/Methods

In recent years, plant ecologists have focused on identifying trait patterns thought to reflect adaptive differentiation in order to understand the role of specific plant traits as drivers of community structure and responses to change. One of the most cited patterns of plant trait variation is the “worldwide leaf economics spectrum” (WLES), a set of correlations among leaf traits that is surprisingly consistent across several thousand plant species from a global gradient of habitats. The relationships among the six leaf traits making up the WLES (photosynthetic rate, dark respiration rate, nitrogen content, phosphorus content, leaf mass per area, and leaf lifespan) are thought to reflect universal physiological and ecological trade-offs. The different trait combinations in the spectrum represent different ecological strategies, ranging from fast-growing, resource-acquisitive species to slow-growing, resource-conservative species. However, the original dataset from which the WLES was derived contained almost no annual or herbaceous perennial species.

This ongoing project uses eight closely related species of the genus Helianthus that differ widely in habitat usage, as well as a member of the sister genus Phoebanthus, to investigate the presence of the WLES trade-offs in a group of annuals and herbaceous perennials using a phylogenetic comparative approach. Seed was collected from wild populations of each of the study species and grown in a greenhouse common garden to minimize environmental variation. Photosynthetic rate, leaf mass per area, leaf lifespan, nitrogen content, and a suite of additional related traits were quantified for individuals of each species. Correlations between traits were identified and corrected using phylogenetically independent contrasts.

Results/Conclusions

Trait values for many of the traits of interest varied greatly among species and were generally found to reflect expectations for source habitats. While some trait trade-offs predicted by the WLES (such as between photosynthetic rate and leaf lifespan) were observed, others (such as between photosynthetic rate and leaf mass per area) were not. Furthermore, trade-offs between WLES traits were generally robust to phylogenetic correction, while those for several other traits measured were not. Planned ongoing work will expand this study to include at least twice as many species and attempt to correlate leaf traits more formally with habitat soil and climate parameters.