LB 10-130 - The plant economic spectrum can be strong where the leaf economic spectrum is weak: lessons from two North American grasslands

Background/Question/Methods:

We tested for coordination between leaf traits, root traits and plant abundance in mixedgrass and shortgrass prairie communities in Wyoming and Colorado, U.S.A. While progress has been made documenting and understanding variation of aboveground traits in these grasslands, root functional traits remain poorly described, despite the fact that the majority of plant biomass in these ecosystems is belowground. Using trait variability among 55 different herbaceous species, we tested the following hypotheses. H1: correlations among leaf traits and among root traits are weak to moderate, given limited variability in leaf lifespan and the relatively narrow range of plant taxa and environmental variability. H2: Correlations among root economic traits are qualitatively similar to correlations among leaf economic traits (e.g., root nitrogen [N] is correlated with root morphology, like the often observed correlations between leaf N and specific leaf area (SLA) or leaf dry matter content (LDMC). H3: Pairs of analogous economic traits of leaves and roots are positively correlated (e.g., SLA and specific root length [SRL], leaf and root [N]). H4: Root diameter is positively correlated with seed size. H5. Common and rare species in shortgrass and mixedgrass prairies possess distinct values for key economic traits.

Results/Conclusions:

The covariance between similar leaf and root traits (e.g., SLA vs SRL, leaf N vs. root N) was tighter than covariance between alternative economic traits within an organ (e.g., SLA vs. leaf N and SRL vs. root N). Tissue dry matter content and nitrogen content are especially tightly coordinated above and belowground. Although global scale analyses suggest there is no link between root morphology and root N concentrations, our study and others show that this relationship does exist within certain contexts (e.g., within herbaceous plants or certain grasslands), and this relationship can exist even in communities where the SLA-leaf N correlation is not apparent. Root diameter is a key trait that appears to constrain the form and function of roots and leaves and seeds in the shortgrass and mixedgrass prairies: species with thicker roots tend to have larger seed mass and leaves with a larger surface area. Rare and dominant species do generally possess different values of economic traits: relative abundance of species is tightly correlated with dry matter content, with more abundant species tending to have higher LDMC and RDMC. These lend further credibility to earlier reports that LDMC is a critical, ‘hub’ trait for these widespread ecosystems.