Independent evolution of leaf and root traits within and among temperate grassland plant communities

Abstract

In this study, we used data from temperate grassland plant communities in Alberta, Canada to test two longstanding hypotheses in ecology: 1) that there has been correlated evolution of the leaves and roots of plants due to selection for an integrated whole-plant resource uptake strategy, and 2) that trait diversity in ecological communities is generated by adaptations to the conditions in different habitats. We tested the first hypothesis using phylogenetic comparative methods to test for evidence of correlated evolution of suites of leaf and root functional traits in these grasslands. There were consistent evolutionary correlations among traits related to plant resource uptake strategies within leaf tissues, and within root tissues. In contrast, there were inconsistent correlations between the traits of leaves and the traits of roots, suggesting different evolutionary pressures on the above and belowground components of plant morphology. To test the second hypothesis, we evaluated the relative importance of two components of trait diversity: within-community variation (species trait values relative to co-occurring species; α traits) and among-community variation (the average trait value in communities where species occur; β traits). Trait diversity was mostly explained by variation among co-occurring species, not among-communities. Additionally, there was a phylogenetic signal in the within-community trait values of species relative to co-occurring taxa, but not in their habitat associations or among-community trait variation. These results suggest that sorting of pre-existing trait variation into local communities can explain the leaf and root trait diversity in these grasslands.

Figure 1. Hypothesized phylogenetic relationships and trait values for 76 plant species growing in Alberta grasslands.

Symbols indicate relative values for species habitat associations (NMDS axis 1 score) and the total (), within-community (α), and among-community (β) components of leaf size variation (all values centered and scaled for visual comparison purposes). Branches are scaled proportional to estimated divergence times, with the root node (monocot – eudicot divergence) estimated at 139 million years ago.

Figure 2. Trait gradient analysis of leaf size for 76 plant species growing in Alberta grasslands.

Gray symbols are leaf size measured on 432 individual plants in 27 communities. Plants are arranged in order of increasing community mean leaf size (dashed line). Black symbols indicate leaf size of individual plants (circles) and the mean within-community (α) and among-community (β) leaf size (square) for Galium boreale. Dotted lines indicate mean leaf size of Galium boreale relative to co-occurring species (α = −0.8) and along the community mean leaf size gradient (β = 0.4), which sum to determine the mean or total leaf size observed for this species ( = 0.4). The solid line indicates the slope of within-species variation in leaf size in Galium boreale along the community trait gradient (b_s).

Figure 3. Results of a non-metric multidimensional scaling (NMDS) ordination for 27 communities in mixedgrass and fescue grasslands in Alberta.

Solid symbols indicate the site and habitat type of individual communities (green = fescue site, blue = mixedgrass sites). Cross symbols indicate species scores; names of selected species are indicated in gray. The first axis of this ordination was used as a measure of the habitat affinity of species.

Figure 4. Correlations between phylogenetically independent contrasts of total (), within-community (α), and among-community (β) components of trait variation for specific leaf area (SLA; cm²/g) versus specific root length (SRL; m/g).

Dashed lines indicate estimated evolutionary correlation through the origin.