Functional differences in fine root morphology of savanna trees and grasses

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Niki Gajjar and Ricardo M. Holdo, Odum School of Ecology, University of Georgia, Athens, GA, Julienne E. NeSmith, School of Natural Resources and Environment, University of Florida, Gainesville, FL

Background/Question/Methods
Soil moisture availability has been shown to be a key driver of tree:grass ratios in savannas, and differences in root morphological traits between trees vs. grasses may reveal water uptake differences in these two functional types. Fine root morphology influences water acquisition and transport, and therefore the ability of plants to take advantage of variable rainfall and water availability. Differences in fine root morphology between trees and grasses have not been comprehensively quantified, especially in the savanna biome. We grew individuals of 21 tree and 18 grass species from a Lowveld savanna ecosystem in South Africa under greenhouse conditions and characterized six key root morphological traits. We scanned representative subsets of fine roots (< 2 mm diameter) at high resolution and processed them with open-source root-tracing software. We used linear mixed models to compare trees and grasses in terms of root diameter, branching intensity, specific root length, root length to leaf area ratio, absorptive to total root length ratio, and root length to plant mass ratio.
Results/Conclusions
We found clear differences in between trees and grasses for all six functional traits, particularly in terms of branching intensity and root diameter. The first axis of the PCA captured 40% of the variation. Grasses tended to have thinner fine roots and more complex branching patterns, while trees generally had thicker roots and simpler root systems. Mean specific root length and branching intensity were, respectively, 2.4 and 2.5 times higher in grasses than trees, and diameter was 1.6 times higher in trees than grasses. Grasses also invested more of their overall root length into distal root orders and had a mean 1.7 times greater distal root length: total root length than trees, but trees were highly variable. Trees and grasses had somewhat comparable values of root length to leaf area ratios, indicating a higher investment in total transpiration area to match the higher investment in water uptake capacity in grasses compared to trees. Finally, root length: total plant mass was twice as high in grasses. Unlike previous work with fine root vascular anatomy, we found no significant phylogenetic signal for traits in the grass clade, and a significant signal for branching intensity only in the tree clade. We expect that our results will contribute to parameterizing models of water uptake in savanna systems and will help improve predictions of tree-grass dynamics under future climate scenarios.