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PremiseBelowground functional traits play a significant role in determining plant water‐use strategies and plant performance, but we lack data on root traits across communities, particularly in the tropical savanna biome, where vegetation dynamics are hypothesized to be strongly driven by tree–grass functional differences in water use. MethodsWe grew seedlings of 21 tree and 18 grass species (N= 5 individuals per species) from the southern African savanna biome under greenhouse conditions and collected fine‐root segments from plants for histological analysis. We identified and measured xylem vessels in 539 individual root cross sections. We then quantified six root vascular anatomy traits and tested them for phylogenetic signals and tree–grass differences in trait values associated with vessel size, number, and hydraulic conductivity. ResultsGrass roots had larger root xylem vessels than trees, a higher proportion of their root cross‐sectional area comprised vessels, and they had higher estimated axial conductivities than trees, while trees had a higher number of vessels per root cross‐sectional area than grasses did. We found evidence of associations between trait values and phylogenetic relatedness in most of these traits across tree species, but not grasses. ConclusionsOur findings support the hypothesis that grass roots have higher water transport capacity than tree roots in terms of maximum axial conductivity, consistent with the observation that grasses are more “aggressive” water users than trees under conditions of high soil moisture availability. Our study identifies root functional traits that may drive differential responses of trees and grasses to soil moisture availability.
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This content will become publicly available on October 1, 2026
Grass functional traits reflect the long history of fire and grazers in the savannas of Texas
Abstract PremiseUnderstanding relationships among grass traits, fire, and herbivores may help improve conservation strategies for savannas that are threatened by novel disturbance regimes. Emerging theory, developed in Africa, emphasizes that functional traits of savanna grasses reflect the distinct ways that fire and grazers consume biomass. Specifically, functional trade‐offs related to flammability and palatability predict that highly flammable grass species will be unpalatable, while highly palatable species will impede fire. MethodsWe quantified six culm and leaf traits of 337 native grasses of Texas—a historical savanna region that has been transformed by fire exclusion, megafaunal extinctions, and domestic livestock. ResultsMultivariate analyses of traits revealed three functional strategies. “Grazer grasses” (N = 50) had culms that were short, narrow, and horizontal, and leaves with high width to length (W:L) and low C to N ratios (C:N)—trait values that attract grazers and avoid fire. “Fire grasses” (N = 104) had culms that were tall, thick, and upright, and leaves that were thick, with low W:L, and high C:N—trait values that promote fire and discourage grazers. “Generalist tolerators” and “generalist avoiders” (N = 183) had trait values that were intermediate to the other groups. ConclusionsOur findings confirm that the flammability–palatability trade‐offs that operate in Africa also explain correlated suites of traits in Texas grasses and highlights that the grass flora of Texas bears the signature of Pleistocene megafauna and the influence of fires that predate human arrival. We suggest that grass functional classifications based on fire and grazer traits can improve prescribed fire and livestock management of savannas of Texas and globally.
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- Award ID(s):
- 1902078
- PAR ID:
- 10645253
- Publisher / Repository:
- American Journal of Botany
- Date Published:
- Journal Name:
- American Journal of Botany
- Volume:
- 112
- Issue:
- 10
- ISSN:
- 0002-9122
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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