Rooting depth varies differentially in trees and grasses as a function of mean annual rainfall in an African savanna

Abstract

A significant fraction of the terrestrial biosphere comprises biomes containing tree–grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree–grass vertical root separation occurs in these ecosystems, how this overlap varies across large-scale environmental gradients, and what these rooting differences imply for water resource availability and tree–grass competition and coexistence. To assess the extent of tree–grass rooting overlap and how tree and grass rooting patterns vary across resource gradients, we examined landscape-level patterns of tree and grass functional rooting depth along a mean annual precipitation (MAP) gradient extending from ~ 450 to ~ 750 mm year−1 in Kruger National Park, South Africa. We used stable isotopes from soil and stem water to make inferences about relative differences in rooting depth between these two functional groups. We found clear differences in rooting depth between grasses and trees across the MAP gradient, with grasses generally exhibiting shallower rooting profiles than trees. We also found that trees tended to become more shallow-rooted as a function of MAP, to the point that trees and grasses largely overlapped in terms of rooting depth at the wettest sites. Our results reconcile previously conflicting evidence for rooting overlap in this system, and have important implications for understanding tree–grass dynamics under altered precipitation scenarios.

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Acknowledgements

SANParks allowed access to Kruger NP for sample collection. We would like to acknowledge Navashni Govender and the Scientific Services staff at SANParks for assistance. Wayne Twine and Wits University provided access to the field site at Wits Rural Facility. Ben Ketter assisted with laboratory work, and Hloniphani Moyo, Deus Rugemalila, and Zak Ratajczak helped with field data collection. This research was partly funded by a grant from the Andrew W. Mellon Foundation. We thank Kevin Mueller and an anonymous reviewer for helpful suggestions on an earlier version of the manuscript.

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Contributions

RMH designed the study. RMH, JBN and MCM conducted the field work. JBN conducted the laboratory analyses, RMH analyzed the data, and RMH, JBN and MCM wrote the manuscript.

Corresponding author

Correspondence to Ricardo M. Holdo.

Additional information

Communicated by Miquel A Gonzalez-Meler.

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Holdo, R.M., Nippert, J.B. & Mack, M.C. Rooting depth varies differentially in trees and grasses as a function of mean annual rainfall in an African savanna. Oecologia 186, 269–280 (2018). https://doi.org/10.1007/s00442-017-4011-4

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Keywords

  • African savanna
  • Environmental gradients
  • Tree–grass coexistence
  • Two-layer model
  • Stable isotopes