First-year Acacia seedlings are anisohydric “water-spenders” but differ in their rates of water use

Scott T. Cory, William K. Smith, T. Michael Anderson

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


Premise: First-year seedlings (FYS) of tree species may be a critical demographic bottleneck in semi-arid, seasonally dry ecosystems such as savannas. Given the highly variable water availability and potentially strong FYS–grass competition for water, FYS water-use strategies may play a crucial role in FYS establishment in savannas and, ultimately, in tree–grass competition and coexistence. Methods: We examined drought responses in FYS of two tree species that are dominant on opposite ends of an aridity gradient in Serengeti, Acacia (=Vachellia) tortilis and A. robusta. In a glasshouse experiment, gas exchange and whole-plant hydraulic conductance (Kplant) were measured as soil water potential (Ψsoil) declined. Trajectory of the Ψleafsoil relationship during drought elucidated the degree of iso/anisohydry. Results: Both species were strongly anisohydric “water-spenders,” allowing rapid wet-season C gain after pulses of moisture availability. Despite being equally vulnerable to declines in Kplant under severe drought, they differed in their rates of water use. Acacia tortilis, which occurs in the more arid regions, initially had greater Kmax, transpiration (E), and photosynthesis (Anet) than A. robusta. Conclusions: This work demonstrates an important mechanism of FYS establishment in savannas: Rather than investing in drought tolerance, savanna FYS maximize gas exchange during wet periods at the expense of desiccation during dry seasons. FYS establishment appears dependent on high C uptake during the pulses of water availability that characterize habitats dominated by these species. This study increases our understanding of species-scale plant ecophysiology and ecosystem-scale patterns of tree–grass coexistence.

Original languageEnglish (US)
Pages (from-to)1251-1261
Number of pages11
JournalAmerican journal of botany
Issue number8
StatePublished - Aug 2022
Externally publishedYes


  • Acacia
  • Serengeti
  • Vachellia
  • drought
  • first-year seedling
  • hydraulic conductance
  • hydraulic vulnerability
  • iso/anisohydry continuum
  • savanna
  • tree–grass competition

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Genetics
  • Plant Science


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