First‐year Acacia seedlings are anisohydric "water‐spenders" but differ in their rates of water use. Issue 8 (12th August 2022)

• Record Type: Journal Article
• Authors: Cory, Scott T.; Smith, William K.; Anderson, T. Michael
• Is Part Of: American journal of botany. Volume 109:Issue 8(2022)
• Page Start: 1251
• Page End: 1261
• Publication Date: 2022-08-12
• Subjects: Acacia -- drought -- first‐year seedling -- hydraulic conductance -- hydraulic vulnerability -- iso/anisohydry continuum -- savanna -- Serengeti -- tree–grass competition -- Vachellia; Botany -- Periodicals; Botany; Electronic journals; Periodicals; 580
• DOI: 10.1002/ajb2.16032
• Languages: English
• ISSNs: 0002-9122

Abstract

Abstract: 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 ( K plant ) were measured as soil water potential ( Ψ soil ) declined. Trajectory of the Ψ leaf / Ψ soil 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 K plant under severe drought, they differed in their rates of water use. Acacia tortilis, which occurs in the more arid regions, initially had greater K max, transpiration ( E ), and photosynthesis ( A net ) 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.