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

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.

Hungry herbivores and thirsty plants: Browsing wildlife shape savanna tree transpiration independently of water use strategies

“…On the other hand, although our extensive sap flow measurements encompassed a wide range of soil moisture contents including dry conditions, we found no evidence suggesting that transpiration was limited by VPD, which suggests a water-spending strategy. This classification is further supported by a previous study that found an anisohydric water-spending strategy in one-year-old S. mellifera seedlings, although strategies may differ across life stages(Cory et al, 2022). We suppose that the water spending strategy of S. mellifera is enabled by a xylem architecture characterised by narrow and short vessels, which prevents xylem embolism and allows sap flow to be maintained under dry conditions(Hacke & Sperry, 2001).Across all three woody species, we found that sap flow is strongly positively correlated with shallow soil moisture (10 cm depth), indicating potentially intense competition with shallow-rooted grasses.A correlation between sap flow and soil moisture at a depth of 50 cm is found for C. mopane and C. alexandri.…”

Tree and grass competition in savannas: Interactive effects of soil moisture, grass defoliation and grass species identity on the growth and survival of tree seedlings

“…A recent experiment by Cory et al. (2022) demonstrated that V. tortilis seedlings transpire and drain soil moisture faster than V. robusta , which contributed to faster drying and, in our case, thinner stems (Figure 2c) and low survival (Figure 3). Alternatively, poor stem growth of V. tortilis relative to V. robusta seedlings could be attributed to differences in water use efficiency under competition.…”

“…The fact that V. robusta seedlings stems did better than V. tortilis, especially under unclipped grasses, but poorly under grass removal may be related to their difference in transpiration rates and water use. A recent experiment by Cory et al (2022) demonstrated that V. tortilis seedlings transpire and drain soil moisture faster than V. robusta, which contributed to faster drying and, in our case, thinner stems (Figure 2c) and low survival (Figure 3). Alternatively, poor stem growth of V. tortilis relative to V. robusta seedlings could be attributed to differences in water use efficiency under competition.…”

“…The dry part of the ecosystem where V. tortilis seems to do well, tend to experience episodic rainfall and very high soil moisture in short time windows which then declines rapidly due to sparse tree cover and edaphic conditions (Reed et al., 2009). Better growth response in grazed/short grass and environment may be a strategy that would benefit young V. tortilis in the dry parts of the ecosystem at the expense of being competitive in the wetter parts (Cory et al., 2022; Stave et al., 2005).…”

“…We were able to show how stem diameter and seedling height of both tree seedlings are impacted by the variability in competition with grasses. Reduced survival probability for V. tortilis seedlings, for the most part, was exacerbated by competition with grasses, probably because they transpire and drain soil moisture quicker than V. robusta, as demonstrated by Cory et al (2022), leading to thinner stems. The question that remains unanswered here is how do higher transpiration rates in V. tortilis help with their endurance in drier conditions.…”