Plant measurements on African tropical Maesopsis eminii seedlings contradict pioneering water use behaviour

- Tropical African pioneer Maesopsis eminii seedlings were hypothesized to cope with drought using strategies known to their ecological strategy cohort.
- Contrary to expectation, we observed low stomatal conductance (gs) in this pioneer species during control and drought stress conditions.
- Conservative water use was further mirrored through hysteresis loops in both sap flow and gs with vapour pressure deficit.
- Seedlings had however limited control over nocturnal water losses.
- Thus, M. eminii's plant-specific traits as opposed to ecological strategy traits governed its drought performance.

With increased drought events affecting forests globally, little is known about their future impact on Africa's forests. In particular, we need to gain a better understanding of how key African forest species will respond to drought stress. In this study, we investigated functional traits and physiological responses to drought of the light-demanding pioneer species Maesopsis eminii Engl. The study involved an experiment on potted M. eminii seedlings with three different drought treatments in which sap flow (SF), stem diameter variation (SDV) and stomatal conductance (gs) were measured. Whereas low gs rates (39 ± 30 mmol m−2 s−1) and pronounced SF-VPD (vapour pressure deficit) and gs-VPD hysteresis loops during well-watered conditions indicated conservative stomatal control on water loss, nocturnal sap flow implied M. eminii is not able to completely block transpiration. At the onset of drought, the pioneer seedlings retained high stem diameter growth despite highly reduced soil moisture (>−0.95 ± 0.03 MPa) and SF rates, indicating that growth was prioritised. Contribution of stored stem water to daily water use was limited both during drought and control conditions, which was confirmed by the absence of time lags between photosynthetic active radiation (PAR)-SDV, PAR-SF and SDV-SF in all treatments. Below a soil water potential of −0.95 ± 0.03 MPa, leaves were gradually shed, but diurnal and nocturnal SF did not turn zero, and absolute stem diameter kept decreasing for the entire experimental period (115 days), portraying that M. eminii seedlings can survive only limited drought periods. In sum, this study demonstrated that species-specific traits, as opposed to species ecological strategy, govern drought performance. For M. eminii, its leaf traits proved pivotal to its drought performance.

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