Abstract
Key messageLeaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stem. However, although it has been intensively investigated these past decades, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Based on a trait-based model, this study theoretically supports that vulnerability segmentation enhances shoot desiccation time across 18 Neotropical tree species.ContextLeaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stems thereby preserving expensive organs such as branches or the trunk. Although vulnerability segmentation has been intensively investigated these past decades to test its consistency across species, the extent to which vulnerability segmentation promotes drought resistance is not well understood.AimsWe investigated the theoretical impact of the degree of vulnerability segmentation on shoot desiccation time estimated with a simple trait-based model.MethodsWe combined data from 18 tropical rainforest canopy tree species on embolism resistance of stem xylem (flow-centrifugation technique) and leaves (optical visualisation method). Measured water loss under minimum leaf and bark conductance, leaf and stem capacitance, and leaf-to-bark area ratio allowed us to calculate a theoretical shoot desiccation time (tcrit).ResultsLarge degrees of vulnerability segmentation strongly enhanced the theoretical shoot desiccation time, suggesting vulnerability segmentation to be an efficient drought resistance mechanism for half of the studied species. The difference between leaf and bark area, rather than the minimum leaf and bark conductance, determined the drastic reduction of total transpiration by segmentation during severe drought.ConclusionOur study strongly suggests that vulnerability segmentation is an important drought resistance mechanism that should be better taken into account when investigating plant drought resistance and modelling vegetation. We discuss future directions for improving model assumptions with empirical measures, such as changes in total shoot transpiration after leaf xylem embolism.
Highlights
The increasing frequency of extreme drought events due to climate change induces global tree mortality and forest dieback (Allen et al 2010; Aleixo et al 2019; Brodribb et al 2020)
Other hydraulic traits related to water absorption and rooting depth, transpiration control, and water storage have been identified in shaping drought resistance of plants (Blackman et al 2016; Martin‐StPaul et al 2017; Brum et al 2019)
When considering all species, including species with both positive (P88,leaf > P88,stem) and negative (P88,leaf < P88,stem) vulnerability segmentation, tcrit was positively related to the degree of P88-segmentation (Fig. 2a) as well as the degree of P50-segmentation (Fig. 2b), such that larger positive segmentation degree was associated with longer shoot desiccation time. tcrit × VPD was positively related to the stomatal-hydraulic safety margin (SHSM; MPa) when considering both P88,stem (Fig. 2c) and P50,stem (Fig. 2d), such that larger safety margin was associated with longer shoot desiccation time
Summary
The increasing frequency of extreme drought events due to climate change induces global tree mortality and forest dieback (Allen et al 2010; Aleixo et al 2019; Brodribb et al 2020). (Brienen et al 2015, 2020; Esquivel‐Muelbert et al 2019) This situation urges a better understanding of the mechanisms underlying drought-induced tree mortality, and the possibility to quantify and predict tree drought resistance (McDowell et al 2018; Brodribb et al 2020). Hydraulic failure of the water transport system due to high water stress is recognised as one of main causes of drought-induced tree mortality (Urli et al 2013; Martin‐StPaul et al 2017; Blackman et al 2019b; Brodribb et al 2020). We still lack a clear understanding of how various drought resistance traits determine drought resistance in a complementary way
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