Residual water losses mediate the trade-off between growth and drought-survival across saplings of 12 tropical rainforest tree species with contrasting hydraulic strategies

Abstract

Knowledge of the physiological mechanisms underlying species vulnerability to drought is critical to better understand patterns of tree mortality. Investigating plant adaptive strategies to drought should thus help to fill this knowledge gap, especially in tropical rainforests exhibiting high functional diversity. In a semi-controlled drought experiment on 12 rainforest tree species, we investigated the diversity in hydraulic strategies and whether they determined the ability of saplings to use stored non-structural carbohydrates during an extreme imposed drought. We further explored the importance of water- and carbon-use strategies in relation to drought-survival through a modelling approach. Hydraulic strategies varied considerably across species with a continuum between dehydration- tolerance and -avoidance. During dehydration leading to hydraulic failure and irrespective of hydraulic strategies, species showed strong declines in whole-plant starch concentrations and a maintenance or even an increase in soluble sugar concentrations potentially favouring osmotic adjustments. Residual water losses mediated the trade-off between time to hydraulic failure and growth, suggesting that it is linked to the 'fast-slow' continuum of plant performances and that dehydration avoidance is an effective drought-survival strategy at the sapling stage. Further investigations on residual water losses may be key to understanding the response of tropical rainforest tree communities to climate change.

Keywords: SurEau model; drought-induced mortality; hydraulic failure; hydraulic strategies; minimum conductance; non-structural carbohydrates (NSC); tropical forests; xylem embolism.