Death from drought in tropical forests is triggered by hydraulics not carbon starvation

Abstract

17Drought threatens tropical rainforests over seasonal to decadal timescales [1][2][3][4] , but the drivers 18 of tree mortality following drought remain poorly understood 5,6 . It has been suggested that 19 reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk 20 through insufficient carbon supply to metabolism ('carbon starvation') 7,8 . However little is 21 known about how NSC stores are affected by drought, especially over the long term, and 22 whether they are more important than hydraulic processes in determining drought-induced 23 mortality. Using data from the world's longest-running experimental drought study in tropical 24 rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the 25 water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from 26 mortality in the experimentally-droughted forest increased substantially after >10 years of 27 reduced soil moisture availability. The mortality signal was dominated by the death of large 28 trees, which were at a much greater risk of hydraulic deterioration than smaller trees. 29However, we find no evidence that the droughted trees suffered carbon starvation, as their 30 NSC concentrations were similar to those of un-droughted trees, and growth rates did not 31 decline in either living or dying individuals. Our results indicate that hydraulics, rather than 32 carbon starvation, triggers tree death from drought in tropical rainforest. 34Drought-response observations from both field-scale experiments and natural droughts have 35 demonstrated increased mortality over the short-term (1-3 years), with notably higher 36 vulnerability for some taxa, and for larger trees 6,9,10 . After several years of drought, 37 recovering growth rates in smaller trees, dbh (diameter at breast height) <40 cm, and reduced 38 mortality have been recorded at different locations 6,11,12 . However, the long-term (>10 yr) 39 sensitivity of tropical forests to predicted prolonged and repeated water deficit [1][2][3] we synthesise these data to test whether long-term soil moisture deficit alters NSC storage 64 and use in tropical rainforest trees, and if this, or hydraulic processes, are most strongly 65 associated with increased mortality rates. 66By 2014, following 13 years of the TFE treatment, cumulative biomass loss through mortality 67 was 41.0±2.7% relative to pre-treatment values (Fig. 1a), and the rate of loss had increased 68 substantially since the previous reported value of 17.2±0.8%, after 7 years of TFE 6 . 69Accelerating biomass loss and failure to recover substantially, or to reach a new 70 equilibrium 13 , has led to a committed flux to the atmosphere from decomposing necromass of 71 101.9±19.1 Mg C ha -1 (Fig. 1a). This biomass loss has been driven by elevated mortality in 72 the largest trees (Fig. 1b), as previously observed over shorter timescales 6 , and has created a 73 canopy that has had a persistently lower average leaf area index during 2010-2014 74 (12.0±1...

Land-use and climate change risks in the Amazon and the need of a novel sustainable development paradigm

“…This behavior is contrary to the hypothesis, previously assumed, that the larger trees would be more resilient to droughts as a result of a deeper root system, allowing them to capture water from the deeper soil layers as a drought survival strategy. It was observed, however, that these large trees could be under water stress due to a significant exposure to solar radiation, eventually dying by cavitation and embolism (113) during extreme droughts.…”

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“…Finally,Eqs. (2a,2b) provides a testable hypothesis explaining why larger trees are more vulnerable to drought (Nepstad et al 2007) or have an apparently lower xylem P50 compared to smaller trees of the same species (Rowland et al 2015).…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…This may provide an advantage during short-term droughts but not necessarily during longer, more intense drought events. The ability to moderate water uptake and growth may be absent in the tropics; Rowland et al [66] found no change in the growth rate of trees in the Amazon under a long-term drought.…”