Abstract
Drought-induced tree death has become a serious problem in global forest ecosystems. Two nonexclusive hypotheses, hydraulic failure and carbon starvation, have been proposed to explain tree die-offs. To clarify the mechanisms, we investigated the physiological processes of drought-induced tree death in saplings with contrasting Huber values (sapwood area/total leaf area). First, hydraulic failure and reduced respiration were found in the initial process of tree decline, and in the last stage carbon starvation led to tree death. The carbohydrate reserves at the stem bases, low in healthy trees, accumulated at the beginning of the declining process due to phloem transport failure, and then decreased just before dying. The concentrations of non-structural carbohydrates at the stem bases are a good indicator of tree damage. The physiological processes and carbon sink-source dynamics that occur during lethal drought provide important insights into the adaptive measures underlying forest die-offs under global warming conditions.
Highlights
Drought-induced tree death has become a serious problem in global forest ecosystems
The destruction of forest ecosystems caused by heat waves and prolonged drought has been widespread over multiple biomes[1,2,3,4], which has resulted in severe damage to the large carbon sink and high biodiversity in forest ecosystems globally
The hydraulic failure hypothesis postulates that tree die-offs are largely a result of dysfunctional water transport caused by xylem embolism[9], and the carbon starvation hypothesis suggests that die-offs are caused by shortages of carbohydrate reserves resulting from a decline in photosynthesis[5,10]
Summary
Drought-induced tree death has become a serious problem in global forest ecosystems. Two nonexclusive hypotheses, hydraulic failure and carbon starvation, have been proposed to explain tree die-offs. The Huber value or the level of defoliation has more advantage for forest managers and policy-makers to find the level of drought-induced damage in adult trees. Such defoliation can contribute to the recovery of hydraulic functions caused by the reduced transpiration area of the whole plant, it results in reduced carbon gain[22]. The physiologically complex pathway is predicted for drought-induced tree dieoffs[19], a greater understanding of tree wilting processes can assist forest managers and policy-makers in developing adaptive measures to protect forest die-offs under future global warming[8]
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