Drought in tropical forests: the role of tree height and wood density for hydraulic efficiency, productivity and vulnerability to cavitation of trees along a lowland precipitation gradient (tropical moist to seasonally-dry)

几项研究报告说，在过去的严重干旱时期，热带潮湿和干燥森林中的树木死亡速度加快。尽管目前仍不清楚热带树木将如何应对几个气候变化情景所预测的干旱频率增加，但可以肯定的是，干旱将导致未来热带森林的群落组成和生态系统功能发生重大变化。导致树木干旱死亡的机制尚不清楚，但似乎在季节性干燥和过湿的热带环境中，高大的热带树木比较小的和林下树木承担着不成比例的更高的干旱导致树木死亡的风险。木材较轻的热带树木似乎也面临着更大的干旱导致枯萎的风险，但相关的发现并不那么一致。树高和木材密度都被认为是影响生长和存活的关键功能性状，但很少有人沿着环境梯度结合水力测量对成年树木进行研究，目的是预测树木对干旱的敏感性。鉴于气候变化，迫切需要确定易于测量的功能特征，以确定不同树木功能类型和群落的热带树木对干旱的敏感性，以评估热带森林的脆弱性。拟议的项目旨在审查水力结构、用水量、生产力、储存池中的碳投资和易受空化的脆弱性之间的相互关系，以及在哥斯达黎加跨降水梯度的大样本热带树木中木材密度和树高对它们的假设控制。在与哥斯达黎加科学家的密切合作下，将在五个过湿到季节性干燥的低地森林中对40种树种(200棵成熟树木)进行比较研究。通过测量树干和树冠树枝的一系列与生产力(木材产量、非结构性碳水化合物、树高)、水力(导水率、P50、液通量密度)和解剖参数(导管尺寸、木材密度)有关的参数，将检验关于不同物候期(常绿与落叶)、生理结构(等渗与各向异性)和干旱暴露的树木的解剖、碳关系和水分关系之间相互关系的12个假说。这些发现将有助于澄清水力衰竭和/或碳饥饿是否是干旱导致枯萎的主要原因，并有助于确定容易获得的关键功能性状，这些性状决定了热带树木在过湿地区对半干旱气候的干旱敏感性。

Several studies have reported accelerated tree mortality in tropical moist as well as dry forests in response to severe drought periods in the past. Although it still remains unclear how tropical trees will respond to increased drought frequency as predicted by several climate change scenarios, it is safe to conclude that drought will cause major shifts in community composition and ecosystem functioning in tropical forests in the future. The mechanisms leading to drought-induced mortality in trees are far from being understood, but it appears that large and tall tropical trees are carrying a disproportionally higher risk for drought-induced tree mortality than smaller and understory trees, both in seasonally-dry and in perhumid tropical environments. Tropical trees with light wood seem also to be at greater risk of drought-induced die-back, but the related findings are less consistent. Both tree height and wood density are thus thought to be key functional traits influencing growth and survival, but they have rarely been studied in adult trees along environmental gradients in combination with hydraulic measurements with the aim to predict the trees susceptibility to drought. In the light of climate change, there is an urgent need to identify easily measurable functional traits that determine the drought sensitivity of tropical trees of contrasting tree functional types and communities in order to assess the vulnerability of tropical forests.The proposed project aims at examining the interrelationship between hydraulic architecture, water consumption, productivity, carbon investment in storage pools and vulnerability to cavitation and their assumed control by wood density and tree height in a large sample of tropical trees across a precipitation gradient in Costa Rica. In close collaboration with Costa Rican scientists, 40 tree species (200 mature trees) will be studied comparatively in five perhumid to seasonally-dry lowland forests. By measuring a set of productivity-related (wood production, non-structural carbohydrates, tree height), hydraulic (hydraulic conductivity, P50, sapflux density) and anatomical parameters (vessel dimensions, wood density) from the stem and sun-canopy branches, 12 hypotheses on the interrelationship between anatomy, carbon relations and water relations in the trees with different phenology (evergreen vs. deciduous), physiological constitution (isohydric vs. anisohydric) and drought exposure will be tested. These findings will help to clarify whether hydraulic failure and/or carbon starvation are the primary causes of drought-induced die-back, and to identify easily accessible key functional traits that determine the drought-sensitivity of tropical trees in perhumid to semi-arid climates.