Facing water limitation: Hydraulic strategies of co-occurring tree species from semiarid subtropical forests and their intra-specific variation along precipitation gradients

树木的分布沿着梯度的季节性和降水量是由其干旱敏感性，即他们的能力，有效地利用资源水和抵御干旱胁迫。树木已经发展出各种各样的策略来科普可用水量的减少，特别是在像季节性干旱森林这样的供水不规律的环境中。世界上最大的连续干旱森林是南美洲的查科，由于牲畜放牧和农业耕作的加剧，查科森林正在破碎化，并进一步受到降水模式变化和气温上升的威胁。因此，量化其关键树种的干旱敏感性，并选择有意义的特征与生理过程的机械联系，确定树木的水力策略，以促进可持续的土地利用管理和重新造林的尝试。树木的生产力和干旱脆弱性与结构和形态特征以及水分运输系统的解剖学特性有关，例如导水细胞之间的凹坑形态和导管大小。然而，在进化的背景下，结合水力测量生理性状的研究很少进行亚热带树木沿着环境梯度。我们目前的知识主要是基于在温带物种中观察到的模式，种内变异往往被忽视。我们将与来自阿根廷的科学家合作，研究大查科地区年降雨量从450毫米到1400毫米的降水梯度内的半干旱森林的四种主要树种。通过测量各种相关的形态（叶形态，导管间的坑形态），生理（光合活性，水分利用效率），木材解剖（导管尺寸，木材密度），和水力变量（潜在的传导性，吸水深度），我们的第一个目标是分析水传导系统和生产力之间的关系，并描述这些重要物种的水力脆弱性。其次，我们将研究在何种程度上所选择的性状的变异性可以与不同种源之间的表型可塑性或当地适应，使用现有的后代试验的树种白牧豆。包括不同白柏种源的生长表现数据将有助于评估其是否适合潜在的重新造林工作。该项目将使我们更好地了解降水减少对主要干旱森林树种的生理后果，以及水力效率和生产力之间的相互作用，并将评估这些树种对干旱强度增加的适应潜力。

The distribution of trees along gradients of seasonality and precipitation is shaped by their drought sensitivity, i.e. their ability to efficiently utilize the resource water and to resist drought stress. Trees have developed a variety of strategies to cope with reductions in water availability, particularly in environments with irregular water supply like seasonal dry forests. The world’s largest continuous dry forest is the South American Chaco, which suffers under fragmentation by intensifying livestock grazing and farming practises and is further threatened by changing precipitation patterns and rising temperatures. Therefore, it is relevant to quantify the drought susceptibility of its keystone tree species and to select meaningful traits with mechanistic links to physiological processes that define the trees’ hydraulic strategies in order to facilitate sustainable land-use management and reforestation attempts. The productivity and drought vulnerability of trees has been linked to structural and morphological features, as well as anatomical properties of the water transport system such as the morphology of the pits between water-conducting cells, and vessel size. Yet, studies integrating physiological traits in combination with hydraulic measurements in an evolutionary context have rarely been conducted in subtropical trees along environmental gradients. Our current knowledge is mainly based on patterns observed in temperate species and within-species variability is often neglected. In collaboration with scientists from Argentina, we will study four dominant tree species of the semi-arid forests in the Gran Chaco Region across a precipitation gradient from 450 to 1400 mm of annual rainfall. By measuring a variety of relevant morphological (leaf morphology, inter-vessel pit morphology), physiological (photosynthetic activity, water use efficiency), wood anatomical (vessel dimensions, wood density), and hydraulic variables (potential conductivity, water uptake depth), we first aim to analyse the relationship between the water conducting system and productivity, and to characterize the hydraulic vulnerability of these important species. Second, we will examine to which extent the variability in selected traits can be related to phenotypic plasticity or local adaptation among different provenances, using existing progeny trials for the tree species Prosopis alba. Including data on the growth performance of different P. alba provenances will allow to evaluate their suitability for potential reforestation efforts. The project will improve our understanding of the physiological consequences of precipitation reduction for dominant dry forest tree species, and of the interactions between hydraulic efficiency and productivity, and will assess the adaptive potential of the species to increasing drought intensity.