Università degli Studi di Padova, Dept Territorio e Sistemi Agro-Forestali, Forest Ecology Research Unit, Viale dell'Università 16, Legnaro (PD), Italy.
Centre for Carbon Water and Food, Faculty of Science, University of Sydney, 380 Werombi Road, Brownlow Hill, NSW, Australia.
Tree Physiol. 2019 Mar 1;39(3):495-502. doi: 10.1093/treephys/tpy110.
Anatomical traits such as xylem conduit diameter and vessel connectivity are fundamental characteristics of the hydraulic architecture of vascular plants. Stem xylem conduits are narrow at the stem apex, and this confers resistance to embolisms that might otherwise be induced by large, negative water potentials at the top of tall trees. Below the apex, conduits progressively widen and this characteristic minimizes effects of path length on total hydraulic resistance. While interconnections among xylem vessels have been noted for decades, their role(s) are not fully clarified. For example, we do not know if they allow water to bypass embolized vessels, or increase the risk of spread of embolisms, or how their arrangement varies within a tree. Here we demonstrate the benefit of removing the independent effect of stem length on assessment of effects of external (e.g., climatic) factors on such xylem traits. We measured the hydraulic diameter (Dh) and vessel conductivity index (VCI) along the stem of 21 shrubs/trees of similar height (1.19 < H < 5.45 m) belonging to seven Acacia species, across a wide aridity gradient in Australia. All trees showed similar scaling exponents of Dh (b = 0.33) and VCI (b = 0.53) vs axial distance from the apex (L), thus conforming with general patterns in woody plants. After de-trending for L, neither Dh (P = 0.21) nor VCI (P = 0.109) differed across the aridity gradient. We found that across a wide gradient of aridity, climate had no effect on xylem anatomy of Acacia spp, which was instead dictated by axial distances from stem apices. We argue that the use of standardization procedures to filter out intrinsic patterns of vascular traits is an essential step in assessing climate-driven modifications of xylem architecture.
解剖学特征，如木质部导管直径和导管连通性，是维管植物水力结构的基本特征。茎木质部导管在茎尖处较窄，这可以抵抗由高大树木顶部的大负水势引起的栓塞。在茎尖以下，导管逐渐变宽，这一特征将导管总阻力受路径长度的影响降到最低。几十年来，人们已经注意到木质部导管之间存在连通，但它们的作用尚未完全阐明。例如，我们不知道它们是否允许水绕过栓塞的导管，或者增加栓塞传播的风险，或者它们的排列在树内如何变化。在这里，我们证明了消除茎长对评估外部（例如气候）因素对这些木质部特征的影响的独立效应的好处。我们测量了澳大利亚广泛干旱梯度中属于七个金合欢属物种的 21 个灌木/树木的相似高度（1.19<H<5.45 米）的茎中沿着茎的水力直径（Dh）和导管传导指数（VCI）。所有树木的 Dh（b=0.33）和 VCI（b=0.53）与距茎尖的轴向距离（L）的比例指数都相似，因此符合木本植物的一般模式。在去除 L 的趋势后，Dh（P=0.21）和 VCI（P=0.109）都没有随着干旱梯度的变化而变化。我们发现，在广泛的干旱梯度中，气候对金合欢属植物的木质部解剖结构没有影响，而是由距茎尖的轴向距离决定的。我们认为，使用标准化程序过滤掉血管特征的内在模式是评估木质部结构气候驱动变化的重要步骤。
