Department of Forest and Ecosystem Science, The University of Melbourne, 221 Bouverie St, Parkville, VIC 3010, Australia.
Tree Physiol. 2014 Feb;34(2):123-36. doi: 10.1093/treephys/tpt125. Epub 2014 Feb 16.
Following disturbance many woody species are capable of resprouting new foliage, resulting in a reduced leaf-to-sapwood area ratio and altered canopy structure. We hypothesized that such changes would promote adjustments in leaf physiology, resulting in higher rates of transpiration per unit leaf area, consistent with the mechanistic framework proposed by Whitehead et al. (Whitehead D, Jarvis PG, Waring RH (1984) Stomatal conductance, transpiration and resistance to water uptake in a Pinus sylvestris spacing experiment. Can J For Res 14:692-700). We tested this in Eucalyptus obliqua L'Hér following a wildfire by comparing trees with unburnt canopies with trees that had been subject to 100% canopy scorch and were recovering their leaf area via resprouting. In resprouting trees, foliage was distributed along the trunk and on lateral branches, resulting in shorter hydraulic path lengths. We evaluated measurements of whole-tree transpiration and structural and physiological traits expected to drive any changes in transpiration. We used these structural and physiological measurements to parameterize the Whitehead et al. equation, and found that the expected ratio of transpiration per unit leaf area between resprouting and unburnt trees was 3.41. This is similar to the observed ratio of transpiration per unit leaf area, measured from sapflow observations, which was 2.89 (i.e., resprouting trees had 188% higher transpiration per unit leaf area). Foliage at low heights (<2 m) was found to be significantly different to foliage in the tree crown (14-18 m) in a number of traits, including higher specific leaf area, midday leaf water potential and higher rates of stomatal conductance and photosynthesis. We conclude that these post-fire adjustments in resprouting trees help to drive increased stomatal conductance and hydraulic efficiency, promoting the rapid return of tree-scale transpiration towards pre-disturbance levels. These transient patterns in canopy transpiration have important implications for modelling stand-level water fluxes in forests capable of resprouting, which is frequently done on the basis of the leaf area index.
受到干扰后，许多木本植物能够重新长出新的叶子，导致叶片与边材的面积比降低，树冠结构发生改变。我们假设这种变化会促进叶片生理的调整，从而使单位叶面积的蒸腾速率更高，这与 Whitehead 等人提出的机械框架一致（Whitehead D, Jarvis PG, Waring RH (1984) 在一片欧洲赤松间距实验中气孔导度、蒸腾和吸水阻力。加拿大林业研究杂志 14:692-700）。我们通过比较树冠未被烧毁的树木和 100%树冠被烧焦并通过重新萌发恢复叶片面积的树木，来检验桉树在野火后的这种情况。在重新萌发的树木中，树叶分布在树干和侧枝上，从而缩短了水力路径长度。我们评估了整树蒸腾量以及预计会改变蒸腾速率的结构和生理特征的测量值。我们使用这些结构和生理测量值来参数化 Whitehead 等人的方程，发现重新萌发和未被烧毁的树木之间单位叶面积蒸腾速率的预期比值为 3.41。这与通过 sapflow 观测测量到的单位叶面积蒸腾速率的观察比值（2.89）相似，即重新萌发的树木单位叶面积蒸腾速率高出 188%。在许多特征中，发现低高度（<2 米）的树叶与树冠（14-18 米）的树叶有明显不同，包括更高的比叶面积、中午叶片水势以及更高的气孔导度和光合作用速率。我们的结论是，这些火灾后重新萌发树木的调整有助于提高气孔导度和水力效率，促进树木尺度蒸腾作用迅速恢复到干扰前的水平。树冠蒸腾的这些瞬态模式对能够重新萌发的森林的林分尺度水分通量建模具有重要意义，而这通常是基于叶面积指数进行的。
