INRA, UMR 1137 Ecologie et Ecophysiologie Forestières, F-54280 Champenoux, France.
Tree Physiol. 2009 Nov;29(11):1395-405. doi: 10.1093/treephys/tpp067. Epub 2009 Sep 10.
Enhanced sapling growth in advance regeneration requires gaps in the canopy, but is often delayed after canopy opening, because acclimation of saplings to the new environment is gradual and may last for several years. Canopy opening is expected to result in an increased transpiration because of a larger climatic demand and a higher stomatal conductance linked to the higher rates of photosynthesis. Therefore, we focused on the changes in water relations and the hydraulic properties of beech (Fagus sylvatica L.) saplings during 2 years after canopy opening. We tested the hypothesis that an increase in leaf-specific hydraulic conductance and a decrease in vulnerability to cavitation occur to sustain an enhanced transpiration. Hydraulic conductance of defoliated shoots, vulnerability to cavitation, size and density of xylem vessels as well as stomatal conductance were recorded on saplings growing in shade (S saplings) or in gaps created by opening the canopy (shade-to-light, SL saplings). Hydraulic conductance per unit cross-sectional area (K(AS)) did not differ in the shoots of S and SL saplings. But a higher ratio stem cross-sectional area/leaf area resulted in a higher leaf-specific hydraulic conductance of the shoots (K(AL)) of SL saplings. Contrary to expectations, vulnerability to cavitation increased transitorily in stems during the first year after canopy opening and no difference was observed between the two treatments in light-saturated stomatal conductance. During the second year, vulnerability to cavitation was similar in the S and SL saplings and light-saturated stomatal conductance increased in SL saplings. These results demonstrate a release of the hydraulic constraints after canopy opening with an adjustment of the ratio stem cross-sectional area/leaf area. But the larger vulnerability to cavitation during the first year could limit stomatal opening and therefore the ability of beech saplings to use the available light for photosynthesis and could therefore partly explain why the growth increase was delayed to the second growing season after canopy opening.
幼树提前在更新芽中生长需要林冠空隙，但通常在林冠打开后会延迟，因为幼树对新环境的适应是渐进的，可能需要持续数年。由于气候需求增加和与较高光合作用速率相关的较高气孔导度，林冠打开预计会导致蒸腾作用增加。因此，我们在林冠打开后的 2 年内，重点研究了山毛榉（Fagus sylvatica L.）幼树的水分关系和水力特性的变化。我们检验了以下假设，即增加叶片比水力导度和降低对空化的易损性可以维持增强的蒸腾作用。在遮荫下（S 幼树）或通过打开林冠形成的空隙中（遮荫到透光，SL 幼树）生长的幼树上，我们记录了去叶枝条的水力导度、对空化的易损性、木质部导管的大小和密度以及气孔导度。在 S 和 SL 幼树的枝条中，单位横截面面积的水力导度（K(AS)）没有差异。但是，较高的茎横截面积/叶面积比导致 SL 幼树枝条的叶片比水力导度（K(AL)）更高。与预期相反，在林冠打开后的第一年，茎中的空化易损性暂时增加，并且在两种处理之间，光饱和气孔导度没有差异。在第二年，S 和 SL 幼树的空化易损性相似，并且 SL 幼树的光饱和气孔导度增加。这些结果表明，林冠打开后，水力约束得到释放，并调整了茎横截面积/叶面积比。但是，第一年较大的空化易损性可能会限制气孔开度，从而限制山毛榉幼树利用可用光进行光合作用的能力，这可以部分解释为什么林冠打开后的生长增加会延迟到第二个生长季节。
