陕北沙地小叶杨“小老树”的水力适应性

The hydraulic acclimation of old and dwarf Populus simonii trees growing on sandy soil in northern Shaanxi Province,China

以黄土高原"小老树"发生面积最大的树种-小叶杨为例,研究了不同水分生境下(水分相对好的沟道和干旱的梁坡片沙地,分别标记为生境A和生境B)小叶杨的生长、光合、水力学特性等,试图探讨小叶杨"小老树"对干旱生境的适应机制。结果表明:生境B小叶杨树高、地径、1 m树高处直径明显小于生境A,同时其主茎顶端枯枝长度大于生境A;生境B小叶杨叶净光合速率和气孔导度明显低于生境A。两种生境下小叶杨黎明前和正午叶水势无显著差异,生境B小叶杨正午时小枝枝干的比导水率明显低于生境A,但两种生境的比叶导水率则无显著差异,生境B小叶杨的Huber值明显大于生境A。生境B小叶杨枝干的P50(导水率损失50%时所对应的木质部水势)比生境A低约0.76 MPa左右,其气孔关闭的水势比生境A晚0.2 MPa左右,生境B小叶杨水分传输安全距离明显大于生境A。表明干旱生境下小叶杨高Huber值和低气孔导度有助于其叶水分关系维持相对稳定,低光合速率和维持大的水分传输安全距离所需的木质部碳投资增加是小叶杨形成"小老树"的重要原因。

One of the most serious problems occurring during the rehabilitation of vegetation in the Loess Plateau in China is the existence of large areas of old and dwarf trees as a result of irrational afforestation measures. These old and dwarf trees not only are not effective in soil and water conservation, as windbreaks and in stabilizing blowing sand, but also do not provide direct economic benefits to local farmers. Populus simonii trees make up the largest area of old and dwarf tree plantations on the Loess Plateau, so information related to how this species adapts to habitats with differing levels of water availability is critical for predicting their survival, growth and water use, for preventing the planting of additional low productivity forest plantations, and for providing theoretical guidance for the future transformation of these old and dwarf trees. Hence, the growth, photosynthesis and hydraulic traits of planted Populus simonii trees were studied with the aim of elucidating the hydraulic acclimation mechanisms of old and dwarf trees growing in two habitats, wet gully-channels （ habitat A） and dry gully-slope sandy soils （habitat B）. The results indicated that tree height, basal diameter and diameter at 1 m of tree height for poplar trees in habitat B were significantly lower and branch dieback length on the major trunk was longer than those for trees in habitat A. Poplar trees in habitat B had a lower leaf photosynthetic rate （Pn） and stomatal conductance （gs） than trees in habitat A, while no difference in leaf-level intrinsic water use efficiency was found. Predawn and midday leaf water potential were the same for trees in both habitats. Field midday specific hydraulic conductivity （Ks） of branches of trees in habitat B was lower than that of those in habitat A, but leaf-level specific conductivity （ K1 ） was almost the same ; the Huber value of poplar trees in habitat B was higher than that for trees in habitat A. Stem xylem water potential at 50% loss of conductivity （Ps0） for poplar trees growing in habitat A was -1.85 MPa, and for two plots in habitat B was -2.54 and -2.68 MPa respectively, indicating that poplar trees on dry gully-slope sandy soils had stronger cavitation resistance. Stomata of poplar trees in habitat B showed lower sensitivity to leaf water potential and closed 0.2 MPa later than trees in habitat A. Poplar trees growing in habitat B had a greater safety margin than trees in habitat A. These results showed that a larger Huber value and a lower stomatal conductance in dry habitats help this species to maintain hydraulic homeostasis in their leaves ; meanwhile, lower carbon assimilation rates and a larger carbon investment of the trees for maintaining a greater safety margin in dry habitat are probably the important reasons of formation of areas with old and dwarf trees in the Loess region.