北方典型沙地樟子松人工林等水/非等水行为特征及适应机制研究

Pinus sylvestris var. mongolica is a tree species involved in afforestation for soil and water conservation in Sandy land in northern China. However, numerous P. sylvestris var. mongolica plantations have been degraded. The survival and growth of plants depend on water loss and carbon capture by stomatal regulation. Further studies should be performed to clarify how stomatal adapts to semiarid climate and develop divergent water regulation strategies, namely, isohydric and anisohydric behaviors, to explore their adaptability to the environment, and to elucidate the decline of P. sylvestris var. mongolica plantations in terms of water relationship and carbon dynamics. In this study, P. sylvestris var. mongolica plantation, which is the dominant species in the Horqin Sandy Land, is investigated. Sap flow, environmental factors, soil/leaf water potential, photosynthetic rate, and nonstructural carbohydrate are then surveyed. The water-regulation classification methods is established to differentiate isohydric and anisohydric behaviors. The characteristics of hydraulic and carbon economic traits between isohydric and anisohydric behaviors are also compared. The coordination degree between the hydraulic responses of whole tree transpiration to changes in soil moisture and whole tree transpiration controlled by canopy stomatal conductance is examined in relation to the changes in isohydric–anisohydric behaviors. The effects of photosynthetic rate responses to changes in soil moisture on the storage and consumption of nonstructural carbohydrate are explored as well. The potential trade-offs and adaptive mechanisms of isohydric and anisohydric behaviors in the maintenance of hydraulic balance and carbon availability are also clarified. This study provide a theoretical basis for the reasonable cultivation and recession prevention of P. sylvestris var. mongolica plantations in Sandy land in northern China.

樟子松是北方沙地重要的水土保持造林树种，近年来出现了大面积枯死。植物生存和生长依赖于气孔对水分损失和碳获取的调节，明晰气孔应对半干旱气候形成的水分调节对策：等水/非等水行为，可从水分关系与碳动态角度探究其对环境的适应性并理解树木衰退的原因。本项目以科尔沁沙地樟子松人工林为研究对象，通过整合树干液流、环境因子、土壤/叶片水势、光合速率和非结构性碳水化合物。利用水分调节分类模型对樟子松等水/非等水行为进行定量分类，从水力性状和碳经济性状比较等水/非等水行为特征差异；在等水-非等水行为变化中，研究整树蒸腾对土壤水分变化的水力响应与冠层气孔导度对整树蒸腾控制相协调的程度，以及光合速率对土壤水分变化的响应及对非结构性碳水化合物贮存和消耗变化的影响，阐明樟子松人工林等水/非等水行为在维持水力平衡和碳可利用性之间潜在的权衡作用及适应机制。研究结果可为北方沙地樟子松人工林的营林建设和防止退化提供理论支撑。

我国北方半干旱地区降雨稀少、气候干燥，生态环境敏感而脆弱。为了改善区域生态环境条件，在“三北”防护林工程建设初期，营建了大面积樟子松人工林。然而全球气候变化导致区域降雨分配不均，使得沙地生态系统无法满足樟子松生长的水分需求，植物生产力持续下降，进而导致其死亡。因此阐明樟子松应对半干旱气候条件的水分调节对策和适应机制，对于深入理解林地水量失衡导致的植物水分损失和碳获取能力下降，解析其衰退死亡原因具有重要意义。项目研究结果表明，生长季内，林木冠层气孔导度与大气之间直接耦联，量纲脱耦联系数均处在较低水平，气孔对蒸腾的控制逐渐增强，气孔导度变化受制于水汽压亏缺变化，证明了大气、气孔和蒸腾三者的强控制关系。相较于成熟林和幼龄林，中龄林冠层气孔导度对水汽压亏缺的变化敏感度较高，说明在逆境环境中，成熟林和幼龄林易偏向维持原有的蒸腾状态，水分调节机制偏向于非等水行为，策略是随着整树蒸腾需求的增加而不断降低其叶水势，维持较高的冠层气孔导度和整树水力导度，以保持支撑水分到达叶片的驱动力，加之针叶树种比阔叶林林冠层降雨截留量较大，穿透雨和树干径流较小水分可利用率更低，使得其生存态势更加脆弱。室内实验也印证了野外实验结果，干旱前期，樟子松幼苗蒸腾作用未完全停止，维持一定的光合作用使得可溶性糖和淀粉含量持续升高，总体NSC呈现先升后降的趋势，并在干旱中期以淀粉的形式进行储存，但是干旱后期，淀粉开始转化为可溶性糖进而对干旱胁迫进行抵御，并修补受损器官，体现在可溶性蛋白、MDA和POD含量上也反映出该趋势变化。同时，沙地樟子松可溶性糖/淀粉比例较高，说明淀粉储量越来越少,可溶性糖获取受限并被逐渐消耗，沙地樟子松碳储存量较低，抵御灾害性天气及病虫害能力较差，长期的干旱胁迫最终会导致叶片蒸腾作用停止、内源物质消耗，叶片衰败、凋落，林冠逐渐稀疏，长期下去可能导致叶片NSC 耗竭而无法维持植株生长代谢活动，最终整株死亡。