沙地樟子松树干液流对土壤水分亏缺响应的机理及阈值研究

The little changes in soil moisture in sandy land may cause large effects on the physiological activities of trees. It is therefore significant in theory and practice to quantify the relationships between soil moisture and stability of the stands. Many past attempts to judge growth conditions of a tree by the physiological activities of leaf or changes of the foliage morphological characteristics were not enough to accurately describe water status of the whole tree. However, the sap flow of the trunk can bring information related to survive, growth, reproduction and a set of comprehensive physiological information of a tree. These information can help with judging stability of a individual tree. Therefore, we hypothesize that the diurnal variation of sap flow with the developing of soil moisture deficit shows three typical forms: nocturnal sap flow, declined peak with time forward, weak peak with time lag. To test this hypothesis, two methods of the thermal dissipation probe (TDP) and the thermal field deformation (HFD) have been combined to analyze the characteristics of sap flow of Pinus sylvestris var. mongolica. A water control trial has been designed in field to reveal the rule of diurnal variation patterns of sap flow with soil water deficit, and the sudden changes of physiological activities of a tree will have been diagnosed. The key thresholds of soil water content during the process will have been found by synchronized measurements of leaf water potential and soil water potential. The results is helpful to pre-judge the stability of stands according to the soil moisture, and provide an effective basis for quantifying the vegetation carrying capacity in sandy land according to rainfall resources.

在沙地，土壤水分含量微小的变化足以引起树木生理活动的显著改变并对植被稳定性产生重大干扰。如何揭示树木生理活动对沙地土壤水分变化的敏感性及响应机制？过去主要集中在对树木枝叶的生理、形态特征变化的观测，并不能反映树木整体的水分状态，而树干液流集合了与树木生存、生长、繁殖等紧密关联的水分信息。本文以沙地樟子松人工林为研究对象，在前期研究基础上提出假说：对应于土壤水分亏缺驱动下樟子松个体生理活动下降的阶段特征，树干液流日变化依次呈现夜间液流发生、峰值下降与提前、峰值微弱与滞后三个典型特征来响应。为验证该假说，设计田间控水试验，采用热扩散（TDP）与热场形变（HFD）技术联合观测单株液流，以明确樟子松液流随土壤水分亏缺进程而变化的规律，辨析响应节点。结合对叶、土水势的同步观测，判断与液流过程显著变化相对应的土壤水分阈值。研究结果为基于土壤水分含量预判个体生长状况及植被的稳定性提供了可靠、简化的方法。

我国首批引种在科尔沁沙地的樟子松林出现的早衰甚至死亡的现象提醒我们，维护樟子松人工林的稳定事关我国“三北”防护林建设工程的成败。目前，有关樟子松生理过程响应土壤水分环境变化的机理、土壤水分阈值等关键问题仍不清楚，这一问题的系统研究有望真正找到沙地樟子松过早衰退的原因。本项研究取得的主要结论如下：.（一）沙质土壤、降水变动大、地下水位持续下降、冠层截留等共同作用形成脆弱的土壤水分生境。该地区降水的年内、年际变化大，每年土壤处于干旱状态的时长占整个生长季的29%~62%，在严重干旱年份，土壤体积含水率最低降到2.3%，接近土壤萎蔫系数。沙质土壤的保水持水能力差，在饱和后的第二天就能流失69%的水分。地下水位持续下降，已经低于樟子松最大根系分布的深度（5.7m）。.（二）沙地1m内土壤的水分状况对樟子松的蒸腾量的大小有极大影响。樟子松的蒸腾量的季节波动与土壤水分的干湿过程同步变化。樟子松蒸腾能力与1m内土层干旱的严重程度呈显著的负线性关系（P < 0.01）。1m内土层对樟子松的用水量贡献最大。.（三）连续、严重的干旱显著降低了樟子松的水分利用能力及其恢复能力。地下水位高低决定了长期土壤干旱被缓解后樟子松的用水能力能否完全恢复。连续两年干旱使得樟子松胸径生长、蒸腾量和冠层气孔导度分别下降了33.8％，51.9％和51.5％。在两年干旱解除之后并得不到完全恢复。.（四）沙地樟子松液流传输对土壤水分响应的临界点为土壤体积含水率约8.4%。樟子松林分自身的蒸腾量约占降水总量的1/3。基于章古台地区的水热条件，樟子松林现实林分的合理林分密度应控制在约393株/hm2以下。.（五）沙地樟子松不同方位液流速率值具有显著差异，但各方位液流速率值间具有高度相关性。以一个方位（北侧）液流速率的测定就能较可靠地估算科尔沁沙地樟子松大树的平均液流速率。.总之，樟子松人工林的退化主要归因于降水时空分布的高度变化，土壤持水保水能力低和地下水位持续下降。