科尔沁沙地主要防护林树种水分利用策略研究

Shelter/protective forests play an important role in establishment of forest plantations in China. However, some protective forest composed of exotic tree species have declined seriously due to various reasons, especially in the Keerqin sandy land, the key area of the shelter forest system project. The decline of the shelter forests composed by the introduced Pinus sylvestris var. mongolica and Populus spp. over large areas has been paid wide attentions by the people in forestry. Previous studies indicate that water deficit is the main reason of the decline, but the native tree species did not present the decline phenomenon in the same water condition. Therefore, it is vital to determine the acclimation mechanisms of both the exotic and native tree species to the current water conditions. On the basis of reported researches by our group, we select the main shelter forest tree species (the exotic species: Pinus sylvestris var. mongolica and Populus spp.; the native species: Pinus tabuliformis, Ulmus pumila and Caragana spp.) as the objects of this study. Water condition, as the most important factor influencing the survival and growth of the trees in the shelter forests in the Keerqin sandy land, is considered as the main topics in this project. Based on the stable isotope and tree ring approaches, water use sources will be mainly studied and the water use efficiency during historical period will be reconstructed for the main shelter forest tree species. The specific objectives of this project are as follows: 1) to determine the difference in water use strategies between the exotic and native tree species; 2) to reveal the mechanisms of the exotic tree species decline and the native tree species acclimation. Besides enriching shelter forestry and guiding the construction of shelter forests, the results obtained in this project certainly will also has both important theoretical and practical significances for promoting rehabilitation and reconstruction of vegetations in sandy land.

防护林是我国人工林的主体，然而，由于各种原因，目前部分引进树种出现严重衰退，尤其在我国防护林建设核心区-科尔沁沙地，引种的沙地樟子松、杂交杨大面积死亡现象引起了林学领域广泛关注；前期研究表明，水分亏缺是主要原因，而相同水分条件下的乡土树种并没有出现死亡现象。因此，明确外来树种和乡土树种对水分条件的适应机制极为重要。本项目以科尔沁沙地主要防护林树种（外来种：樟子松、杂交杨，乡土种：油松、白榆和锦鸡儿）为对象，以影响防护林树种存活和生长的水分因子为主线，利用稳定同位素技术（δD、δ18O和δ13C）和树木年轮学方法，重点研究主要防护林树种水分利用来源，重建历史时期水分利用效率；明确外来树种与乡土树种水分利用策略的差异，揭示科尔沁沙地外来树种衰退机制和乡土树种适应机理。研究结果将丰富防护林学，对指导防护林体系建设、实现植被恢复和重建具有重要的理论和现实意义。

防护林是我国人工林的主体。然而，目前部分引进防护林树种出现严重衰退现象，而相同水分条件下的乡土树种并没有出现衰退。前期研究表明，水分亏缺是造成引进树种衰退的主要原因。因此，引进树种和乡土树种水分利用策略的对比研究是揭示引进树种衰退机制的关键。本项目以科尔沁沙地主要防护林树种（引进树种：樟子松、杂交杨；乡土树种：油松、白榆和小叶锦鸡儿）为对象，利用稳定同位素技术（δD、δ18O 和δ13C）和树木年轮学方法，研究主要防护林树种水分利用来源，重建主要防护林树种历史时期水分利用效率。主要结果如下：.1）明确了科尔沁沙地引进树种和乡土树种水分利用来源：樟子松生长季利用0~100 cm土壤水和地下水，而杂交杨生长季利用0~200 cm土壤水。油松、白榆和小叶锦鸡儿生长季利用0~200 cm土壤水和地下水。在干旱月份，地下水对樟子松耗水的贡献为39%，而地下水对油松、白榆和小叶锦鸡儿耗水的贡献分别为20%, 10% 和24%，表明与乡土树种相比樟子松在干旱条件下更加依赖地下水。与乡土树种相比，在相对干旱年份，杂交杨生长季仅利用100~200 cm土壤水。.2）重建了引进树种和乡土树种历史时期水分利用效率：与油松（无显著变化）和白榆（略有增加）年轮δ13C相比，樟子松和杨树年轮δ13C随时间变化显著增加，表明引进树种在生长发育过程中受到的水分胁迫程度不断增加。引进树种和乡土树种水分利用效率均随时间变化显著增加，但是樟子松和杨树的增加幅度显著高于油松和白榆，表明引进树种可通过提高水分利用效率适应不断增加的水分胁迫。.3）揭示了引进树种衰退机制和乡土树种适应机理：与乡土树种相比，引进树种在栽植以后，随着林龄增加，受到的水分胁迫程度逐渐增加。在极端干旱年份，土壤水不能满足樟子松和杨树生长、存活需要。地下水位由于干旱急剧下降，樟子松根系难以利用；深层土壤水由于干旱得不到补充，杨树利用难度增加，从而极易引发樟子松和杨树人工林衰退。而乡土树种在利用深层水源方面表现出较高的可塑性，从而维持正常生长。