陕北沙地不同树龄小叶杨的水分利用策略

在黄土高原树木生长经常遭受水分不足的制约,导致形成大面积＂小老树＂。选取＂小老树＂形成面积最大的小叶杨为研究对象,利用热扩散探针技术研究不同树龄（31~34年生和49~54年生）小叶杨生长季内树干液流的动态变化及与环境因子（太阳辐射、空气温度、相对湿度和降雨量）和土壤含水量的关系,揭示不同树龄小叶杨水分利用策略的差异,为黄土高原＂小老树＂形成机制的阐明及其改造提供理论基础。结果表明：1）31~34年生小叶杨一日中液流出现峰值的时间较早,而49~54年生小叶杨维持液流峰值的时间较长。2）旱季31~34年生小叶杨液流密度低于49~54年生小叶杨,且超过最大值后液流密度出现下降现象,随着雨季土壤含水量的增加液流密度会显著增加;49~54年生小叶杨雨季的液流密度低于31~34年生小叶杨,其液流密度随环境条件达到饱和后就不再发生变化,其液流对土壤含水量变化反应不敏感。3）2个树龄小叶杨整株耗水量差异不大,但31~34年生小叶杨单位面积的耗水量显著大于49~54年生小叶杨。表明低龄小叶杨液流对环境变化的敏感性较大,而高龄小叶杨的水分利用趋于保守化,对干旱表现出更强的适应性。

Water use strategies of different ages of Populus simonii on sandy land of northern Shaanxi

Tree growth on the Loess Plateau is often limited by water deficiency,thus resulting in formation of large area of ＂old and dwarf trees＂. In this study,we chose Populus simonii,a tree species that takes up the largest area of ＂old and dwarf trees＂on the loess plateau as an example,and studied changes of xylem sap flow of different ages of poplar trees（ 31- 34 a and 49- 54 a） during the growing season using thermal dissipation probe technique and the concomitant changes of environmental factors（ solar radiation,air temperature,relative humidity and rainfall） and soil moisture,aimed to elucidate water use strategy for different ages of poplar trees in this area and provide theoretical basis for clarifying the mechanism of formation of dwarf trees and their transformation. We found that： 1） 31- 34 a old poplar trees showed an earlier peak of sap flow density while 49- 54 a old trees kept a relative widerpeak. 2） The sap flow density of 31- 34 a old trees during dry seasons was lower than that of 49- 54 a old trees,but the trend was opposite during wet seasons. Sap flow density of 31- 34 a old trees during the dry seasons showed a parabolic change with environmental variates while during wet seasons it showed a sigmoildal change,but sap flow density of 49- 54 a old trees showed a sigmoildal change during both seasons. Sap flow density of 31- 34 a old trees was more sensitive to the variation of soil moisture than that of 49- 54 a old trees. There was no difference for the whole tree water use but 31- 34 a old trees had higher water use per unit leaf area. These results indicated that young poplar trees had greater sensitivity to environmental change,while water use strategy of old poplar trees tended to be more conservative and showed stronger drought adaptation.