干热河谷主要造林树种气体交换特性的坡位效应

植物光合与水分生理特征是植物逆境适应能力评价的重要参考指标.极端生境下,坡位的选择有时成为造林成败的关键.探讨了元谋金沙江干热河谷9个主要造林树种光合与蒸腾作用在不同坡位间的空间差异及干热季向湿润季转换的时序差异.结果表明,低的坡位有助于树种维持相对高的净光合速率,且树种不同,由坡位引起的光合增益效应亦具有较大差别；低坡位的光合增益效应在干热季更为明显,而在湿润季,干热胁迫解除,低坡位的光合增益效应具有较大程度的降低；在不同坡位间发生的光合限制主要受非气孔因素主导.无论在干热季或湿润季,与净光合速率的变化情形一致,低的坡位均促进了树种的蒸腾速率.水分利用效率受坡位的影响较为复杂,因树种、季节而异.

Effects of slope position on gax exchange charateristics of main tree species for vegetation restoration in dry-hot valley of Jingsha River

Plant photosynthesis and transpiration are important reference indices for evaluating adaptable ability in adverse habits. In extreme growth environment, the selection of slope position is often becoming the key factor determining the success or failure of reforestation. Valley-type savanna is a unique type of savanna that develops in deep-incised river valleys, such as those of the dry-hot valleys in the Hengduan Mountains, southwest China, and the total area of this type in China is approximately 3.0×10⁶ hm². In the dry-hot valleys of China, the dry-hot valley of Jingsha River is typical, where the environment shows the changes from dry-hot to wet weather in one year, and because of its long dry-hot season, tree species often are difficult to survive and grow well. In this study, based on the consideration that different slope positions have different soil and atmosphere condition in the extreme habitat, the effects of slope position on photosynthesis and transpiration of nine main tree species for vegetation restoration in dry-hot season and wet season of the dry-hot river valleys were analyzed. The trial tree species include nine species, there are Azadirachta indica A.Juss, Leucaena glauca L. Benth., Tephrosia candida DC., Eucalyptus camaldolensia Dehn., Acacia auriculiformis A. Cunn., Melia toosenden Sieb.et Zucc., Dodonaea viscosa L. Jacq., Phyllanthus emblica L. and Jatropha curcas L. These tree species all grow in fields. The results showed that: comparing with upper slope position, lower slope position was in favor of keeping leaf net photosynthetic rate (Pn) a relative high level, but this favorable effect of lower slope position on Pn had obvious difference among different tree species. The effect of slope position on Pn was more obvious in dry-hot season than in wet season, while in wet season the favorable effect of lower slope position on Pn decreased at great extent because the dry-hot stress got relieved. Photosynthesis restriction, induced by raise of slope position, was mainly controlled by non-stomatal factor. Whether in dry-hot season or wet season, lower slope position both promoted transpiration rate (Tr) of the tested tree species, which was accordant with the change of Pn. The change of water use efficiency(WUE) with different slope positions was relatively complicated, the effects were different among different tree species and between the two seasons. It is obvious that the lower slope position benefits the photosynthesis of the tested tree species, and which helps trees to spend the dry-hot season. Because water deficit is one of the main factors that restrict the growth of tree species for vegetation restoration in the dry-hot valley of Jingsha River, and the leaf water potential of trees in lower slope position is more higher than in upper slope position, it can be knowed that the difference of water condition may be the main factor that cause the difference of photosynthesis in different slope positions.