Dynamics of water-use efficiency of tree species for vegetation restoration in dry-hot river valleys

In order to reveal the dynamic characteristics of water-use efficiency of major tree species for vegetation restoration under the condition of extremely dry and hot environment in the dry-hot valleys of the Jinsha River, Yuanmou County, we compared the water-use efficiency of 14 natural tree species on typical sunny days during the dry season (March), dry-hot season (May) and wet season (October).Combining the practical growth conditions, the adaptability and application of the tested tree species were analyzed.The water-use efficiency of tree species in different seasons all could be divided into three classifications, i.e. high, medium and low water-use efficiency.The classification of some tree species varied among different seasons.The dynamic response mechanism of daily water-use efficiency displayed significant seasonal effects in the dry-hot river valleys with obvious seasonal dry-hot stress.When growth environment varied from dry in March to dry and hot in May,the peak value of daily water-use efficiency occurred earlier and the water-use efficiency declinded earlier under intensified dry-hot stress.During the stage from dry-hot season to wet season,the peaks of daily water-use efficiency presented at a relatively later time;the phenomenon of “midday depression” of some tree species vanished; the water-use efficiency of Azadirachta indica (A.) Juss, Leucaena glauca (L.) Benth., Acacia glauca (L.) Moelichl and Cajanus cajan (L.) Millsp. got improved greatly,indicating that these tree species had a higher photosynthesis capability under the condition of enough water supply,which was beneficial for their survival during the wet season in the dry-hot river valley.Five other tree species,Phyllanthus emblica L., Dodonaea viscosa L., Tephrosia candida DC., Jatropha curcas L. and Albizia kalkora (Roxb.) Prain,presented relatively high or medium water-use efficiency both during the dry-hot season and wet season.