Study on Physio-Ecological Characteristics of Platycladus Orientalis under Different Soil Water Condition on Loess Plateau

Soil moisture graduations of signal Platycladus orientalis on Loess Plateau were established through artificial control of water amount.The transpiration rate,net photosynthesis rate,stomatal resistance and environmental factors were measured by portable steady porometer(Li-1600) and portable photosynthesis system(Li-6200) under natural conditions in Fangshan Country,Shanxi Province.The effects of soil water on physio-ecological characteristics were studied by pot experiment.The results show that the diurnal changes of transpiration displays a double-peak curve.When the water situation is in worse condition,the time of reaching trough is much earlier than that of in better condition and the transpiration rate is lower.The diurnal changes of net photosynthetic rate displays a double-peak curve and displays a signal-peak curve in worse water condition.The predominant limiting factor of photosynthesis are stomatal conductance of stomatal limitation in the morning and low carboxylation capacity of the leafmesophyll in the afternoon.Optimum soil water content keeping better transpiration rate,net photosynthesis rate and water use efficiency is 19.90%,15.75% and 10.65% respectively.The soil water content maintaining the best water use efficiency can be chosen a critical value for tree density control and be shown as 9.5%～11% for Platycladus orientalis.