Factors affecting transpiration of Pinus tabulaeformis in a semi-arid region of the Loess Plateau

The effects of soil water and meteorological factors affecting transpiration of Pinus tabulaeformis were studied under different levels of soil water content to offer a scientific basis for increasing efforts in afforestation survival and management of soil water in forested land. Under artificial control methods for soil water and potting experiments, the transpiration rate (T r) of P. tabulaeformis and environmental factors were measured using a portable steady porometer (Li-1600) and a speedy weight method (BP-3400) during a representative fine day in the growing season of 2004. The results indicated that the diurnal course of T r and R st of P. tabulaeformis displayed a double-peaked curve and a “W” curve under different levels of soil water content. Given a representative fine day, the T r could be represented as a cubic relation with soil water content (SWC). The SWC which caused maximum T r values of P. tabulaeformis was 17.7%, 19.8%, and 17.5% in July, August and October respectively. T r was affected not only by physiological characteristics, but also by SWC and meteorological factors. T r was significantly correlated with meteorological factors when the soil water was sufficient, but this correlation would decrease under conditions of serious water stress. Under such stress conditions, air temperature was the primary factor to affect T r in July and August and photosynthetically active radiation (PAR) was the primary factor in October. When soil water is sufficient, the main factors affecting T r were relative humidity (RH), air temperature (T a) and leaf temperature (T l) in July, August and October respectively.