Observation And Simulation Of The Effects Of Environmental Factors On Transpiration For Three Tree Species In The Dongting Lake Basin

The responses of transpiration to environmental factors(solar radiation, vapor pressure deficit and soil water potential) were examined for three tree species(Osmanthus fragrans, Cinnamomum camphora and Liquidambar formosana) over three years of distinguished precipitation amount(drought summer in 2013 and wet in 2014 & 2015) based on a recently developed transpiration model(the modified Jarvis-Stewart model(3)). The model calculated transpiration from a maximum value scaled down by three functions of individual environmental factors. The study compared the effects of different environmental factors on three species’ transpiration, proved the dual impact of VPD to transpiration, analyzed the temporal variation of the constraints of three factors on transpiration, and identified the main factor in different years. Performance of transpiration simulation using three models-- the Modified Jarvis-Stewart model(4), the Modified Jarvis-Stewart model(3) and Buckley model were compared and evaluated. An advanced optimization algorithm(DREAM) was used in the study to calibrate the models.The results indicate that:(1) The response of Osmanthus fragrans and Liquidambar formosana transpiration to solar radiation is more sensitive than Cinnamomum camphora, while Cinnamomum camphora transpiration appears to be more sensitive to VPD than Osmanthus fragrans and Liquidambar formosana. These results suggest that the response of plant transpiration to environmental factors varies from species to species.(2) The dual effects of VPD on transpiration are observed for 2013, in which an optimal VPD for transpiration appears. Away from this optimal VPD value, transpiration decreases. Compared to 2013, VPD in 2014 and 2015 is smaller, transpiration increases with VPD.(3) The changes of the limitation of all factors in 2013 can be divided into three periods: pre-drought, during the drought and after the drought. Water potential has little effects on transpiration in the pre-drought condition, while its influence becomes dominated during the drought. Over the three years, the limitation of water potential for transpiration is more obvious in summer than in other seasons. While the stress of solar radiation and VPD fluctuate in a year and both of them have a similar pattern of temporal variation, which is mainly affected by rainy days.(4) Averaged over the whole growing season, the primary impact factor of plant transpiration is water potential in 2013, VPD in 2014 and 2015. The result shows that the main factor of the plant transpiration is the difference of hydrological and meteorological conditions between different years, rather than the difference of tree species.(5) By comparing the effects of three transpiration model(MJS(4), MJS(3) and Buckley model), we find that the suitable models of three target tree species are different. For Osmanthus fragrans, the MJS(4) model is more suitable than the MJS(3) model and Buckley model, the MJS(3) model is the best for Cinnamomum camphora, and the Buckley model for Liquidambar formosana. The NSE values of all suitable models are bigger than 0.5, which shows the effects of the transpirations model in this study are reliable.