Characteristics And Modeling Of Canopy Transpiration Of Main Tree Species In Semi-arid Region Of Chinese Loess Plateau

Estimating plant water use is an important step in assessing the effects of increasing vegetation cultivation on the hydrological cycle especially in the semi-arid Loess Plateau of China. In this study, water movement monitoring in Soil-Plant and Plant-Atmosphere system combined with sap flow techniques provided a low-cost option to study the canopy physiological transpiration of main tree species-Platycladus orientalis, Pinus tabulaeformis, Ulmus pumila and Robinia pseudoacacia response to environmental factors on a continuous basis. This research was conducted atTuqiaogou watershed of Fangshan County in Shanxi province. With the guidance of SPAC theory, we analyzed the dynamic changes of precipitation resource in plot, the soil water in forest land, the physiological moisture characteristics of different trees and the content of undergrowth. After modeling the water transpired by tree canopy, we studied the stand evaportranspiration and water requirements of different trees, consequently the relationship between water consumption and forest density was concluded based on water balance theory. These studies could provide the scientific bases for assessment of water environment capacity and the potentiality of forest constructions in semi-arid region.The major findings and conclusions are as follows:(1) Dynamic analysis of precipitation in study site. The average annual precipitation (1945～2009) in study site is 503 mm, large amplitude between more and less yearly precipitation. Above 70% of precipitation concentrated in midsummer. Inter-annual variability of precipitation reached 20.01% and 23.86% in growth season.(2) Analysis of soil water under different forest land and characteristics of undergrowth diversity. As the planting density increase, the average height, DBH and canopy area will be decreased. A number of trees in Ulmus pumila has been a serious phenomenon of dieback, reflecting that dense forests consumes more water, with the result that catchment yield may decline and even soil desiccation occurred.(3) Analysis of the physiological moisture characteristics in different trees, such as leaf water potential, the wither index, soil water depleted index etc. Physiological characteristics of main tree species in Fangshan were discussed. The results show that the mechanisms of leaf water potential response to environmental variables are different, Platycladus orientalis survivals much longer time than others. The wither index reflected as follow:Ulmus pumila (4.58%)> Pinus tabulaeformis (4.25%)> Platycladus orientalis (4.08%); Soil water use efficiency ranked from high to low as Platycladus orientalis (80.57%)> Pinus tabulaeformis (79.76%)> Ulmus pumila (78.19%).(4) Analysis of the sap flow characters and canopy conductance & transpiration in main tree species.Canopy transpiration (Ec) calculated from sap flow monitoring of TDP technology, the results show that solar radiation (Rs) and vapor pressure deficit (VPD) is the dominant environmental factors for Ec. The dimensionless decoupling coefficient (Ω) showed transpiration of main tree species in this trial was strongly controlled by stomatal conductance, and the canopy was highly coupled to the atmosphere. Canopy conductance (gc) variation was to be strongly related to atmosphere vapor pressure deficit, solar radiation and weakly to air temperature.(5) Model established for predicting the canopy conductance and (or) canopy transpiration with two different methods. The first method was of sap flow based transpiration measurements to parameterize the Jarivis-type model of canopy conductance, and simulate the canopy transpiration using the Penman-Monteith equation. Cross validation shows that this model provided good predictions of canopy conductance and transpiration for Platycladus orientalis, Pinus tabulaeformis and Ulmus pumila, in which 85%,90%, and 80% of the variability was explained respectively. The average relative error of this modeled was found to be less than the traditional porometer method.Another method based on Jarvis-Stewart model express Ec as a function of its driving environmental variables, directly estimate the Ec rather than canopy conductance, was found suitable for predicting the canopy transpiration to variation in vapour pressure deficit, solar radiation and air temperature. However, these functional forms ignored law of turbulent exchange and energy balance, the resulting error was not correctly predicted.(6) Assessment of actual tree water consumption within growing season and analysis of actual & potential evaportranspiration to estimate water environment capacity in plot. The canopy transpiration of Platycladus orientalis, Pinus tabulaeformis, Ulmus pumila were separately 156.95 mm,171.72 mm and 238.91 mm during the growing season. Actually, evaportranspiration were separately 440.10 mm, 463.08 mm and 527.92 mm. And under the theory of water balance, water environment capacity were calculated as follow:the suitable stand density volume of Platycladus orientalis and Ulmus pumila on tamped slope should be less than 3236 plant hm-2 and 1382 plant hm-2 separately (the rational catchment area should be more than 3 m2 and 7 m-2every tree). The reasonable stand density volume of Pinus tabulaeformis on natural slope should be less than 1689 plant hm-2 with the rational catchment area should be more than 6 m2.