Characteristics Of Photosynthesis And Water Use Of Dominant Tree Species In Two Typical Forests And Related Factors In The Semiarid Region Of Loess Plateau, China

The semiarid region of Loess Plateau of China is characterized by its extremely fragile ecosystems. Severe soil erosion and drought are the two major ecological problems in this region. Sustainable restoration of vegetation is the fundamental approach to the control of soil erosion and ensurance of the regional ecological security. According to the contradiction between revegetation and water deficiency occurred during the process of vegetation restoration, two typical forests in the semiarid region of Loess Plateau, i.e. Robinia peseudoacacia plantation and Quercus liaotungensis forests, were selected in this study. In forests of Mt. Gonglushan of Yan'an city, the leaf scale characteristic of photosynthesis and transpiration of the dominant tree species of the two typical forest types and the corresponding influencing factors were studied using the LI-6400 portable photosynthesis system. With the thermal dissipation probe sap flow measuring system and automatic meteorological station, the stem level water use of each tree species of the two typical forests and relevant environmental factors were monitored in situ in the same forests region. The influencing factors of sap flow of each tree species in the two typical forests were analyzed systematically. The stand level water use of the two typical forests in the growing season of 2008 was assessed. The main results are as follows:1. The photosynthesis diurnal courses of the dominant tree species of the two typical forests, i.e. R. peseudoacacia and Q. liaotungensis, were bimodal curve or single-peak curve during clear days of the growing season. Both the two dominant tree species had a significant photosynthetic midday depression phenomenon; and the main reason for this depression of the two tree species included stomatal factor and non-stomatal factor. High solar radiation, high VPD and high temperature are the major ecological factors that induced the photosynthetic midday depression of the two tree species. The maximum net photosynthetic rate of R. peseudoacacia was higher than it of Q. liaotungensis while the daily mean photosynthetic rate of R. peseudoacacia was less than it of Q. liaotungensis. The net photosynthetic rate of R. peseudoacacia and Q. liaotungensis had extremely significant correlation or significant correlation with stomatal conductance, intercellular CO2 concentration, available photosynthetic radiation, air temperature, air CO2 concentration, air vapour pressure deficit (VPD) and air relative humidity during growing season. The physiological factors, including stomatal conductance and intercellular CO2 concentration, were the major factors influencing the net photosynthetic rate of the two tree species. The ecological factors had indirect effect on the net photosynthetic rate of the two tree species by influencing stomatal conductance and intercellular CO2 concentration. The analysis of light response curve of R. peseudoacacia and Q. liaotungensis showed that the ability of carbon fixation and ability of using low light and high light of the two tree species were decreased obviously with the enhancing of soil water stress.2. The diurnal courses of transpiration of leaves of R. peseudoacacia and Q. liaotungensis were single-peak curve or irregular multi-peak curve during clear days of growing season. The seasonal differences of the diurnal courses of transpiration of leaves of the two tree species during growing season may be related to the different biologic characteristics of the two tree species or the differences of ecological factors of the different stages of growing season. The transpiration rate of R. peseudoacacia and Q. liaotungensis leaves had extremely significant correlation or significant correlation with stomatal conductance, available photosynthetic radiation, air temperature, air relative humidity and VPD during growing season. The ecophysiological factors influencing the transpiration rate of the two tree species varied with the different months during growing season.3. The diurnal courses of sap flux density of each tree species of R. peseudoacacia plantation and Q. liaotungensis forests was similar to the diurnal courses of solar radiation and VPD in clear days. The average sap flux density of each diameter at breast height (DBH) class of dominant tree species (R. peseudoacacia, Q. liaotungensis and Arrmeniaca sibirica) of the two typical forests differed among DBH classes significantly. However, we did not find a correlation between DBH and average sap flux density of R. peseudoacacia, Q. liaotungensis and A. sibirica in this study. This implies that sap flux density of each dominant tree species of the two typical forests may be controlled by multiple ecophysiological factors. The diurnal courses of the stand average sap flux density of each tree species of the two typical forests were single-peak curve in clear days and bimodal curve in cloudy days and overcast days during growing season. The daily mean stand average sap flux density of each tree species in different weather conditions was: Clear days > cloudy days > overcast days. This may be resulted from the significant difference of ecological factors among different weather conditions. The daily sap flow starting time, duration, peak value and the time of peak value of each tree species of the two typical forests differed evidently among the different months during growing season. In general, the daily maximum stand average sap flux density was lower at the beginning and the end of growing season and higher in the peak of growing season respectively. The stand average sap flux density of each tree species was influenced by the combined factors of solar radiation, air temperature, air relative humidity and VPD during most of growing season. The stepwise multiple regression model of ecological factors and stand average sap flux density of each tree species of the two typical forests in different months during growing season were all extremely significant. And, ecological factors of the stepwise multiple regression models are able to explain the most variation of the stand average sap flux density of each tree species in different months during growing season.4. The power function model can be used to fit the relationship between sapwood area and DBH of the dominant tree species of R. peseudoacacia plantation and Q. liaotungensis forests. Sapwood area of each tree of the dominant tree species in experimental plot can be calculated by DBH of tree and the corresponding fitted model. The stand average sap flux density of each tree species of R. peseudoacacia plantation and Q. liaotungensis forests were differed obviously among different months of growing season. The seasonal dynamics of stand average sap flux density of each tree species may be related to the differences in leaf phenology, meteorological factors and availability of soil water. The missing stand transpiration data of the two typical forests can be estimated on the basis of the nonlinear regression model between stand level transpiration of each tree species and the mean daily daytime VPD (VPDm) of each month. Stand transpiration of the two typical forests were higher from May to July. Total stand transpiration of R. peseudoacacia plantation and Q. liaotungensis forests during the growing season of 2008 were 73.8 mm and 127.85 mm, respectively. Daily mean stand transpiration for R. peseudoacacia plantation and Q. liaotungensis forests in the region were 0.41 mm day-1 and 0.63 mm day-1, respectively. The relatively lower daily mean stand transpiration for the two typical forests may be related to the relatively low leaf area index and a low ratio of sapwood area to ground area.5. Except for the 0-10cm layer, the soil bulk density of Q. liaotungensis forests was higher than it of R. peseudoacacia plantation. Soil bulk density of the two typical forests remained constant below underground 100cm. The variations of soil moisture of the two typical forests were related with the climate factors changes and stand transpiration dynamics. The variations of soil moisture in 0-300cm soil layer of the two typical forests were higher while the soil moisture of 300-500cm soil layer of the two typical forests kept a relatively stable low value. The mean soil moisture and cumulative soil water storage of underground 300-500cm of Q. liaotungensis forest was higher than the corresponding value of R. peseudoacacia plantation. Soil moisture of Q. liaotungensis forests dominanted by the native tree species was better than it of the plantation which was mainly composed by the exotic tree species (R. peseudoacacia).