Response Of Sap Flow Density To Throughfall Reduction In Baotianman Nature Reserve Quercus Aliena Var.acuteserrata And Its Influencing Factors

The changes in precipitation pattern is one of the important characteristics of global climate change.Some regions at mid-latitude might experience more frequent and severe drought.However,it is unclear how the precipitation change will affect the forest vegetation in those regions.The vegetation in the transition zone between northern subtropical and warm temperate zone in China has obvious north-south intersection and transitional characteristics,and the vegetation in the transition zone is more sensitive to climate change.Under the influence of climate change,water use strategy of forest ecosystem is a hot topic of research.The Baotianman Forest Ecological Station in the transitional area is distributed with deciduous broad-leaved forests dominated by Quercus aliena var.acuteserrata.The study of transpirational water metabolism of Q.aliena in this region is carried out.In this study,a field control experiment platform for simulated throughfall reduction experiment was conducted in Baotianman Nature Reserve of Henan Province since 2013.Using Granier thermal diffusion probe technology,combined with LI-6400XT portable photosynthetic instrument,the sap flow density of Q.aliena with different diameter classes and photosynthetic physiological indexes of leaves of Q.aliena were measured in the growth season of 2018.The effects of throughfall reduction on sap flow density of Q.aliena of different diameter classes and photosynthetic physiological indexes of leaves of Q.aliena were studied.The results showed:?1?The throughfall reduction in forests has effects on soil moisture content,but due to the seasonal pattern changes of rainfall,there are significant differences in different months.During July,September and October,the soil moisture content of the throughfall reduction plots?18.36%,14.73%,12.31%?was significantly lower than that of the control plots?31.38%,28.14%,24.06%?.There was no significant difference in other observational months.?2?The throughfall reduction in forests in early growing season?July?significantly reduced the sap flow density of Q.aliena.However,in the late growing season?October?,the throughfall reduction increased the sap flow density of Q.aliena significantly.Therefore,the sap flow density of Q.aliena in throughfall reduction plots and control plots responded differently to soil water content in different seasons.?3?The sap flow density of Q.aliena of different diameter sizes showed different responses to drought during the growing season,especially the sap flow density of Q.aliena of the small-trees?diameter at breast height,DBH?21 cm?was different from that of medium?21cm<DBH<30 cm?and large?DBH?30 cm?trees.For example,the sap flow density of the small trees in the throughfall reduction plots in May and July was significantly smaller than that in the control plots,and in September,the throughfall reduction plots were significantly larger than the control plots.The sap flow density of the medium trees in the throughfall reduction plots was significantly larger than that of the control plots in May,and it was significantly smaller than the control plots in July.The sap flow density of the large trees in the throughfall reduction plots was significantly smaller than that of the control plots in June,July and September,and the throughfall reduction plots in October were significantly larger than the control plots.?4?The photosynthesis physiological indexes after the throughfall reduction were lower than the control plots during the peak season,but did not reach significant levels,and the photosynthesis rate of the leaves was not synchronized with the stomatal conductance and transpiration rate.In the growing season,the highest photosynthetic rate of leaves was in July.The throughfall reduction plots and the control plots were 4.3?mol·m^-2·s^-1 and 5.1?mol·m^-2·s^-1,respectively.The highest stomatal conductance was in June.The throughfall reduction plots and the control plots were 0.062 mol·m^-2·s^-1 and 0.067 mol·m^-2·s^-1,respectively.The highest leaf transpiration rates were in June,which were 1.20 mmol·m^-2·s^-1 and 1.22mmol·m^-2·s^-1 in the throughfall reduction plots and the control plots,respectively.Stomatal conductance and transpiration rate showed a certain synergistic effect,but in June,when the stomatal conductance was large,the photosynthetic rate did not reach the maximum.?5?There was a significant correlation between sap flow density and Photosynthetic Physiological Indexes?photosynthetic rate,stomatal conductance and transpiration rate?.The strongest correlation between the mean value of sap flow density and environmental factors was the relative humidity of the air?throughfall reduction plot:R²=0.55;control plot:R²=0.57?and photosynthetically active radiation.?throughfall reduction plot:R²=0.41;control plot:R²=0.59?;There was a significant correlation between sap flow density and soil water content?R²=0.06?in the throughfall reduction plot,but there was no correlation between sap flow density and soil water content in the control plot.