Transpiration Characteristics Of Robinia Pseudoacacia And Its Response To Driving Factors Under The Simulated Rainfall Reduction

Under the backdrop of global climate change,the Loess Plateau region experiences a decreasing trend in yearly precipitation.As a result,the water factor could potentially impact the function and stability of the ecosystem in this area.To deeply investigate the adaptation strategies of trees in the Loess Plateau to drought conditions and provide a theoretical basis for formulating rational vegetation restoration measures,this study examines Robinia pseudoacacia plantations in the sub-humid area of the Loess Plateau.Artificial rainfall exclusion treatment experiments are established,and the long-term monitoring of typical tree trunk sap flow using Granier thermal diffusion probes is conducted alongside the monitoring of key environmental factors.The study aims to analyze the effects of continuous rainfall exclusion treatment on R.pseudoacacia’s transpiration water consumption and the response characteristics of transpiration driving factors,ultimately revealing the sensitivity of canopy stomatal conductance responses to transpiration drivers under varying rainfall exclusion treatment.Revealing the sensitivity of canopy stomatal conductance to the response of transpiration driving factors that may change due to the rainfall exclusion treatment.The main results are as follows:（1）The exclusion of rainfall treatment slows down the growth rate of R.pseudoacacia stands.The growth rate of R.pseudoacacia’s diameter at breast height（DBH）in the treated plots was notably slower.Long-term rainfall exclusion treatment can increase the difference in the peak diurnal variation of trunk sap flow between the two plots.On a daily scale,the variation of R.pseudoacacia trunk sap flow in sunny conditions was similar to that of transpiration factors in both the control and treated plots.The peak time of trunk sap flux density occurred after the peak of solar radiation,but before the peak of vapor pressure deficit.In the treated plot,the peak diurnal variation of R.pseudoacacia trunk sap flow was consistently lower than that of the control plot,and the time to reach the peak was slightly delayed.As the years of the rainfall exclusion treatment increased,the gap between the two plots also increased.During the first year（2015）and third year（2017）of the rainfall exclusion treatment,there was no significant difference in the average sap flux density between the two plots.However,in the fifth year（2019）and seventh year（2021）of the treatment,there was a notable difference in the average sap flux density between the plots.This finding confirms that prolonged rainfall exclusion treatment can lead to a reduction in the trunk sap flux density of R.pseudoacacia.A strong positive correlation between daily average sap flux density and transpiration driving factors.However,the treated plot was lower compared to the control plot,indicating a weakened response of tree trunk sap flow to transpiration driving factors due to the treatment.However,the fitting slope of the treated plot was lower than that of the control plot,indicating that the rainfall exclusion treatment weakened the response of trunk sap flow to transpiration driving factors.（2）The rainfall exclusion treatment can reduce the peak value of R.pseudoacacia’s canopy stomatal conductance.The diurnal variation of stomatal conductance in R.pseudoacacia canopy maintains the same trend as solar radiation and air vapor pressure deficiency.The peak time of canopy stomatal conductance was typically later than solar radiation and earlier than vapor pressure deficit.The peak value of stomatal conductance of R.pseudoacacia canopy in the treated plot was significantly lower.From different months,the highest stomatal conductance peaks in both plots occurred in July.The average stomatal conductance of R.pseudoacacia canopy in two sample plots during the growth season（May-August）showed an upward trend with the experimental years,but the treated plots were significantly lower than the control plots（p<0.05）.The responses of the daily average canopy stomatal conductance of R.pseudoacacia to daily solar radiation or vapor pressure deficit are linear and logarithmic curve fitting relationships,respectively.The fitting slope between the stomatal conductance and transpiration driving factors of R.pseudoacacia canopy in the treated plot is always lower than that in the control plot.Due to the influence of phenological rhythms such as changes in leaf area of R.pseudoacacia stands and environmental factors such as soil moisture,the fitting slope is also higher in the early growth stage when the soil moisture is sufficient,further confirming the important role of soil moisture conditions on the average level of canopy stomatal conductance.（3）Long term arid environments can weaken the sensitivity of canopy stomatal conductance to air vapor pressure deficiency.Continuous rain exclusion treatment significantly reduced the sensitivity（m）and reference canopy stomatal conductance(G_sref)of R.pseudoacacia canopy stomatal conductance in response to air vapor pressure deficiency.Similarly,the reference canopy stomatal conductance(G_sref)is similar at different stages,but the sensitivity（m）is relatively high in the early stages of the growing season with more rainfall.On an annual scale,the sensitivity of both plots was higher in years with more rainfall.The sensitivity of the stomatal conductance of R.pseudoacacia canopy to air vapor pressure deficiency（m）is linearly correlated with the stomatal conductance of the reference canopy(G_sref).During the experimental period,the m/G_sref of both plots was greater than 0.6,indicating that the R.pseudoacacia forest in the experimental area has a strong ability to respond to air vapor pressure deficiency.After implementing rainfall exclusion treatment,the stomatal regulation ability decreased,but remained at a normal level.The results indicate that continuous rainfall exclusion treatment can decrease R.pseudoacacia’s transpiration water consumption and weaken its response to transpiration driving factors.With the extension of the rainfall exclusion treatment period,the influence mechanism of long-term soil water stress on the adjustment of transpiration and water consumption of R.pseudoacacia can be explored on a larger time scale.The research results can also serve as a reference for optimal ecosystem management in the Loess Plateau,specifically with regards to water resource supply.The research results also provide reference for the optimal management of ecosystems based on water resource supply in the Loess Plateau.