Characteristics Of Photosynthesis And Water Use Of Caragana Intermedia Plantation And Related Factors In Alpine Sandy Land

The increasing desertification is a serious threat to China’s economic and social development,especially in arid and semi-arid regions.The Gonghe Basin is one of the most severely desertified areas in the Qinghai-Tibet Plateau.As a typical representative of desertification in Qinghai Province,artificial measure is the most important measure to combat desertification in the region to achieve vegetation restoration and reconstruction of degraded ecosystems.In view of the contradiction between vegetation growth and water supply and demand during the ecological restoration process in this area,Caragana intermedia plantation in different ages（5a,7a,10a,12a,18a,32a）in the Gonghe Basin were selected as the research object.The photosynthesis and response curves to light and CO₂,δ¹³C andδ¹⁵N isotope,leaf traits and sap flow were measured,and the dynamic changes of rainfall and soil moisture were monitored simultaneously.The photosynthetic water consumption characteristics of Caragana intermedia in different ages were studied,and its influencing factors and response methods were analyzed.Research indicated that:（1）Forest age was significantly positively correlated with net photosynthetic rate（Pn）,stomatal conductance（Gs）,intercellular CO2 concentration（Ci）,and transpiration rate（Tr）,negatively correlated with stomatal limit value（Ls）and instantaneous water use efficiency（WUEi）.Pn was significantly associated with physiological indicators.The soil water content（VWC）of leaf Pn and different depths decreased first and then increased with the increase of forest age.The depth of soil layer with the highest correlation gradually became shallower.The correlation of Pn and VWC showed a trend of decreasing first and then increasing with the increase of soil depth.The response of plants to environmental changes changed from passive physical defense to active biochemical defense with the increase of forest age,and physiologically active reduction of environmental damage to themselves,the accumulation of dry matter gradually increased as well as drought resistance.The utilization efficiency of light energy of Caragana intermedia leaves decreased firstly and then increased with the forest age.The utilization efficiency of CO2 increased first and then decreased.The light energy and CO2utilization capacity of Caragana intermedia of 12a were the strongest among all ages.（2）Theδ¹³C,Δ¹³C and long-term water use efficiency（WUE）of plant leaves increased with the age of the forest,and the monthly change trend gradually became obvious.δ¹³C and WUE of the leaf not only originated from the plant’s own genes,but also showed a significant positive correlation with relative humidity（RH）and rainfall,and a significant negative correlation with total radiation（Rs）and potential evapotranspiration（ET₀）,whileΔ¹³C was the contrast.δ¹⁵N depended mainly on its genetic material and physiological condition.As the age of the forest increased,the ability to obtain water from the soil enhanced and strongly negative correlated with Rs.It was found that Caragana intermedia showed strong adaptability in leaf traits,water use and dry matter accumulation.（3）In the growing season,the sap flow of Caragana intermedia showed an irregular multimodal curve.The daytime sap flow rate was significantly higher than that at night,and the sap flow rate of the side branches was significantly lower than that of the trunk.The peak width of the branch sap flow showed a narrowing trend with the increase of the forest age.The flow rate of the old-aged trunk sap flow rate was significantly higher than that of other forests.The trunk sap flow rate of Caragana intermedia increased first and then decreased in different months,and the branch sap flow rate did not decrease significantly.The difference between age groups increased gradually while which between the same age group reduced.The trunk sap flow rate of all stands was negatively correlated with relative humidity（RH）,positively correlated with temperature（Ta）,total radiation（Rs）,wind speed（WS）,potential evapotranspiration（ET0）,soil temperature（Ts）,and water vapor pressure and deficit（Vpd）.The peak value of the branch sap flow after rainfall increased significantly.The start time of the branch flow of each forest age was delayed by 1 h,the increment of the start-up sap flow increased significantly with the narrower peaks,and the daytime flow stop time was postponed0.5h-1h.The rainfall events impacted on the branch sap flow lightly with the daytime sap stop time delayed by 0.5 h-2 h.（4）The soil moisture increased-decreased-increased with the soil depth,and different soil layers was significantl different and correlating.The correlation decreased with the space of soil layers.The variability of soil moisture content decreased with soil depth.The soil water storage capacity of plantations in different growth stages increased first,then decreased and then increased with the increase of stand age.As the depth increases,the soil water storage decreases first and then increases.The rainfall mainly concentrated in May to September in the Alpine Sandy Land.And rainfall event was mainly small rain,but the heavy rain contributed to the soil moisture more.The soil depth of 0-30cm,0-40cm and 0-60cm fluctuated while the rainfall was less than 5mm,.5-18mm and more than 18mm separately.The lag of soil moisture in response to rainfall increased with the increase of forest age,and the increment of soil moisture content decreased relatively.Therefore,attention should be paid to the density of seedling planting in the afforestation area,and soil and water conservation measures should be strengthened during the afforestation to reduce the impact of drought stress on the survival rate of afforestation.（5）With the increase of forest age,the water consumption of plantation shrubs increased first and then decreased.The rainfall in the study area during the test period counld meet the water demand of the artificial forests of 5a,7a,10a and 12a,but could not fully meet the water demand of the 18a and 32a plantations.Therefore,it is suggested that the plantation forest with forest age above 12a should be properly leveled to improve the quality of the forest,improve the growth of vegetation,and balance the water supply and demand of vegetation growth in the process of plantation management,.During the test period,Caragana intermedia of 12a was the most adaptable to the environment in the study area.The planting area should be appropriately reduced according to the growth period in the management process of the plantation,and the planting density should be loose.The research is of great significance in solving the management and consolidation,reconstruction and restoration of vegetation,and also provides theoretical support for the scientific management of plantation in alpine sandy land and the restoration of degraded land in ecologically sensitive areas of the Qinghai-Tibet Plateau.