Research Of Carbon-water Coupling Mechanisms Of Typical Vegetation In An Arid Oasis Ecosystem

Oasis plays a very important role in regulating regional climate and feeding population in the arid area of northwest China,although it covers less than 5% of the total area.Water resource is the most valuable natural resource in oasis ecosystem,which has a significant impact on restricting vegetation survival and evolution.A better understanding of the carbon-water coupling mechanism of diverse vegetation which grow in environment with limited water can not only be helpful for guiding local scientific planting and irrigating,but also be of great significance in predicting the feedback mechanism of vegetation-atmosphere in the context of climate change.However,the mechanisms on the coupled relationships between photosynthesis and transpiration at leaf scale have not been reported in detail,especially in arid oases.Thus,we conducted a series of field experiments in Nanhu Oasis,supported by Dunhuang Eco-hydrological Observation Station.Meanwhile,some laboratory tests and related models were also performed in this study.Firstly,we revealed photosynthetic characteristics and photosynthetic capacity of 36 species which belonging to 6 plant functional types?PFTs?,respectively;then,explored the behavior of photosynthesis-transpiration coupling and its influence factors;meanwhile,revealed the impact of stomatal control on carbon-water coupling behavior;finally,compared the water use efficiency?WUE?among species.The main conclusions are as follows:?1?Characteristics of diurnal courses and seasonal variation of the net photosynthesis rates?A_n?among species were basically identical,while the carbon fixation capacity(A_nmax):the diurnal courses were similar to parabolic curves,with single-peak or double-peak;affected by both environmental factors and physiological conditions,A_n showed obvious seasonal variation which firstly increased and then decreased from May to October,and reached the maximum value in August;C4vegetation had the strongest carbon fixation capacity,A_nmax can reach up to 38.77 mol m^-2 s^-1,while ENF had the lowest carbon fixation capacity,only about 6.39 mol m^-2 s^-1.?2?Sensitivity analysis effectively identified key parameters among all photosynthetic parameters.Based on the result of sensitivity analysis,we created the hierarchical Bayesian structure which had the ability to estimate parameters across diverse species at different levels?i.e.,plant,species,and PFTs level?simultaneously.The result showed that model with the optimized parameter sets had higher accuracy than original model;meanwhile,each parameter can be ensured its inherent biochemical meanings.Otherwise,the posterior distributions of key parameters showed significant differences among species.Parameter variation at species level should be paid more attention in the carbon-water coupling model in the further.?3?Stomatal conductance?g_s?was regulated by both external environmental factors and and their own hydraulic characteristics:first,there were obvious clockwise hysteresis loops between stomatal conductance?g_s?and the main environmental factors?e.g.,photosynthetic effective radiation,PAR;water vapor pressure deficit,VPD;and leaf temperature,T_a?,respectively,which caused by the hysteresis effect between g_s and these environmental factors;second,the regression equation between g_s and leaf water potential??_L?was similar to the Weibull cumulative distribution function,which suggested the relationships between g_s and?_L were highly varied at different period of time.On the basis of stomatal behavior analysis,this study estimated parameters of the optimal stomatal conductance model.The results showed that model with optimized parameters can well reflect the dynamics of g_s in arid oasis.Additionally,the stomatal conductance slope?g₁?showed significant variability among species,g₁ values were obviously lower in C4 species than in other species;for C3 species,g₁ values were much higher in cropland?CRO?and grassland?GRA?than in open shrubland?OSH?and evergreen needleleaf forest?ENF?,intermediate in deciduous broadleaf forest?DBF?.This result suggested g₁ variation be considered in the carbon-water coupling model in the further.?4?The leaf photosynthesis and transpiration were tightly coupled of plants in the arid oasis:first,photosynthesis and transpiration were driven by the same environmental factors,which was demonstrated by the similar daily course and linear relationships between A_n and T;second,stomata controlled the exchanges of CO₂ and vapor between plants and atmosphere simultaneously,A_n-g_s and T-g_s exhibited stably positive relationships.Meanwhile,the photosynthetic and transpiration coupling behavior showed difference characteristics among PFTs:for ENF,OSH,and C4 plants,A_n-T had fairly linear relationships,suggesting that stoma had strong restriction to photosynthesis and transpiration;while for DBF,GRA,CRO,relationships between A_n and T would vary with the change of environment.With the increasing of PAR,T_a,and VPD,g_s increased gradually.When g_s reached a certain level,restriction of stoma to photosynthesis became weaker,and photosynthesis-transpiration process began decoupling.?5?WUE,a quantitative measurement of relationships between carbon and water coupling,is affected by both environmental and physiological factors.First,WUE had significant variation among PFTs:WUE were significantly higher in C4 species than in other species;for C3 species,WUE were highest in evergreen needleleaf forest,then open shrubland,and relative low in cropland and grass land,intermediate in deciduous broadleaf forest;WUE was exponentially related with leaf dry matter content?P<0.01?,which suggested that WUE was affected by the trade-off between rapid growth and nutrient storage.Second,WUE can be influenced by temperature increase,drought,and CO₂ concentration enrichment.WUE showed negative feedback to T_a and soil water content?SWC?,and positive feedback to C_a.Relative contribution of T_a,SWC,and C_a to WUE was different among PFTs:the relative contribution of T_a to WUE was bigger in DBF,GRA,and CRO than that in OSH and C4;the relative contribution of C_a to WUE was obvious bigger in C4 than that in other PFTs.