Photosynthetic Characteristics And Carbon Fixation And Oxygen Release Capacity Of Typical Plants In Helan Mountain

Forest ecosystem is an important part of terrestrial biosphere,and its carbon sink function plays a vital role in regulating climate,carbon cycle and mitigating climate warming.As one of the three natural forests in Ningxia,Helan Mountain Nature Reserve has well protected the ecological environment of Yinchuan Plain,and the ecological benefits such as photosynthesis and carbon sequestration of vegetation in Helan Mountain have played an important role in protecting the ecological environment of Helan Mountain.In view of this,in this paper,four tree species(Picea crassifolia,Populus davidiana,Pinus tabuliformis,Ulmus glaucescens),three shrubs(Berberis thunbergii,Cotoneaster hissaricus,Lonicera microphylla)and three herbaceous species(Thermopsis lanceolate,Agropyron cristatum,Carex muliensis)were selected as the research objects in the eastern foot of Helan Mountain.The functional traits and gas exchange parameters of its leaves were measured to estimate the carbon fixation and oxygen release rates of plants at different scales,and then the plants with strong stress resistance and ability of carbon fixation and oxygen release were selected to provide a scientific basis for coping with global climate change and ecological restoration in Helan Mountain.The main research results are as follows:(1)The leaf area,leaf dry weight and leaf area index of P.crassifolia were significantly higher than those of other plants.In terms of different life forms,tree plants were significantly higher than shrub and herb plants.C.muliensis had the largest specific leaf area,and from the different life forms,herbaceous plants were significantly higher than shrubs and trees,broadleaf species was larger than coniferous species.(2)The daily mean value of net photosynthetic rate(Pn)changed in a rank of T.lanceolata>U.glaucescens>P.davidiana>C.hissaricus>A.cristatum>P.tabuliformis>L.microphylla>B.thunbergii>P.crassifolia>C.muliensis,and from the perspective of different life types,it was represented as tree>herb>shrub.The diurnal variation curves of Pn of P.crassifolia,P.tabuliformis,U.glaucescens,B.thunbergii,C.hissaricus and L.microphylla were single peaks,while the Pn diurnal curves of P.davidiana,T.lanceolata,C.muliensis and A.cristatum were double peak shape,with obvious photosynthetic "noon break" phenomenon.The phenomenon of P.davidiana and A.cristatum were mainly caused by nonstomatal factors,while T.lanceolata and C.muliensis were mainly caused by stomatal factors.Compared with other plants of the same life type,P.crassifolia,P.tabuliformis,L.moicrphylla and T.lanceolata had higher daily mean water use efficiency(WUE).The correlation analysis between net photosynthetic rate and main environmental factors of different life forms showed that light and effective radiation(PAR)had the greatest effect on Pn of trees,shrubs and herbaceous plants.Leaf temperature(Ti)was the secondary influencing factor for Pn of trees and shrubs,and relative air humidity(RH)was the secondary influencing factor for Pn of herbaceous plants.Path analysis showed that light intensity(PAR)was the main determinant factor for trees and shrubs,while leaf temperature(T1)and saturated vapor pressure deficit(VPD)were the main limiting factors.The main limiting factors for herbaceous plants were air temperature(Ta)and T1.(3)The order of daily carbon fixation and oxygen release per unit leaf area is P.crassifolia>U.glaucescens>P.davidiana>C.hissaricus>A.cristatum>P.tabuliformis>L.microphylla>B.thunbergii>P.crassifolia>C.muliensis;The order of daily carbon fixation and oxygen release per unit canopy projection area is P.davidiana>P.crassifolia>U.glaucescensn>P.tabuliformis>C.hissaricus>B.thunbergii>L microphylla;The order of daily carbon fixation and oxygen release per plant is P.crassifolia>P.davidiana>P.tabuliformis>U.glaucescens>B.thunbergii>C.hissaricus>L.microphylla>T.lanceolata>A.cristatum;The order of daily carbon fixation and oxygen release per unit forest area is P.crassifolia forest>mixed forest>P.tabuliformis forest>U.glaucescens forest.In conclusion,compared with other plants of the same life type,P.crassifolia,P.tabuliformis,L.microphylla and T.lanceolata are better adapted to the environment of drought,less rainfall and poor soil in Helan Mountain,and P.crassifolia and P.davidiana have higher carbon sequeiling and oxygen releasing capacities at different scales.In the future ecological restoration of Helan Mountain,the emphasis should be placed on P.crassifolia,P.davidiana and P.tabuliformis,and shrubs and herbs with strong carbon sequestrating and oxygen releasing capacity such as P.crassifolia and C.hissaricus can be used to improve the benefits of ecological restoration measures to a greater extent.