Effects Of The Exclosure On The Photosynthetic Characteristics Of Dominant Species And Plant Communities And Simulation Of Productivity In Alpine Meadow Of Qilian Mountains

Alpine Meadow is important grassland resources in our country that has the reputation of?Plateau Meatpacking?since ancient times.Qilian Mountains have important strategic meanings in ecological and livelihood.But due to the dual effects of natural and human factors in recent years,some degradation appeared in the alpine meadow of Qilian Mountains which influences sustainable development of alpine meadow and the ecological service function of Qilian Mountains grassland system.Grazing and enclosure are the main ways of human disturbance to the alpine meadow system.How does human disturbance affect the productivity of the grassland?To answer the question,the recognition of photosynthesis has become an important research topic,and it is also the basis of the research on the carbon cycle of the alpine meadow.On the one hand,the plant leaf photosynthetic capacity,community soil respiration and photosynthesis rate of dominant population under enclosure and grazing conditions are the precondition to understand the growth state and succession process of alpine meadow ecosystem.On the other hand,it is basis to estimate the whole ecosystem carbon sequestration of the meadow.The study will provide theoretical basis for the ecological management and ecological assessment in the study area.In order to reveal the photosynthetic capacity of the meadow community,the research focus on two dominant species,that is,Elymus nutans and Potentilla anserina under conditions of five-year exclosure and perennial grazing.Research is conducted on the differences of photosynthetic physiological parameters through field investigations.We introduce plant physiological parameters to modify the photosynthetic model and simulate the community productivity by scale transformation from leaf scale to community scale.The main results are as follows:?1?The diurnal patterns of leaf gas-exchange parameters show that the net photosynthetic rate variation of E.nutans and P.anserina has obvious double-peaked characteristics,and the second peak appears at approximately 17:30.?2?The photosynthetic efficiency,CO₂ and H₂O assimilation rate of E.nutans and P.anserina in the five-year exclosure areas are significantly higher than that in grazing areas,there is better promoting effect on the photosynthesis of E.nutans than on that of P.anserina.This effect is especially obvious at the end of the growing season.?3?The diurnal variation pattern of CO₂ flux of the community ecosystem is unimodal,and the peak appears approximately 14:00.The seasonal variation pattern indicates that the vegetation community has a strong CO₂ absorption period around August.The above-ground biomass,gross ecosystem photosynthesis rate,ecosystem dark respiration,net ecosystem CO₂ exchange of plant community in the five-year exclosure areas are significantly higher than that in grazing areas,however,the values of RES/GEP in the exclosure areas are lower than that in grazing areas.?4?The modified NRH?H-NRH?equation by introducing plant height performs better than the general NRH equation,and the H-NRH equation is more convenient and faster to predict the PLR curve of different growth states.?5?The respiration rate of a single species community is nearly unchangeable at the beginning of the growth season,and decreased at noon in the middle and late stages.The soil respiration of the communities of P.anserina and E.nutans accounts for 65.9%and 60.7%of the ecosystem respiration on both the daily scale and the growth season scale.?6?The respiration rate of a single species community increases firstly and then decreases slowly on the growth season scale.The respiration rate of the ecosystem and soil at different periods of the growth season are successively:the middle growth period,the later growth period and the early growth period.?7?The net photosynthetic rate of a single species community is estimated based on leaf net photosynthetic rate and four key parameters?i.e.,the average proportion of available light to the community,the number of leaves in the community,the leaf area index and the proportion of healthy leaves to total leaves?.The correlation coefficients of the E.nutans and P.anserina communities at different periods of the growth season are 0.7607,0.8866,0.7807 and 0.9130,0.9001,0.9144,respectively,and the model performs better for the P.anserina community than the E.nutans community.