Effects Of Seasonal Yak Grazing On Structure And Function Of Plant Community In An Alpine Meadow On The Qinghai-tibet Plateau

Alpine meadow is one of the grassland types with the largest continuous grazing area and the most sensitive response to human activities such as grazing globally,accounting for 46.7% of the grassland area on the Tibetan Plateau,and 34.5% of the degraded grassland due to improper grazing.Seasonal imbalance of forage supply in global pastures is the root cause of grassland degradation,and seasonal grazing can improve grassland productivity,but there is insufficient research on the functional characteristics of plant community structure and populations by seasonal grazing.Therefore,it is important to investigate how seasonal grazing affects the plant community structure and population functional characteristics of alpine meadows for sustainable development of alpine meadows.In this study,based on the Maqu Grassland Agricultural Experiment Station of Lanzhou University,the effects of four seasons of grazing on the species diversity,productivity and population functional characteristics of alpine meadow plant communities were investigated in an alpine meadow-yak grazing system,and the main findings were as follows:1 Effects of seasonal grazing on plant community characteristics in an alpine meadow:The species richness,Shannon-Wiener index and Pielou evenness index of the plant community were the highest in summer and the lowest in winter(P < 0.05).Pielou index in 2021 was greater than in 2022(P < 0.05).In terms of functional structure composition,the alpine meadow community structure was characterized as Gramineae > Cyperaceae > Leguminosae in spring and summer,and Cyperaceae >Gramineae > Leguminosae in autumn and winter.In terms of nutritional quality,grazing in summer and autumn reduced the neutral detergent fiber and acid detergent fiber of the plant community and increased the content of crude plant fat(P < 0.05),but the content of crude ash in the plant community was not affected by grazing season(P > 0.05).The species richness and Shannon-Wiener index of the plant community were significantly negatively correlated with the content of acidic detergent fiber and neutral detergent fiber of the plant community and positively correlated with the content of crude fat(P < 0.05).2 Effects of seasonal grazing on morphological characteristics and reproductive allocation of plants in an alpine meadow:Grazing decreased plant height of Elymus nutans and Poa pratensis in autumn,increased their basal diameter size in summer,and decreased their crown size in winter(P < 0.05).Grazing increased height,basal,and crown size of Koeleria cristata in winter,summer,and spring respectively(P < 0.05).Meanwhile,summer and autumn grazing decreased plant height and basal diameter of Kobresia graminifolia,and winter grazing increased crown size(P < 0.05).In terms of reproductive characteristics,grazing in spring and winter reduced the number of nutritional branches of the four dominant plants,while grazing in summer and autumn increased the number of reproductive branches of the four dominant plants(P < 0.05).In terms of biomass allocation,the allocation pattern of Elymus nutans and Poa pratensis was followed the order as stem > leaf > reproductive organ,and the allocation pattern of Kobresia graminifolia was followed the order as leaf > stem > reproductive organ,while the allocation pattern of Koeleria cristata was seasonally influenced by spring and autumn: stem > reproductive organ > leaf,and summer and winter: stem > leaf >reproductive organ.In terms of nutritional quality,grazing in spring and winter increased the acidic detergent fiber,neutral detergent fiber and crude ash content of Elymus nutans 、 Koeleria cristata and Kobresia graminifolia(P < 0.05),while grazing in summer and autumn increased the crude fat content(P < 0.05).3 Mechanisms of seasonal grazing on plant communities’ structure and function:Seasonal grazing rotation affected the above-ground biomass of Poaceae and Cyperaceae plants in alpine meadow plant communities by regulating their species diversity and nutritional quality.In terms of plant populations,grazing season significantly influenced the growth and reproductive characteristics of plants and thus the nutritional quality of Elymus nutans,Koeleria cristata and Kobresia graminifolia(P < 0.05),but seasonal grazing indirectly influenced growth and reproductive characteristics by affecting the nutritional quality of Poa pratensis.Therefore,grazing in summer and autumn seasons is beneficial to the development of species diversity and population reproductive characteristics of alpine meadow plant communities,and grazing in spring and winter seasons has positive effects on plant nutritional quality.The four-season grazing system effectively improves the alpine meadow plant community structure and functional characteristics of dominant populations,providing new insights into the restoration and management of grassland ecosystems.Long-term seasonal dynamic monitoring of alpine meadow ecosystems is needed in the future to explore the mechanisms of plant communities in respond to different seasons alpine meadow.