Study On Plant Functional Trait And Community Phylogenetic Assembly Based On Different Grazing Intensity In Alpine Steppe Habitat Of The Qinghai-Tibet Plateau

Alpine steppe ecosystem on the Qinghai-Tibet plateau plays a very important role in water conservation, biodiversity conservation, greenhouse gas mitigation, etc. However, in recent decades, a larger number of areas are degraded in alpine steppe habitats of the Qinghai-Tibet plateau. How grasslands are used by grazing reasonably, which always been concerned by many scientists. Numerous studies showed that the unreasonable grazing was the main cause of alpine grassland degradation. Many studies demonstrated using plant functional trait (PFT) can predict plant response to grazing. Besides, grazing also affects grassland ecosystem function, and AGB (above-ground biomass) and soil chemical property are considered as the important indexes for evaluation of grassland ecosystem function. Grazing also affects phylogenetic structure of community, which includes historical information of co-existed species and reflects evolutionary and ecological process of communiy assembly. However, so far, relatively little is known about PFTs changing trend of plant species, the relationship between plant functional traits and above-ground biomass and soil chemical property, phylogenetic assembly of community under grazing in alpine steppe habitat of the Qinghai-Tibet plateau.Alpine steppe of Qinghai-Tibet plateau was used as the research object in this paper. A grazing experiment was conducted in Yikewulan town, Gangcha County, Qinghai Province, located at the northeastern part of the Qinghai-Tibet plateau. Grazing experimental site was fenced off from public pasture with relatively consistent pasture quality and was a total area of 10.11 ha in 2009. The grazing experiment began in June 2010 using six grazing intensity (0,1.72,2.29,2.87,4.13 and 5.62 sheep/ha) with three replicates. The research in this paper was carried out in 2012 and 2013. Six dominant plant species including Elymus nutans, Stipa purpurea, Poa crymophila, Kobresia humilis, Kobresia pygmaea and Medicago ruthenica and plant functional group including Grasses, Sedges and Legume were studied in this paper. These indexes including plant height (PH), plant weight (PW), leaf area (LA), leaf dry mass (LDM), specific leaf area (SLA), nitrogen content per unit mass (Nmass), leaf phosphorus content (LPC), leaf nitrogen content:leaf phosphorus content (LeafN/P), AGB, soil organic carbon (SOC), soil total nitrogen (STN) and soil total phosphorus (STP) were measured, in order to address the questions as follows:(1) Response of PFTs of these six dominant species(Elymus nutans, Stipa purpurea, Poa crymophila, Kobresia humilis, Kobresia pygmaea and Medicago ruthenica) and plant functional group (PFG) including Grasses, Sedges and Legume to different grazing intensity; The relationship between PFTs and AGB and soil chemical property; (2) The effect of different grazing intensity on community structure and species’ diversity; (3) The effect of different grazing intensity on phylogenetic strcture of community. The main findings were as follows:(1) With the increase of grazing intensity, these PFTs including PH, LA and LDM of these six species (E. nutans, S. purpurea, P. crymophila, K. humilis, K. pygmaea and M. ruthenica) and PFG including Grasses, Sedges and Legume showed consistent responses and decreased significantly. These six species and PFG tended to become miniaturization and showed the characteristic of small plant and leaf size under heavy and extreme grazing pressure. Although the PW of K. humilis and Sedges increased firstly from ungrazed to very light grazing intensity and then decreased, PW of other species and PFG showed decreased trend significantly. SLA of Grasses and Sedges decreased firstly then increased, but the SLA of Legumes showed not consistent change trend. Meanwhile, SLA of these four dominant species (E. nutans, S. purpurea, P. crymophila and K. humilis) reached lowest value under moderate grazing intensity and therefore 2.87sheep/ha was recommended as the optimal grazing intensity in this region. Nmass of these six species and PFG showed not consistent change trend. With the exception of M. ruthenica, LeafN/P of other five species showed a significant response to grazing but there was no consistent change trend. Variation tendency and range of LeafN/P are not consistent for these six dominant species and PFG.(2) PH, LA and LDM of these six dominant plant species showed a significant positively relationships with AGB and SOC. In addition, PH, LA and LDM of E. nutans, S. purpurea and P. crymophila had a positive relationships significantly with STP. Meanwhile, there were positive relationships between PW of other five dominant species and STP, with exception of PW of E. nutans. There was no relationship between LPC of these six dominant plant species and STP.(3) With the increase of grazing intensity, species composition and species number of community changed significantly, and species number of Grasses at ungrazed sites and very light grazing pressure was more than other grazing pressures. Meanwhile, species number of forbs was less than other grazing intensity. Community coverage (CC) and AGB of community reduced significantly with increasing grazing intensity, and there was no significant variation between light and moderate grazing intensity, but they declined very significantly under heavy and extreme grazing pressures. Important value (IV) and summed dominance ratio (SDR) of Grasses, Sedges and Legume reduced very significantly from light to extreme grazing intensity. And IV and SDR were higher at light grazing intensity, meanwhile moderate grazing intensity came second, however, they reduced very significantly under heavy and extreme grazing pressures. Shannon-Wienner diversity and Pielou evenness indexes had consistent trend and reached the highest value at light grazing intensity, meanwhile moderate grazing intensity ranked the secend place. The lowest value was showed at ungrazed and extreme grazing pressure. Taking a comprehensive view of change trend and range of CC, AGB, IV, SDR and Shannon-Wienner diversity and Pielou evenness indexes of community, grazing intensity ranging from 2.29 to 2.87 sheep/ha was recomended as the optimal grazing intensity range in this region.(4) Through the methods in combination of phylogenetic researching and community investigation, phylogenetic structure in different grazing intensity was assessed through comparing observed phylogenetic distances with phylogenetic distances in null communities obtained from making phylogenetic relationships of the regional pool of species randomized. Phylogenetic Supertree of exsisting species was built in alpine steppe in this region. Phylogenetic structure of community varied significantly with increasing grazing intensity, phylogenetic overdispersion pattern was observed from ungrazed to light grazing intensity including SRO, SRI.72, SR2.29. However, phylogenetic clustering pattern was found from moderate to extreme grazing intensity including SR2.87, SR4.13, SR5.62.