Drought may impact the net ecosystem exchange of CO2 （NEE） between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under drought stress in the hinterland of Tibetan Plateau （Damxung, Tibet, China） were investigated. Data were obtained using the covariance eddy technique in 2009. Severe drought stress appeared in the early growing season （May to early July） and September. Drought conditions during the early growing season limited grass production and the green leaf area index （GLAD increased slowly, with an obvious decline in June. When encountering severe water stress, diurnal patterns of NEE in the growth season altered with a peak carbon release around 16：00 h or a second carbon uptake period before sunset. NEE variations in daytime related most closely with O other than PAR when daily averaged @〈0.1 m3 m 3. Seasonal patterns of gross primary production （GPP） and NEE were also influenced by drought： the maximum and minimum of daily-integrated NEE were 0.9 g C m-2 d-1 on 3 July 2009, and -1.3 g C m-2 d-1 on 12 August 2009 with a GPP peak （-2.3 g C m-2 d-1） on the same day, respectively. Monthly NEE from May to July remained as carbon release and increased gradually; peak values of monthly NEE and GPP both appeared in August, but that of ecosystem respiration （R^co） was reached in July. Annual NEE, GPP and Reco of the alpine grassland ecosystem were 52.4, -158.1 and 210.5 g C m-2, respectively. Therefore, the grassland was a moderate source of COs to the atmosphere in this dry year. Interannual variation in NEE was likely related to different water conditions in the growing season. The three greatest contributors to seasonal variation in NEE, GPP and R^co respectively were GLAI〉Ta〉O, GLAI〉O〉PPT, and Ta〉GLAI〉PAR. Seasonality of GLAI explained 60.7% and 76.1% of seasonal variation in NEE and GPP, respectively. GPP or NEE was more sensitive than Reco to variation in GLAI, and ecosystem
Species richness and diversity indices （Shannon-Wiener index, Simpson dominance index and Pielou evenness index） in alpine grassland ecosystems （alpine meadow, alpine steppe and desert steppe） under grazing-excluded and freely grazed sites were investigated along the Northern Tibetan Plateau Alpine Grassland Transect during summer 2009 and 2010. We found that species richness and diversity have not been significantly altered by short-term grazing exclusion since 2006 at vegetation and regional scales. Species richness and diversity were mainly driven by growing season precipitation （GSP）, which accounted for over 87 % of the total variation observed, Species richness and diversity at grazing- excluded and freely grazed sites appear to respond to growing season precipitation in parallel. Species richness exponentially increased with GSP while diversity indices showed positively linear relationships with GSP. This indicates that GSP on the Northern Tibetan Plateau is crucial in regulating species richness and diversity and should be taken into account in future studies on alpine grassland conservation.
To better understand the ecological and economic benefits of short-term grazing exclusion on the aboveground net primary productivity(ANPP)of alpine pastures,we conducted annual multi-site transect surveys in the summers from 2009 to 2011 and calculated the aboveground biomass discrepancy(ABD)between grazed and ungrazed pastures at plant community and economic group levels for three zonal alpine grassland types—meadow,steppe,and desert-steppe—across the northern Tibetan Plateau.Our results indicated that aboveground biomass(AGB)significantly differed among grassland types and declined northwesterly from 64.07 to11.44 g m-2with decreasing precipitation and increasing temperature.The mean ABD exhibited considerable community dependency,with meadow(12.47 g m-2)[steppe(6.91 g m-2)[desert steppe(2.54 g m-2),and it declined from 25.42 to 1.29 g m-2with decreasing precipitation and increasing temperature.‘Good forage’,i.e.grasses and sedges,benefited most from grazing exclusion,followed by edible forbs.With longer grazing exclusion durations(GEDs),the aboveground biomass of poisonous locoweeds initially decreased and then increased compared with the adjacent grazed sites.In the nested analysis of co-variances with a general linear model,growing season precipitation(GSP;from May to September)accounted for 52.67%ofthe observed variation in AGB,followed by AGT(9.77%)and pasture management systems(PMSs;grazing or grazing-excluded,5.31%).The variation in ABD was explained primarily by AGT(16.52%),GED(20.25%),and the interaction of AGT 9 GED(19.58%).Our results confirm that precipitation is the primary factor controlling the ANPP of alpine grasslands on the Northern Tibetan Plateau and that the ecological benefits arising from grazing exclusion are also partly dependent on grassland type and exclusion duration.Therefore,spatial and temporal variations in growing season precipitation and plant functional traits or economic group composition should be jointly considered when developing policies concerning the management
