Effects of livestock exclusion and climate change on aboveground biomass accumulation in alpine pastures across the Northern Tibetan Plateau

To better understand the ecological and eco- nomic 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--mea- dow, 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 to 11.44 g m-2 with 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-2 with 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 x 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 tr