C4 abundance in an Inner Mongolia grassland system is driven by temperature-moisture interaction, not grazing pressure

Global warming and CO ₂ rise is expected to change the balance of C3 and C4 plants in grassland vegetation, but disturbance, including grazing, could also affect C3/C4 community structure. We used a six-year (2005-2010) grazing experiment to test the prediction that the relative abundance of C4 plants (P _C4) in the semi-arid grassland of Inner Mongolia is related to growing-period temperature and disturbance by grazing. Paddocks were stocked with sheep at six rates (0.375-2.25sheepha ^-1a ^-1) between June and September. P _C4 was estimated from the relative ¹³C/ ¹²C composition (δ ¹³C) of wool grown during the grazing period of each year and a two-member mixing model of the δ ¹³C of the C3 and C4 communities in the paddocks. Stocking rate had a slight effect (<0.5‰) on the δ ¹³C of the C3 and the C4 end-members. Annual average P _C4 varied between 5% and 24%. Stocking rate had no significant effect on P _C4 but the growing-season temperature had a large effect, and P _C4 increased with temperature. Importantly, the growing season shifted between years due to changing seasonal distribution of rainfall and soil moisture availability from winter precipitation causing large variation in temperature of the effective growing season. Changes in temperature and in amount and seasonal distribution of rainfall as a consequence of inter-annual fluctuations and long-term change can thus have a large impact on the C4 abundance and its spatial pattern.