Effects of temperature increase and grazing on stomatal density and length of four alpine Kobresia meadow species,Qinghai-Tibetan Plateau

Low temperature is widely recognized as the primary limiting factor in plant growth in alpine meadows of the Qinghai-Tibetan plateau.Warming is thus supposed to alleviate temperature limitation in the area.Livestock grazing is the predominant land use in the area and is the main anthropogenic factor affecting plant growth.Grazing,while diminishing total leaf area,may raise the level of nutrient elements in the soil,and thus have the opposite effect on plant growth.Stomata apparatus play important roles in controlling the passage of CO2 into and out from plants.Since it is difficult to directly measure photosynthesis rates for most species in the area,changes in stomatal characteristics could be used as proxies to understand how the assimilation rates of plants change under warming or grazing treatments.A two-way factorial design(warming×grazing) experiment was carried out at the Haibei Research Station in Qinghai province,China.A temperature free-air controlled enhancement(T-FACE) system with arrays of infrared radiation heaters deployed by Kimball et al.was used to elevate vegetation canopy temperatures.The setpoint differences in summer were 1.2℃ in daytime and 1.7℃ at night-time,while in winter daytime and night time differences were 1.5℃ and 2.0℃ respectively.Two sheep per plot(3 m2) grazed once a month from July to September,representing a moderate level of above ground biomass utilization.Four alpine meadow species were selected in August 2008 after two years of treatment: Thalictrum aplinum,Kobresia humilis,Gentiana straminea,Elymus nutans Griseb.Fully mature leaves were selected to measure stomatal density(SD) and stomatal length(SL) and the length between the junctions of the guard cells at each end of the stoma using a Motic microscope system(Motic BA200,China).Warming decreased the SL of all four species(P=0.037),by 3.3%,1.9%,3.9%,2.7% in T.aplinum,K.humilis,G.straminea.E.nutans,respectively.Warming affected SD in a species-specific way,as indicated by a marginally significant(P=0.086) interaction between warming and species in ANOVA.Grazing increased the SD of all four species,by 12.5%,15.7% and 15.9%,2.4% in T.aplinum,K.humilis,G.straminea.E.nutans respectively.Grazing had no statistically significant effects on SL(P=0.907).Potential conductance index(PCI) was also calculated for all four species using the formula(stomatal length)2 stomatal density.Grazing increased the PCI for all four species(P=0.000),while warming decreased the PCI,but with a lower level of significance(P=0.068).There were no significant interaction effects between warming and grazing on SD,SL and PCI.The decline in SL subjected to warming might be the result of warming-generated soil drought.The rise in SD and PCI induced by grazing may imply an increase in assimilation rate per leaf area at a given grazing intensity.Since both SD and SL contributed to maximum stomatal conductance,their responses to grazing and warming counteracted each other in the case of maximum stomatal conductance.It may be the tendency that SD increase and SL decrease in grazing alpine meadow under future warming.