Effects of temperature increase and grazing on stomatal density and length of four alpine Kobresia meadow species,Qinghai-Tibetan Plateau
Effects of temperature increase and grazing on stomatal density and length of four alpine Kobresia meadow species,Qinghai-Tibetan Plateau
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作者：
Zhao Xinquan
低温被广泛认为是青藏高原高寒草甸植物生长的主要限制因素。因此，温暖被认为是缓解该地区温度限制的主要因素。牲畜放牧是该地区主要的土地利用方式，也是影响植物生长的主要人为因素。放牧在减少总叶面积的同时，可能会提高土壤中的营养元素水平，从而对植物生长产生相反的影响。气孔器在控制二氧化碳进出植物方面起着重要作用。由于该地区大多数物种的光合作用很难直接测量，在青海省海北研究站进行了双向析因设计(增温×放牧)试验。利用金博尔等人部署的红外辐射加热器阵列的温度自由空气控制增强(T-FACE)系统来提高植被冠层温度。夏季设定点差白天为1.2℃，夜间为1.7℃。在冬季，昼夜时差分别为1.5℃和2.0℃。从7月到9月，每块地(3m2)每月放牧两只羊，地上生物量的利用处于中等水平。经过两年的处理，于2008年8月选择了4种高寒草甸植物：唐松草、矮嵩草、草龙胆、垂穗披碱草。选择完全成熟的叶片，用光学显微镜(Motic BA200)测量气孔密度(SD)、气孔长度(SL)和气孔两端保卫细胞连接之间的长度(中国)。暖化使四种植物的SL分别降低3.3%、1.9%、3.9%和2.7%(P=0.037)。暖化对SD的影响具有物种特异性，表现为气候变暖与物种间的交互作用(P=0.086)。放牧使四种植物的SD分别增加12.5%、15.7%和15.9%、2.4%。用气孔密度公式计算了4种植物的气孔导度指数。放牧使4种植物的气孔导度增加(P=0.000)，而变暖使气孔导度降低，但差异不显著(P=0.068)。放牧引起的SD和PIC增加可能意味着单位叶面积的同化速率在一定的放牧强度下增加，由于SD和SL都贡献了最大的气孔导度，所以在最大气孔导度的情况下，二者对放牧和增温的响应是相互抵消的。未来气候变暖可能是放牧高寒草甸SD增加而SL减少的趋势。
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.