北京市绿化树种紫玉兰的蒸腾特征及其影响因素

The characteristics of Magnolia liliflora transpiration and its impacting factors in Beijing City

温带和热带的干旱半干旱地区不断加快的城市绿化,对水的大量需求给未来水资源的合理利用提出了严峻挑战。了解树木蒸腾特征及环境和生物因子对蒸腾的调控有助于城市绿化树种的选择和养护。利用热消散技术研究不同时间尺度上15年生紫玉兰(Magnolia liliflora)的耗水特征,并同步监测环境因子。在昼夜尺度上,春夏季节紫玉兰树干液流密度多呈双峰型曲线,蒸腾午休现象明显;秋季树干液流密度呈单峰型曲线;冬季树干液流密度日变化格局不明显。于年内不同月份,从3月开始上升,5月达到峰值,9月开始显著降低;于不同的季节,春季>夏季>秋季>冬季,显示紫玉兰属春季耗水型树种;于物候期,从发芽到开花展叶阶段,整树耗水迅速增加,叶速生期达到最大值,休眠期整树耗水显著降低。不同时间尺度上紫玉兰整树耗水对环境因子和生物因子的响应不同。昼夜尺度上,液流密度与总辐射和水汽压亏缺显著相关,且有显著时滞;日尺度上,总效辐射、水汽压亏缺以及大气温度是整树耗水的决定因子,由于城市中的适时灌溉和研究期间降水充足,土壤水分对紫玉兰的耗水没有显著影响;月尺度上,大气温度是紫玉兰的整树耗水的决定因子;开花展叶期,叶面积指数是影响紫玉兰耗水的关键因子,同期环境因子对整树耗水的解释量只有25.5%。

The fast urban greening in the arid and semi-arid lands in temperate and tropical regions increased the consumption of water resource greatly.Identifying the characteristics of tree transpiration,as well as the environmental and biological variables controlling water use in urban environment can help us choose the reasonable tree species and manage them scientifically.Magnolia liliflora is a typical urban greening species and distributes widely due to its high ornamental value.However,the water use characteristics of M.liliflora in the urban environment are poorly understood.Thermal dissipation method therefore was used to quantify the sap flow variations of M.liliflora trees at multi-temporal scales.The environmental factors including total radiation（Rs）,vapor pressure deficit（D）,air temperature（Ta）,air humidity（RH）,soil water content at the depth of 10cm,30cm,50 cm,80 cm（SWC10,SWC30,SWC50,SWC80）,wind speed（w） and precipitation（P） were also monitored at the same frequency as sap flow measurement.Sapwood cross-sectional area（AS,cm2） was estimated from increment borer samples.Sap flux density measurements made in stems were scaled to each individual tree by its AS.Measurements of whole-tree leaf area index（LAI） were measured using a Plant Canopy Analyzer（LAI2000,USA） every two or three days during leaf expansion and defoliation,and once a week in other period under diffuse light conditions on cloudy days or at dusk.The diurnal,daily,monthly and seasonal variations of whole-tree sap flow were analyzed.Diurnal sap flux density（Js） of M.liliflora had the double peak pattern in spring and summer and single peak pattern in autumn.The monthly variation of the whole tree transpiration（Et） increased gradually from March to May and decreased rapidly after September.The seasonal variation of Et was the highest in spring and decreased in the sequence of summer,autumn,winter.Changes in plant phenologyphase directly related to the change of Et.Et rose rapidly from sprouting stage to flowering and leaf expansion stage,reached the maximum value in the fast growing period and the minimum value in the dominant period.Great variations in Et observed at different temporal scale were due to environmental factor changes.On the diurnal scale,hourly mean Js with significant time lag was significantly related to Rs and D（R2=0.87,P0.001;R2=0.66,P0.001）.On the daily scale,Et significantly related to Rs,Ta,and D,while there were no obvious relationships between Et and SWC due to the irrigation and amplitude rainfall during the research period.On the monthly scale,Ta was the most important factor affecting Et.During the flowering and leaf expansion stage,LAI was the most important factor affecting Et,while the environmental factors only explained 25.5% variance.