北京绿化树种油松、雪松和刺槐树干液流的空间变异特征

Characteristics of spatial variations in xylem sap flow in urban greening tree species Pinus tabulaeformis,Cedrus deodara and Robinia pseudoacacia in Beijing,China

城市绿化树木具有多重生态效应,其耗水量不容忽视。在不了解树干液流空间变异的前提下,将点的测定值推广到整树或者林段尺度会产生很大的误差。为准确地确定整树耗水,采用热消散探针法研究了夏秋季北京成年常绿树种油松(Pinus tabulaeformis)、雪松(Cedrus deodara)和刺槐(Robinia pseudoacacia)树干液流的空间变异特征及产生原因。各树种树干液流存在方位变异,受树干靠南的方向受光较多、木材解剖特征和枝下高高度的影响,油松和雪松液流密度与方位之间的关系较为固定,而刺槐液流密度与方位之间的关系表现出随机性。不同方位每小时液流密度之间高度相关(p<0.0001)。因此,可以基于这种关系准确地计算其他方位的液流(R2>0.91,p<0.0001)。油松和雪松树干液流的径向变异显著,较深处和较浅处树干液流的日变化格局相似,但是较深处的液流明显滞后于较浅处的树干液流,且较浅处树干液流对环境因子的响应远高于深处的液流。不同深度树干液流之间密切相关,因此可以利用较浅处的液流外推其他深度的液流(R2>0.89,p<0.0001)。然而,同一棵树不同方位径向剖面特征不同,雪松南向较深处的液流明显高于其他方位,且滞后不显著,这与树冠南向受光较多有关。结合误差分析,采取北向15mm和75mm深处的液流密度均值来估算整树耗水较为准确。

Aims Water consumption of urban plants with multiple ecological effects is important. However,large errors may occur when sap flow is scaled from single point measurement to whole tree without knowledge of spatial sap flow profiles in the trunk. Our objective was to investigate the spatial variation of sap flux density （Js） and its possible cause to estimate whole-tree water use more precisely. Methods Spatial patterns of sap flux density in the sapwood of Pinus tabulaeformis,Cedrus deodara and Robinia pseudoacacia were investigated with thermal dissipation probe from June to November 2008 in Beijing,China. Important findings Axial variation was substantial. Due to greater exposure to sun in the south aspect,the anatomy of the xylem structure and lower branch height,there was an apparent relationship between sap flux density and aspect in P. tabulaeformis and C. deodara,but no apparent relationship in R. pseudoacacia. Hourly Js over 24 h at different aspects were highly correlated; therefore,mean Js may be accurately estimated based on measurement obtained on one aspect. Js showed marked radial variation within the trunk. Js at different depths show similar diurnal pattern,while Js at deeper depth lagged behind and was more sensitive to evaporative demand than the shallower depth. Hourly Js over 24 h at different depths were highly correlated,so Js at a particular depth could be extrapolated as a multiple of Js at the depth of 15 mm. However,depth profiles of Js differed among aspects within a tree. Js at the deeper depth on the south aspect of C. deodara was greater and had no time lag compared to other aspects. In conclusion,sap flux density on the north side at depths of 15 and 70 mm could give an accurate estimation of whole-tree transpiration.