West、Brown和Enquist提出的树木水分传导的分形网络模型(简称WBE模型)认为,树木连续分枝之间的导管或管胞直径按照一定的比率均匀变细,其总的水力阻力与水分传导的路径长度无关,从而使不同部位叶片获得基本相当的水分供应。该模型对树木高生长的水力限制假说提出了置疑。为了验证WBE模型中树木导管或管胞均匀变细的假说,该文研究了云南哀牢山中湿性常绿阔叶林中6种常绿阔叶树, 腾冲栲(Castanopsis wattii)、景东石砾(Lithocarpus chintungensis)、木果石砾(L. xylocarpus)、长尾青冈(Cyclobalanopsis stewardiana)、滇木荷(Schima noronhae)和舟柄茶(Hartia sinensis)木质部解剖特征随树高和年龄的变化。对这6个树种共14株样木进行了不同高度树干圆盘和边材生长轮取样,样木的高度为15~25 m,按照常规木材解剖的处理和分析方法,在显微镜下测定木材切片的导管直径和密度等特征。结果表明:在14株样木中,有4株树木导管直径随树木高度增加呈线性减小, 1株没有明显变化,其它9株树木导管直径在树冠以下的树干部分变化幅度较小或没有明显变化,而从树冠基部往上直到树木顶端导管直径显著减小。同一植株随着高度的增加,导管密度增加并且在树冠内增加更显著。有三分之一的树木导管占边材面积的比例随树高增加没有明显变化,其余树木导管占边材面积比在树冠以上有所减小。多数树木理论比导率在树冠以下没有明显变化而在树冠基部往上显著降低。在从髓芯开始往外的20~40个年轮范围内导管直径增加显著,但大部分植株导管直径在40个年轮后趋于稳定。不同高度圆盘导管直径随形成层发育时间的变化呈相似的趋势,并且相同发育年龄的导管直径没有明显差异。该文的研究结果说明,导管直径的轴向和径向变化一定程度上补偿了水分运输阻力随树木个体增大而增加的缺陷,但是6种常绿阔叶树树干的导管基本不按一定比率均匀变细,不支持WBE模型。

The model of West, Brown and Enquist shows that total hydraulic resistance in trees can be independent of path length provided that vascular conduits taper upward sufficiently. This model contradicts the hydraulic limitation hypothesis on tree height growth. To test the validity of this model, we investigated axial and radial changes in xylem anatomical characteristics of six evergreen broadleaved tree species in a subtropical forest in Yunnan. The six species studied included Castanopsis wattii, Lithocarpus chintungensis, L. xylocarpus, Cyclobalanopsis stewardiana, Schima noronhae, and Hartia sinensis. The first four species are from the Fagaceae and the other two species are from Theaceae. Fourteen trees (15-25 m) were sampled from the six species. Sapwood cores were taken from each tree at intervals of 1 m along the trunk to study the axial variation in xylem anatomy. Stem cross sections were collected at three heights from the four Fagaceae species to characterize radial variation. Transverse sections of 50-80 μm thickness were made using a sliding microtome. Our analysis of transverse microscopic sections showed that the conduit lumen diameters increased from the top to the base of the crown in all trees and to the base of the bole in four trees. Conduit lumen diameter was approximately constant from the crown base to the tree base in the remaining trees. Vessel density increased with height, especially within the crown. The lumen/total sapwood area ratio was constant along the trunk in four of the twelve trees and decreased from the base of the crown to its top in most of the remaining trees. The theoretical specific hydraulic conductivity decreased substantially from the base of the crown to its top and was constant below the crown base in most trees. Conduit lumen diameters increased linearly for the first 20 - 40 years of cambial age and then stabilized in most of the eight trees of Fagaceae. There was no difference in conduit lumen diameter produced by the same aged cambium at different aboveground heights. Our results suggested that axial and radial trends in conduit lumen diameter of the six evergreen broadleaved species were consistent with partial buffering of hydraulic resistance from path length effects. The uniform size of conduit lumens below the crown base contradicted the critical assumption of constant conduit taper along the trunk in the model of WBE.