Meng Shawn X, Lieffers Victor J, Reid Douglas E B, Rudnicki Mark, Silins Uldis, Jin Ming
Department of Renewable Resources, ESB 4-42, University of Alberta, Edmonton, Alberta, Canada T6G 2H1.
J Exp Bot. 2006;57(12):3175-82. doi: 10.1093/jxb/erl079. Epub 2006 Aug 14.
The hypothesis was tested that upper limits to height growth in trees are the result of the increasing bending moment of trees as they grow in height. The increasing bending moment of tall trees demands increased radial growth at the expense of height growth to maintain mechanical stability. In this study, the bending moment of large lodgepole pine (Pinus contorta Dougl. Ex Loud. var. latifolia Engelm.) was reduced by tethering trees at 10 m height to counter the wind load. Average bending moment of tethered trees was reduced to 38% of control trees. Six years of tethering resulted in a 40% increase in height growth relative to the period before tethering. By contrast, control trees showed decreased height growth in the period after tethering treatment. Average radial growth along the bole, relative to height growth, was reduced in tethered trees. This strongly suggests that mechanical constraints play a crucial role in limiting the height growth of tall trees. Analysis of bending moment and basal area increment at both 10 m and 1.3 m showed that the amount of wood added to the stem was closely related to the bending moment produced at these heights, in both control and tethered trees. The tethering treatment also resulted in an increase in the proportion of latewood at the tethering height, relative to 1.3 m height. For untethered control trees, the ratio of bending stresses at 10 m versus 1.3 m height was close to 1 in both 1998 and 2003, suggesting a uniform stress distribution along the outer surface of the bole.
该假说得到了验证，即树木高度生长的上限是树木随着高度增加而弯矩增大的结果。高大树木不断增加的弯矩要求增加径向生长并以牺牲高度生长为代价来维持机械稳定性。在本研究中，通过在10米高处系住树木以对抗风荷载，降低了大型黑松（Pinus contorta Dougl. Ex Loud. var. latifolia Engelm.）的弯矩。系住的树木的平均弯矩降低至对照树木的38%。六年的系住处理导致相对于系住前的时期，高度生长增加了40%。相比之下，对照树木在系住处理后的时期内高度生长下降。系住的树木沿树干的平均径向生长相对于高度生长有所减少。这有力地表明，机械约束在限制高大树木的高度生长中起着关键作用。对10米和1.3米处的弯矩和基部面积增量的分析表明，在对照树木和系住的树木中，添加到树干的木材量与这些高度处产生的弯矩密切相关。系住处理还导致在系住高度处晚材的比例相对于1.3米高度有所增加。对于未系住的对照树木，1998年和2003年10米与1.3米高度处的弯曲应力之比均接近1，表明沿树干外表面的应力分布均匀。
