University of Maine, School of Forest Resources, Orono, Maine, 04469-5755, USA.
University of Alberta, Department of Renewable Resources, Edmonton, Alberta, T6G2R3, Canada.
Am J Bot. 2018 Oct;105(10):1617-1630. doi: 10.1002/ajb2.1171. Epub 2018 Oct 9.
In 1757 Leonhard Euler demonstrated that to avoid bending tall columns needed to be stiffer but not stronger than shorter columns of equal diameter and material density. Many researchers have concluded that trees have a fixed stiffness to basic density ratio, and therefore, trees adjust for increasing height by adding mass to adjust stem form. But the wood science literature points to considerable variance in stiffness with respect to green wood density.
Using the vast global repository of green wood mechanical properties, we compared relative stiffness and relative strength between taller and shorter species. For North American trees, we examined stem moisture distribution.
For all regions of the world, taller species on average possessed greater stiffness, but not strength, than shorter species of equal basic specific gravity. We looked for a possible universal mechanism that might allow taller tree species to adjust stiffness without affecting xylem specific gravity and concluded that the evidence points to a decrease in cellulose microfibril angle in structural cell walls combined with possible increases in holocellulose percentage. The evidence is strongest for conifers. We also showed that tall conifers have the ability to adjust the distribution of xylem moisture to maximize conduction while minimizing column load.
Our research reveals that taller trees have developed internal stem adjustments to minimize diameter increase while attaining ever-greater heights, thus enabling these taller species to reduce energy expended on biomass accumulation while gaining greater access to solar radiation.
1757 年，Leonhard Euler 证明，为避免弯曲，高柱需要比直径和材料密度相等的短柱更硬但不更强。许多研究人员得出结论，树木具有固定的刚度与基本密度比，因此，树木通过增加质量来调整茎的形态以适应高度的增加。但是木材科学文献表明，刚度与新鲜木材密度之间存在相当大的差异。
利用全球大量的新鲜木材力学性能数据库，我们比较了较高和较低物种之间的相对刚度和相对强度。对于北美树木，我们研究了茎干的水分分布。
在世界所有地区，平均而言，较高的物种比基本比重相等的较短物种具有更大的刚度，但强度却没有。我们寻找了一种可能的通用机制，使较高的树种能够在不影响木质部比重的情况下调整刚度，并得出结论，证据表明结构细胞壁中纤维素微纤维角的降低与可能的全纤维素百分比的增加相结合。这一证据在针叶树中最为明显。我们还表明，高大的针叶树有能力调整木质部水分的分布，以最大限度地提高传导能力，同时最小化柱负载。
我们的研究表明，较高的树木已经在内部茎干上进行了调整，以最小化直径的增加，同时达到更高的高度，从而使这些较高的物种能够减少在生物质积累上消耗的能量，同时获得更多的太阳能辐射。
