只要有足够的时间，美国加州红杉、美洲杉和花旗松能够长到120米高。问题来了，它们为什么不能再长高一些？
据美国《科学》杂志在线新闻报道，研究人员将这归结于高度极限与气泡斗争的结果。细胞膜防止气泡进入树木的毛细管系统，这是由于气泡会切断水分的传送。细胞在树木中的位置越高，它们阻挡气泡的能力就越强。然而最终，这种能力变得如此强大，以至于连水都无法通过，树木便也就因此停止了生长。研究人员表示，这一发现将有助于增进植物学家对于树木如何应对干旱和气候变化的理解。研究人员在本周出版的美国《国家科学院院刊》（PNAS）上报告了这一研究成果。（生物谷Bioon.com）
生物谷推荐原始出处：
PNAS，doi: 10.1073/pnas.0710418105，Jean-Christophe Domec, Katherine A. McCulloh
Maximum height in a conifer is associated with conflicting requirements for xylem design
Jean-Christophe Domec*,†, Barbara Lachenbruch†,‡, Frederick C. Meinzer†,§, David R. Woodruff§, Jeffrey M. Warren¶, and Katherine A. McCulloh‡
+Author Affiliations
*Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27795;
‡Department of Wood Science and Engineering, Oregon State University, Corvallis, OR 97331;
§Forestry Sciences Laboratory, United States Department of Agriculture Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331; and
¶Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Edited by Michelle Holbrook, Harvard University, and accepted by the Editorial Board June 5, 2008 (received for review November 1, 2007)
Abstract
Despite renewed interest in the nature of limitations on maximum tree height, the mechanisms governing ultimate and species-specific height limits are not yet understood, but they likely involve water transport dynamics. Tall trees experience increased risk of xylem embolism from air-seeding because tension in their water column increases with height because of path-length resistance and gravity. We used morphological measurements to estimate the hydraulic properties of the bordered pits between tracheids in Douglas-fir trees along a height gradient of 85 m. With increasing height, the xylem structural modifications that satisfied hydraulic requirements for avoidance of runaway embolism imposed increasing constraints on water transport efficiency. In the branches and trunks, the pit aperture diameter of tracheids decreases steadily with height, whereas torus diameter remains relatively constant. The resulting increase in the ratio of torus to pit aperture diameter allows the pits to withstand higher tensions before air-seeding but at the cost of reduced pit aperture conductance. Extrapolations of vertical trends for trunks and branches show that water transport across pits will approach zero at a heights of 109 m and 138 m, respectively, which is consistent with historic height records of 100–127 m for this species. Likewise, the twig water potential corresponding to the threshold for runaway embolism would be attained at a height of ≈107 m. Our results suggest that the maximum height of Douglas-fir trees may be limited in part by the conflicting requirements for water transport and water column safety.
