只要有足夠的時(shí)間,，美國加州紅杉,、美洲杉和花旗松能夠長到120米高。問題來了,，它們?yōu)槭裁床荒茉匍L高一些,？
據(jù)美國《科學(xué)》雜志在線新聞報(bào)道,，研究人員將這歸結(jié)于高度極限與氣泡斗爭的結(jié)果。細(xì)胞膜防止氣泡進(jìn)入樹木的毛細(xì)管系統(tǒng),，這是由于氣泡會(huì)切斷水分的傳送,。細(xì)胞在樹木中的位置越高，它們阻擋氣泡的能力就越強(qiáng),。然而最終,，這種能力變得如此強(qiáng)大，以至于連水都無法通過,，樹木便也就因此停止了生長,。研究人員表示，這一發(fā)現(xiàn)將有助于增進(jìn)植物學(xué)家對于樹木如何應(yīng)對干旱和氣候變化的理解,。研究人員在本周出版的美國《國家科學(xué)院院刊》（PNAS）上報(bào)告了這一研究成果,。（生物谷Bioon.com）
生物谷推薦原始出處：
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.
