But in tall, low density conifers compressive hoop stress might be counteracted by adjustment of the MFA ( Boyd, 1985). In higher density woods, wall collapse may be thwarted by thick walls ( Hacke et al., 2001). MFA is measured, not only to relate it to mechanical properties, but also to assess its possible role in counteracting compressive hoop stress in tracheid walls caused by negative hydraulic pressures.
Microfibril angle (MFA) of tracheids is often cited as a correlate for mechanical properties ( Meylan and Probine, 1969 Cave, 1972 Astley et al., 1998 Booker et al., 1998 Nakada et al., 1998). In this study, xylem specific gravity, strength and stiffness for green wood, as found in living trees, were measured, and these are compared with other low‐to‐moderate density conifers to assess probable habitat preferences. These studies have focused on properties of seasoned wood in relation to utilization. The most comprehensive study is that of Polman et al. Previously, selected properties of Metasquoia wood have been determined from trees growing in China, The Netherlands, Poland and Russia ( Li, 1948 Liang et al., 1948 Brazier, 1963 Jaroslavcev and Visnjakova, 1965 Linnard, 1966 Hejnowicz, 1973 Surminski and Bojarczuk, 1973 Wu and Chern, 1995 Polman et al., 1999). During the Palaeocene and Eocene periods, the range of Metasequoia extended at least to 80°N latitude, and often dominated wet, lowland forests, comprising more than 90 % of stands, and reaching estimated heights of greater than 30 m ( Francis, 1991 Greenwood and Basinger, 1994). Based on the fossil record, Metasequoia forests were once widely distributed in the northern hemisphere ( Momohara, 1994). In some situations, it has demonstrated a very high growth rate potential, with trees in the eastern United States reaching heights of 38 m in 50 years ( Kuser, 1999). Since its description by botanists in 1945 ( Florin, 1952 Li, 1957), Metasequoia has been widely planted throughout the world. In this region trees can reach 50 m in height and 13 m in diameter ( Florin, 1952). The current natural range of Metasequoia is restricted to a small, remote area of Hupeh province, near the border of Szechuan, China, at an approximate latitude of 30☁0′N ( Chu and Cooper, 1950). In this paper, the xylem of the main stem is examined in relation to support and hydraulic conduction, and the question is asked whether these provide any evidence to suggest adaptability to a wet palaeoenvironment of continuous but weak intensity light. In a previous study, the canopy and leaf design characteristics of Metasequoia glyptostroboides Hu et Cheng have been discussed in relation to light interception and gas exchange ( Jagels and Day, 2003). Utilizing the principles of ‘adequate design’ ( Rashevsky, 1973), it is possible to construct a hypothetical woody plant that incorporates the compromises needed for mechanical support, hydraulic conduction, light interception and gas exchange ( Niklas, 1992 Romberger et al., 1993 McCulloh et al., 2003). It is proposed that xylem anatomy and chemistry are among these strategic features. A number of ecophysiological features, such as requirements for seed germination, canopy shading and root competition will determine whether a tree species can capture and maintain site dominance. During the Palaeogene period, Meta sequoia may have exhibited similar characteristics at high latitudes ( Francis, 1991 Greenwood and Basinger, 1994). The adaptability of these features to a high‐latitude Eocene palaeoenvironment is discussed.Īlthough many tree species can be readily categorized as either short‐lived, early succession or longer‐lived, late succession cohorts a few, like redwood, seem to have characteristics of each type and, hence, can colonize a site, grow rapidly, exclude competitors and produce long‐lived pure stands of tall trees ( Ornduff, 1998). The design strategy for Metasequoia is to produce a mechanically weak but hydraulically efficient xylem that permits rapid height growth and crown development to capture and dominate a wet site environment. These xylem properties are referenced to the measured bending properties of modulus of rupture and modulus of elasticity, and compared with other low‐to‐moderate density conifers. The heartwood is moderately decay resistant, helping to prevent Brazier buckling. In some cases, this compressive stress is converted to irreversible strain (plastic deformation), as evidenced by cell wall corrugations. The microfibril angle in the S 2 layer of tracheid walls is large, even in outer rings, suggesting a cambial response to compressive rather than tensile stresses. The xylem of Metasequoia glyptostroboides Hu et Cheng is characterized by very low density (average specific gravity = 0♲7) and tracheids with relatively large dimensions (length and diameter).
0 kommentar(er)
