Wood specific gravity gradients in tropical trees.

摘要

Pith to bark wood specific gravity (SG) trends were measured in tropical trees and compared with trends found in temperate trees. Extraordinary increases were found across the stems of some tropical wet forest colonizing species, with most tropical species showing at least moderate increases. In contrast, temperate species showed little change, and decreases were almost as common as increases.; In Chapter One (published in Wood Fiber Sci. 20(3):344-349), three colonizing species in tropical wet forest were found to produce low SG wood when small and to linearly increase SG with diameter growth. Trends in small trees were similar to those in the inner portions of large trees. Increases were {dollar}>{dollar}200% in the largest trees, with no apparent maximum SG.; Chapter Two (Forest Sci. 35(1):197-210) reports on an additional 20 tropical wet forest species and 17 temperate species. The greatest increase in any temperate species was 40% from pith to bark. In contrast, the greatest increase in tropical species was 270%; the average was 60%. The largest gains were found in colonizers.; Intermediate results were found in tropical dry and montane rain forests, and are reported in the third chapter (Amer. Jour. Botany 76(6): 924-928). For dry forest, the largest increases (60%) were in species also found in wet forest.; Chapter Four (unpublished) reports on mechanical properties of wood from four colonizers. Static bending showed strong correlations between SG and moduli of rupture and elasticity. Microscopic examination indicates that greater proportion of fibers and thicker cell walls account for increased SG.; The overall conclusions to be drawn from this study are that some light-demanding tree species of highly competitive tropical environments allocate resources toward production of low SG, weak wood and rapid height growth when young. With age, these species gradually shift resources toward production of stronger, stiffer wood with more fibers and thicker cell walls. Shade tolerant species and species from less competitive environments show little change. Thus biomass allocation toward the competing goals of height growth and stem strength may be related to species light requirements and degree of competition.