Does canopy position affect wood specific gravity in temperate forest trees?

Abstract

The radial increases in wood specific gravity known in many tree species have been interpreted as providing mechanical support in response to the stresses associated with wind loading. This interpretation leads to the hypothesis that individuals reaching the canopy should (1) be more likely to have radial increases in specific gravity and (2) exhibit greater increases than individuals in the subcanopy. Wood specific gravity was determined for three species of forest trees (Acer rubrum, Fagus grandifolia and Tsuga canadensis) growing in central Massachusetts, USA. Acer rubrum shows radial increases in specific gravity, but these increases are not more pronounced in canopy trees; the other two species show a pattern of radial decreases. The degree of radial increase or decrease is influenced by tree height and diameter. Of the dominant tree species for which we have data, A. rubrum, Betula papyrifera and Pinus strobus show radial increases in specific gravity, whereas F. grandifolia, T. canadensis and Quercus rubra show decreases. The occurrence of radial increases in B. papyrifera and P. strobus, which are often canopy emergents, suggests that it is overall adaptive strategy that is important rather than position (canopy vs. subcanopy) of any individual tree. It is suggested that radial increases in specific gravity are associated with early-successional status or characteristics and decreases with late-successional status or persistence in mature forest.