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Wood for the trees
Those who work with wood, and even those who just burn it, know of the immense variation in texture and density amongst the many tree species. It is density, or more correctly, specific gravity, that has been investigated by Woodcock and Shier (University of Hawaii-Manoa, pp. 529-537). Across the range of tree species, values of specific gravity between 0.12 and 1.4 have been recorded. However, wood specific gravity also varies within a species and within individual trees. It is widely assumed that changes in wood density reflect changes in conditions experienced by a tree during its lifetime. In many species, wood specific gravity increases radially from inside to outside; this is thought to be a response to greater mechanical stresses such as wind loading as the tree enlarges. The authors reasoned that in any one species, canopy trees were likely to be more exposed than sub-canopy individuals; thus it would be expected that individuals that form or protrude from the canopy would show a greater radial increase in wood specific gravity than those in the sub-canopy. But their data from a mixed hardwood forest in Massachusetts, USA, revealed a more complex situation. Of three species studied in detail, only Acer rubrum exhibited a radial, inner to outer, increase in wood specific gravity. In Fagus grandiflora and Tsuga canadiensis an opposite trend was very marked. There was some relationship between canopy position and specific gravity: the inner wood of canopy trees had a higher specific gravity than that of sub-canopy trees, but the reverse was true for the outer layers. Also, the maximum specific gravity of the wood was higher in canopy trees than in sub-canopy trees. As the authors so rightly put it: ‘The number of factors that may influence specific gravity is quite large . . .’. There is extensive scope here for further research.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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