Wood with real polish

The Ecology of Trees in the Tropical Rain Forest
November 29, 2002

Deep in the basement of my own institute is a dark room marked "Xylarium". Inside are shelves of wood blocks, catalogued like the volumes in a library, dusty plaster models of wood fibres and draws of microscope slides, relics of an era when wood structure was a key element in the botany curriculum. It is a room devoted to the description, but not the interpretation, of biological variation, an approach that I have always found to stifle my interest.

So it was only when held reluctant prisoner by yet another late-running home-bound train that I settled down to read the lengthy section on the structural and mechanical properties of wood that opens chapter two of Ian Turner's book. Turner examines the basis for ecological variation among tropical tree species, often from a strongly physiological perspective. His approach is meticulous. The book is packed with recent research detail, but it is all woven into a scholarly critical analysis of each subject that grips the attention.

Take the killer subject of wood anatomy. Even I can recall, from deadly dull lectures I have endured, that trees suck water from the soil through tracheids or vessel elements in the xylem. Most textbooks will embellish this with the information that water is pulled up the tree, rather than pushed, by the suction caused by evaporation of water from the leaves, the so-called cohesion-tension theory. But trees experience the same problem that any child has encountered trying to drink through several straws connected together - the longer the column of liquid that has to be lifted, the greater the suck required. There comes a point when the suck required to lift the water exceeds the strength of the water column and it tears apart, creating an embolism.

Turner synthesises evidence that suggests that tropical rainforest trees, although they grow in areas with an ever-wet climate, may often experience difficulties in lifting water more than 50m to the forest canopy. A pressure of at least 0.1 megapascals is required to lift water each 10m, and recent research suggests that the column of water inside xylem tubes may tear apart under as little as 0.2 megapascals of suction. Intriguingly, tropical tree species from drought-prone environments seem as prone to embolism as those from ever-wet ones. A new theory suggests that the anatomy of roots, wood and vascular bundles is arranged so that the water column is protected from cavitation. I scarcely dare admit it, but the next morning I quietly slipped into the Xylarium to re-examine those plaster models of wood structure and see if I could unravel these new ideas.

This is an excellent book that does not restrict itself to a dry description of how tropical trees work. It bravely tackles the bigger issue of why different species of tropical tree adopt different solutions to complex environmental problems and looks for patterns that may have adaptive or evolutionary significance.

Nick Brown is lecturer in forest ecology, University of Oxford.

The Ecology of Trees in the Tropical Rain Forest

Author - Ian M. Turner
ISBN - 0 521 80183 4
Publisher - Cambridge University Press
Price - £55.00
Pages - 298

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