Kanin Rungwattana and Peter Hietz
It takes a lot of wood to make a tree and wood thus constitutes the largest share of Earth’s biomass. At first sight, wood is needed to provide the mechanical structure that exposes leaves to sunlight. But leaves have to be supplied with water from the soil, which is also transported through wood. And wood can store the energy produced by the leaves, which is used to produce more wood. Somewhat simplified, these functions are provided by three cell types: dead fibres for strength, dead vessels for water transport, and living parenchyma cells that store carbohydrates, but also serve for defence and can repair blockages in water-transporting vessels. Trees come in a great variety and differ in life strategies such as growth rates, which is also reflected in their wood structure. In addition, small and large trees make different demands on wood, so that wood structure is also related to tree size. The change in wood structure during a tree’s life, which reflects the changing demand as the tree grows, can be studied by looking at the variation in wood structure from the center of the stem to the outermost part of wood close to the bark. We studied this in five trees from a Thai seasonal forest that differ in their shade tolerance and growth rates, and whether they drop their leaves in the dry season or not. Strength is provided by fibres and hard (dense) wood is more costly to build than light wood, thus trees with hard wood tend to have slow growth rates. We also found that when growth rate changes within a tree’s life time, wood density changes in parallel, often increasing with tree size. But larger trees also need to transport more water to their leaves, thus water transport capacity also increases in all trees. This would appear to squeeze out the function for parenchyma cells, but far from it: the amount of parenchyma actually can increase too because water transport and strength can be increased by changing cell wall thickness and cell sizes. Thus by modulating its rather simple cell structure, wood can support trees from sapling to giant.
Image provided by authors.