ROOTS - The Soil - page 46

F IG. 15
Sitka spruce tree blown over by a storm in Kielder Forest, Northumberland, showing the shallow root
plate. (Photograph G.P.Buckley.)
The typical shape of a tree root system is very different from its above-ground
crown. Whereas the latter normally has a well-defined trunk, the main roots of most
trees radiate out from the base of the trunk; there may also be a few downward grow-
ing roots, though they rarely penetrate more than a few metres. When a tree does blow
over, this pattern is readily seen: on good, deep soils a pit as much as a metre or two
deep may be excavated by the falling tree, the roots pulling the soil up with them.
On poorer soils, and especially on waterlogged peat, a fallen tree may scarcely leave
any hollow at all: downy birch Betula pubescens , one of the commonest trees on peat,
has a wholly superficial root system, and Sitka spruce Picea sitchensis , which is often
grown on peat, is similar and so very susceptible to windthrow ( Fig. 15 ).
Nevertheless, the underground parts of plants can be remarkably extensive, and
can make up a considerable fraction of the total mass of a plant. Trees and annual