The Nutrient Cycle - Soil: Reflections on the Basis of our Existence - page 96

Waiting for fire . The likelihood of lightning striking and causing a forest fire is much lower on
small islands than on large ones. Applying this probability, researchers have studied how fire fre-
quency affects soil processes on different-sized islands in the Uddjaure and Hornavan lakes in
northern Sweden. The humus layer is only a few centimetres thick on the large islands, as in stand-
ard forest soil, but can reach a depth of 1 m on the small islands
scientists took their samples. The forest was in fine condition and it occurred to me
that unscrupulous-minded people might be tempted to fell a few trees and float them
over to the mainland. Fortunately, no such incidents have occurred and the islands
remain undisturbed and a scientists' paradise. There is a clear link between tree
growth and the islands' size, with trees growing best on the largest islands and vice
versa. Spruce dominates the canopy on the smallest islands and crowberry the under-
growth. Fallen spruce needles and crowberry leaves decompose slowly and the soil
here is so deep because the addition of organic matter outpaces the rate of decom-
position—a process that has continued at an annual growth rate of two millimetres
since the Ice Age and which will continue so long as the islands remain undisturbed.
On the smallest islands, the soil reaches a depth of more than one and a half metres.
The islands have never been immune to disruption—even before humans set
foot there. Fierce forest fires would burn away the topsoil and signs of old blazes
can be seen on some of the ancient pines, especially on the larger islands. This
is because the larger the island, the higher the chance of a lightning strike. The
bigger islands experience blazes on average once every five hundred years, and
because pine is equipped to withstand forest fires (its thick bark offers decent pro-
tection against flames) it predominates there. Pine seeds that germinate after a fire
grow especially well because the nutrients from the humus are concentrated in the
ash. Bilberry bushes also benefit because they grow faster than lingonberry and
crowberry in nutrient-rich soil. Fire frequency is much lower, at around once every
three thousand years, on the smaller islands and here spruce predominates, while
lingonberry and crowberry are commoner in the under-storey because they fare