The Nutrient Cycle - Soil: Reflections on the Basis of our Existence - page 95

Scandinavian Cold Enables Humus Layers to Grow Thick
Leaves and needles that have not decomposed fully can accumulate to form thick
layers of soil. This happens if the temperature is sufficiently low to prevent full
decomposition of fallen leaves and twigs before the process starts again the fol-
lowing autumn. I saw this with my own eyes on a trip to northern Sweden.
The summit of Akkeli mountain lies midway between lakes Uddjaure and
Hornavan near the town of Arjeplog in Swedish Lapland. The sweeping view from
the peak takes in a myriad of islands, most of them covered in forest and largely
untouched by human hand. The islands' inaccessibility makes exploitation for log-
ging unprofitable, in stark contrast to the clear-fells and young plantations that
pockmark the mainland. Not all the islands are untouched, however: some have
red wooden cabins with green lawns. Here and there are patches of bright purple,
where fireweed has taken hold. This stately plant is common throughout most of
Sweden, especially in the north.
Along with Professor David Wardle and his colleagues at the Swedish University
of Agricultural Sciences, I visited some of the islands to take samples. Together we
filled a motorboat with soil augers, insect nets, collection jars and quadrats. Scientists
have taken an interest in the islands ever since a forester in the 1930s noticed that
several of the smaller ones had much thicker soil than typically found on the main-
land, with a layer sometimes more than a metre thick on the scree that constitutes the
geological substratum. So thick was the soil that by the water's edge it formed banks
high enough for sand martins to nest in. The forester reported his findings to the
National Council for Forest Preservation, but they did not come to the attention of
university researchers until many years later. David Wardle and colleagues from all
over the world now visit the islands every summer to take samples and record data.
Our first port of call was a medium-sized island dominated by birch trees, with
lesser numbers of pine and spruce. Lingonberry, bilberry and crowberry bushes cov-
ered the ground, though the berries had yet to ripen. I settled down on the shore and
watched the augers drill into the soil and the insect nets sweep through the under-
growth. Within minutes I was covered in ants and, as I jumped up frantically trying
to dust them off, I gave the insect net-wielding Michael Olsson a good laugh. He
called the place “Ant Island” and told me he had caught more than twenty thousand
ants in his traps there, compared to just a few hundred on the other islands.
“Why is that?” I asked him.
Michael had no firm answer, having been unable to establish any link between
the ants and the type of vegetation or island size. He had, however, noticed that
aphids were commoner and spiders less common compared to the other islands.
Ants are known for “milking” aphids for the honeydew they secrete when eating
plant sap, and everywhere I looked I saw ants scaling the birch trunks to reach
aphids in the leaves and others descending, laden with honeydew to take back to
their nests. The ants show their appreciation for the aphids by protecting them
from spiders, which they attack and kill.
We moved on to the next island, which was much larger and dominated by pine
and bilberry bushes, on which the fruit was ripe. I gorged on the berries while the