Leaf litter in Cantabrian and Pyrenean beech forests

Environmental conditions

Water availability varies within a single forest. On steep slopes, runoff carries away leaf litter; small depressions at the foot of slopes accumulate more leaves, water and nutrients. A survey of 240 bags of leaf litter collected between May and July 2009 found higher total faunal abundance and more detritus-feeding and predatory macrofauna in these accumulation sites than in dry sites. This is an observation from that campaign, not a current census.

Distribution and scope

Beech forests on the Iberian Peninsula are concentrated mainly along the Cantabrian–Pyrenean axis, in mountain environments with high annual rainfall. Their dominant tree, Fagus sylvatica, forms a dense canopy and often forests with a single tree species. Leaf litter was studied in Picos de Europa, Ordesa y Monte Perdido and Aigüestortes i Estany de Sant Maurici; in the latter case, some sampling sites were also located around the park.

Structure

The leaf layer is not an inert carpet: its structure provides spaces for a diverse community of invertebrates. Among the groups studied, enchytraeid worms and springtails feed on decomposing organic matter or plant-like microorganisms; earthworms, woodlice and millipedes belong to the larger detritus-feeding group. Spiders, pseudoscorpions, harvestmen and centipedes occupy positions as consumers of other animals. These differences in feeding connect the community into a food web.

Ecological processes

Fallen organic matter and soil organisms contribute to nutrient recycling in the forest. They do not all act in the same way: some consume detritus or microorganisms, while others feed on those consumers. Changes in moisture and feeding relationships can therefore affect how leaf litter functions, as well as altering the distribution of its fauna. Predators are also part of this web, not just animals that eat remains.

Seasonal changes

Each autumn, falling beech leaves replenish the organic layer. Slow decomposition and its structure provide an environment relatively sheltered from the cold, allowing active fauna to be present for much of the year. In Picos de Europa, drizzle and fog favour relatively constant humidity; the beech forests studied in Ordesa and Aigüestortes show a stronger seasonal contrast.