ecological study of water-filled tree-holes and their position in the woodland ecosystem

A study of the ecology of water-filled tree-holes in beech trees in Wytham Woods, Berkshire has been made and some of the results are presented here. A review of work and observations on similar aquatic habitats associated with plants is included and the paper concludes by considering the position of water-filled tree-holes in the woodland ecosystem. (1) Water-filled tree-holes are the most common aquatic habitats occurring in plants in the temperate zone. They vary greatly in size and occur at all heights from the ground. They are especially common in beech trees but are recorded from a large variety of other species. They are of two types basically, pans and rot-holes, and contain layers of rotting vegetation, fallen leaves and rain-water. (2) Six species of insect pass their immature stages in the tree-holes studied. Five of these are Diptera and the sixth, a beetle. All these larvae are saprophagous and largely restricted to tree-holes. (3) All species of larvae studied are present at all times of the year but all except the beetle, Prionocyphon serricornis, show marked seasonal fluctuations. The most numerous species, the chironomid Metriocnemus martinii, appears to have up to three generations each year while the other species of Diptera have only a single one. It is suggested that Prionocyphon may take two or more years to complete a single generation. (4) Most species pass the winter as quiescent larvae or adults but the ceratopogonid, Dasyhelea dufouri, has larvae which actively feed, grow and develop during the winter. (5) Densities of all species are higher in holes in the canopy layer (i.e. above c. 2 m) possibly because the quality of the detritus as food is higher in these holes. (6) Densities of larvae in the smaller holes are generally higher than in the larger holes. This is unexplained but possible reasons connected with food availability, microclimate and density of eggs are suggested. (7) Tree-holes are put forward as examples of `discrete habitats' and their fauna show characteristics which are also attributable to the inhabitants of other discrete habitats. (8) Water-filled tree-holes and their fauna have many connections, both direct and indirect, with other parts of the woodland ecosystem. There are connections with the canopy-layer via leaf-fall, with the leaf-litter via drifting, and with the soil via rain-splashing. The adult insects that emerge from tree-holes must feed and require vegetation, flowers or, in some species, other