The "Length of branches with leaves (LbT)", a new index to compare insect densities on trees of different expansion

Some hundred small-sized Aspen (Populus tremula) and Willow trees (Salix caprea) occurring at the forest edge were examined since 1997 in Lower Saxony. Several thousand phytophagous insects, mainly larvae of the family Lepidoptera, were found on the trees. The objective was to compare the habitat preferences of the insects in relation to the type of forest edge and one practical problem which presented itself was find a way of describing the size of each examined tree. The shape of trees at the forest edge differs from the ideal tree shape encountered inside a forest, and the commonly used parameters like height or crown volume are not helpful when describing the volume of leaves and branches which may be used as habitat by phytophagous insects (Fig. 2 and 3). Therefore, a new parameter, the "Length of branches with leaves (LbT)", has been developed. All branches with leaves longer than 10 cm are measured from the basis of the first leaf to the top of the branch. The sum of all the leafed branch lengths, measured in metres, is designated as LbT for every tree (Fig. 1). The number of larvae of Lepidopterans on the examined tree, divided by LbT is used as an index of preference, for a single taxon, a group of taxa or all individuals found on a single tree or either a special group of trees (Fig. 5, Tab. 1 and 2). This index, known as the "edge habitat preference" enables the comparison of the settling density of the taxon or the insect group of interest in relation to defined tree classes/species. The quantity LbT is relatively easy to assess in the field and can be used to compare shrubs and trees with very different shapes and sizes (Fig. 4). In addition the changes of tree shapes resulting from natural growth or possible damage by browsing or cutting may be assessed, if trees are examined repetitively.