Altitudinal and within-crown distribution of insect grazing on Mountain birch (Betula pubescens coll.) in SW Greenland and NW Norway

Abstract

The distribution of grazing on birch leaves over different altitudes and levels above the ground was studied on valley slopes at three Greenland localities with different degree of oceanity, and compared with three Norwegian localities previously studied.

In the Greenland valleys, grazing was concentrated to the lower half of the slopes and to the near-ground foliage, except at the most continental locality where it extended up into the crowns of the tree-sized birches. In the Norwegian valleys, grazing comprised most of the forest-clad slope and the whole crown of the birch trees. In Norway, the grazing insects were Epirrita autumnata and Operophthera brumata (Lep., Geometridae), their larvae being in outbreak numbers, in Greenland the larvae had left the foliage and therefore remained unidentified. They were in non-outbreak numbers. The importance of these differences for the results is discussed, as well as the effect of climate on grazing and the effect of grazing on growth of birches.