Invertebrate predators in Sitka spruce plantations managed by clear-cutting or continuous-cover silvicultural systems

Abstract

Invertebrate predators of aphids were sampled at ground level and in the canopy in 13 even-aged Sitka spruce Picea sitchensis forest stands managed by clear-cutting and replanting, and in 11 Sitka spruce stands managed as continuous cover forestry using either shelterwood or group selection silvicultural systems. The expectation was that stands managed using shelterwood or group selection systems would support a greater abundance and diversity of invertebrate predators, because of their more complex internal structure, but results were mixed. Hoverflies (Syrphidae), predatory bugs (Hemiptera), ladybird beetles (Coccinellidae) and spiders (Araneae) were more abundant in shelterwood and group selection stands, but soldier beetles (Cantharidae), lacewings (Neuroptera) and harvestmen (Opiliones) were more abundant in mid-rotation and mature even-aged stands, and vespid wasps (Vespidae) showed a similar abundance across all stand types. In contrast, species richness and diversity of the insect predator groups were higher in shelterwood and group selection stands, even for groups that were less abundant in those stands. It was not clear, however, whether the greater species richness of insect predators in shelterwood and group selection was the result of these stands supporting breeding populations of a wider range of species, including less common species, or was because the more open structure of these stands allowed introgression of species from other habitats within the forest.