Factors influencing herbivory by insects on oak trees in pure stands and paired mixtures.
An investigation was made of factors affecting the distribution and abundance of insect herbivores on oak (Quercus petraea) in pure stands or in mixtures with alder (Alnus glutinosa) or spruce (Picea abies) recorded in a previous study. The stands were in Lancashire, UK; measurements were made in 1984-87. The N content of oak leaves from the oak/alder mixture was greater than that from the pure oak plot throughout the season (June-August) and the oak/spruce plot in July-August 1985. The consumption of oak leaves and the growth of a generalist herbivore (Spodoptera littoralis) were greater on oak leaves from the oak/spruce mixture, despite the fact that leaf N and herbivore abundance were both low in the oak/spruce plot. Incident light was greater in the oak/alder and pure oak plots than in the oak/spruce plot, especially in the middle and lower canopy where oak leaves were always deeply shaded relative to other stands. The C content of oak leaves from the oak/alder plot was greater than in leaves from the oak/spruce plot. There was a positive linear relation between leafhopper [cicadellid] density and leaf apparency. In 1985 there was a negative relation between total insect damage and leaf apparency. It is suggested that this is because leaf apparency is related to height within the tree canopy and hence the amount of incident light. The amount of incident light is an important factor controlling the production of oak leaf defences. Bud burst of oak trees in the oak/spruce plot was later than bud burst in the pure oak and oak/alder plots.