Factors affecting the natural regeneration of Quercus in Scottish oakwoods. II. Insect defoliation of trees and seedlings.
Defoliation of oak trees and seedlings (Quercus petraea, Q. petraea and putative hybrids of the 2 species) by various lepidopteran species was investigated in 2 upland semi-natural oakwoods (Dinnet and Ariundle National Nature Reserves) within contrasting climatic zones in the Scottish Highlands. Experiments with artificially planted seedlings of Q. robur and hybrids in 1989-91 were designed to test 4 hypotheses: (i) the failure of oak to regenerate naturally is partly attributable to defoliation of seedlings by insects; (ii) seedling defoliation is linked to canopy density and composition; (iii) the degree of seedling defoliation is linked to the extent of canopy defoliation; and (iv) defoliation is positively correlated with insect population density in the canopy. The extent of canopy defoliation of oak was significantly different between the 2 woodlands, with trees at Ariundle, located within a wetter climatic zone, more defoliated than those at Dinnet, which is in a drier zone. Defoliation also varied significantly between individual trees and between years. Defoliation was positively correlated with degree of infestation by the larvae of several lepidopteran species. Leaf samples at Dinnet were dominated by Operophtera brumata, and those at Ariundle by Erannis defoliaria. The degree of infestation was higher at Ariundle. Experimental seedlings were significantly more defoliated under oak canopy than in the open or under a Betula spp. canopy. Canopy defoliation was positively correlated with defoliation of seedlings growing directly beneath, but seedling defoliation was negatively correlated with oak canopy density. A group felling system using coupe sizes of over 0.5 ha is recommended for encouraging oak regeneration in woods more susceptible to insect defoliation such as Ariundle, where seedlings need to be kept free from the influence of an overhead oak canopy. Shelterwood systems, where seed trees are retained after extensive thinning, are not recommended. In woods similar to Dinnet, both group felling and shelterwood systems are appropriate management options. Woodland managers should be aware of defoliation problems within their woods before deciding on which silvicultural option to choose.