FACTORS AFFECTING THE NATURAL REGENERATION OF QUERCUS IN SCOTTISH OAKWOODS .2. INSECT DEFOLIATION OF TREES AND SEEDLINGS

1. Defoliation of oak trees and seedlings (Quercus petraea and e. robu r x e. petraea putative hybrids) by various lepidopteran species was i nvestigated in two upland semi-natural oakwoods (Dinnet and Ariundle N ational Nature Reserves) within contrasting climatic zones in the Scot tish Highlands. 2. Experiments were designed to test four hypotheses: (i) the failure of oak to regenerate naturally is partly attributable to defoliation of seedlings by insects; (ii) seedling defoliation is l inked to canopy density and composition; (iii) the degree of seedling defoliation is linked to the extent of canopy defoliation; and (iv) de foliation is positively correlated with insect population density in t he canopy. 3. The extent of canopy defoliation of oak was significantl y different between the two woods, with trees at Ariundle, located wit hin a wetter climatic zone, more defoliated than those at Dinnet, whic h is in a drier climatic zone. Defoliation also varied significantly b etween individual trees and between years. 4. Defoliation was positive ly correlated with degree of infestation by the larvae of several lepi dopteran species. Leaf samples at Dinnet were dominated by Operophtera a brumata, and those at Ariundle by Erranis defoliaria. The degree of infestation was higher at Ariundle. 5. Experimental seedlings were si gnificantly more defoliated under oak canopy than in the open or under a Betula spp. canopy. Canopy defoliation was positively correlated wi th defoliation of seedlings growing directly beneath, but seedling def oliation was negatively correlated with oak canopy density. 6. Defolia tion of oak seedlings by insects poses more of a threat to the natural regeneration of oak at Ariundle than at Dinnet, possibly due to clima tic differences between these two woods. 7. A group-felling system usi ng coupe sizes of over 0.5 ha is recommended for encouraging oak regen eration in woods such as Ariundle, where seedlings need to be kept fre e from the influence of an overhead oak canopy. Shelterwood systems, w here seed trees are retained after extensive thinning, are not recomme nded. In woods similar to Dinnet, both group-fell and shelterwood syst ems are appropriate management options. 8. Woodland managers should be aware of defoliation problems within their woods before deciding on w hich silvicultural option to choose.