INFLUENCE OF REPEATED DEFOLIATIONS BY INSECTS ON WOOD INCREMENT IN COMMON OAK (QUERCUS-ROBUR L)

The dynamics of radial wood increments were analysed over a 30 year pe riod in oak stands (Quercus robur) of the forest-steppe zone in the Vo ronezh region (eastern-central Russia). Relative losses in wood increm ent, due to insect-caused defoliations of diverse extents and periodic ity, were quantitatively assessed. Such estimates were made for trees with early and late phenology, as well as with respect to growth class es and to local forest types. The intensity of the responses of domina nt and codominant trees were shown to vary in different forest types. No differences in increment losses were detected between early and lat e oaks after one to three moderate defoliations. A factor two variatio n was observed in the intensity of the responses, depending on actual climate. In response to repeated defoliations, the ratio latewood to e arlywood was reduced by a factor 5-8 in a stand and even 20 in an indi vidual tree. This suggests that insect defoliation severely affects oa k growth.