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 period in oak stands (Quercus robur) of the forest-steppe zone in the Voronezh region (eastern-central Russia). Relative losses in wood increment, due to insect-caused defoliations of diverse extents and periodicity, were quantitatively assessed. Such estimates were made for trees with early and late phenology, as well as with respect to growth classes and to local forest types. The intensity of the responses of dominant and codominant trees were shown to vary in different forest types. No differences in increment losses were detected between early and late oaks after one to three moderate defoliations. A factor two variation was observed in the intensity of the responses, depending on actual climate. In response to repeated defoliations, the ratio latewood to earlywood was reduced by a factor 5-8 in a stand and even 20 in an individual tree. This suggests that insect defoliation severely affects oak growth.