Late leaf development in pedunculate oak (Quercus robur): An antiherbivore defence?

Two phenological forms of the pedunculate oak co-occur in the same habitats throughout the species range: the early trees (Quercus robur var. praecox) develop leaves up to 5 weeks before the late ones (Quercus robur var. tardiflora). This study tests the idea that late leaf flushing serves as an antiherbivore defence, i.e. late trees, which develop leaves asynchronously with eclosion of folivorous caterpillars, avoid the costs of defoliation, which could offset the costs of a later onset of the growing season. Effects of folivorous caterpillars foraging on oaks were observed in 1998-2006 in remnants of primeval temperate lowland forest preserved in the Biaowieza National Park (eastern Poland). Observations covered trough and outbreak years of the major defoliator, the winter moth [Operophtera brumata L. (Geometridae)]. In seven out of nine seasons, including all peak caterpillar years, the amount of frass produced by folivorous caterpillars on late trees (n=8-18) was significantly (up to 7.1 times) lower than on the early ones (n=12-32). Assessment of the degree of defoliation in 2002-2006 showed that the late oaks were visibly defoliated only during a caterpillar peak (2003), while the early trees were affected in all years and in 2003 almost completely lost their leaves. The results confirm the effect of late budburst on lowering herbivore damage, and give support to the idea that late leaf flushing acts as an antiherbivore defence.