Tree Seed Production and Food Conditions for Rodents in an Oak Wood in Southern England

Seed production and seed losses were monitored in an oak (Quercusrobur) wood in southern England between 1975 and 1988. In additionto acorns, seeds from several large beech (Fagus sylvatica)and sweet chestnut (Castanea sativa) trees which occurred withinthe wood, and from the understorey of hazel (Corylus avellana)trees, were also monitored. The seeds from these four tree speciesare all subject to heavy predation by rodents, as well as otherseed-eaters such as birds and insects. Seed fall tended to be irregular and three years, 1976, 1985and 1987 were considered oak mast years. There were too fewyears with large acorn crops to examine whether resource depletionin these years resulted in poor crops in subsequent years. Therewas no correlation in seed production between the four treespecies which does not agree with one of the predictions ofthe predator satiation theory of masting behaviour. The effectsof weather on seed production were investigated but few significantcorrelations were found. In general the results tended to supportprevious findings. Between 1980 and 1982 a serious outbreakof Tortrix moth caterpillars occurred within the wood and mayhave affected acorn production. A negative relationship wasfound between oak defoliation and acorn production in the subsequentyear. The loss of seeds from the forest floor was initially rapidand in 9 years no seeds survived into the following year andfood conditions were poor for granivores. In 3 of the remaining4 years acorns survived well into spring and early summer providinggood food conditions for forest rodents through to the timeswhen alternative food supplies became available. Although oaktrees contributed most seed energy in good years, the othertree species, especially beech, became important when acornproduction was poor. Predispersal losses due to predators werefrequently high and averaged 80 per cent, 40 per cent, 38 percent and 33 per cent in hazel, sweet chestnut, oak and beechrespectively. On average, hazel nuts tended to fall first, some6 weeks before acorns which tended to be the last seeds to fallof the four tree species. Knopper galls and weeviled acornsfell earlier than sound acorns. A key-factor analysis indicatedthat a failure to mature, predispersal predation and insectinfestation from weevils and Knopper galls contributed equallyto changes in numbers of acorns among years.