Latitudinal gradients in herbivory and plant antiherbivore defenses for North American hardwood species

Latitudinal gradients in herbivory and plant antiherbivore defenses are widely accepted, based primarily on evidence from interspecific studies comparing tropical and temperate taxa. Much less research has been conducted on intraspecific latitudinal variation and local adaptation in plant defenses over a species range. This research tests whether: 1) plant defense (palatability, nitrogen content, toughness, lignification, and tannin concentrations) against herbivorous insects is better associated with latitude than with local adaptation or with genetic differences among subpopulations, 2) plants from low latitudes have higher constitutive defenses than plants from high latitudes, and 3) plants from low latitudes have lower inducible defenses than plants from high latitudes. The study focuses on six important hardwood species of eastern North America: white oak, chestnut oak, willow oak, red oak, tulip poplar, and red maple. Populations from throughout the range of each species were planted in three common gardens at different latitudes (Rhode Island, north Alabama, and north Florida) during 2006. Preliminary analyses of field-collected leaves have yielded no latitudinal trends in herbivory or chemical defenses within particular species thus far. However, the glass house studies show a clear trend of populations from lower latitudes initiating bud burst before those from higher latitudes. This poster also includes field data and results obtained during 2007 in Rhode Island. The multiple common garden approach permits large sample sizes, promotes interactions with local herbivores, and removes environmental variation inherent in field surveys. It also allows separation of the effects of genetic variation and local adaptation. The approach permits an investigation into the level of plasticity in defenses, as well as gene by environment interactions. Because the study includes phenological studies of some species across a latitudinal gradient outside their current range, the results may also shed light on potential effects of climate change on North American tree species.