Freeze and folivory together affect oak woody growth

Plant-insect herbivore interactions are well-studied, but how other environmental factors influence these interactions is often poorly understood. Two important stresses to Quercus populations are freeze and herbivore outbreak; correlative data suggest that together these stresses can cause oak decline (localized chronic dieback and mortality of oak caused by multiple stresses). This study considers the effects of a natural late-spring freeze in 2005 and simulated herbivore outbreak on the woody growth of white Quercus alba and chinkapin Q. muehlenbergii oak saplings in Southeastern Missouri in a 2x2 factorial design. Herbivore outbreak was simulated by removal of 80% of the leaf area of all sapling leaves. Leaf reflush occurred following freeze but not following simulated herbivory; reflush leaves had higher water contents and were smaller than typical leaves. Both frost and herbivory decreased woody growth, but frost only reduced growth when coupled with herbivory, consistent with previous oak decline studies. A positive correlation between leaf area and woody growth suggests the effects of treatments on growth may be partially mediated by their effects on leaf area. Leaf measurements in 2006 indicate that treatment effects persist across years; we will therefore measure 2007 woody growth. This research has several ramifications. Climate change may increase stochasticity in weather, perhaps making freeze events following budbreak more common. The invasive gypsy moth will likely enter Southeastern Missouri within the next 20 years, presumably increasing outbreak herbivory events. The non-additive nature of these stresses makes them particularly important considerations in Quercus forest management and restoration strategies.