Developing a stand hazard index for oak decline in upland Oak forests of the Ozark Highlands, Missouri
Black oak (Quercus velutina Lam.) and scarlet oak (Quercus coccinea Muenchh.)--two major components (44% of total stand basal area) of upland oak forests--are suffering severe decline and mortality in the Ozark Highlands, Missouri. However, factors influencing their survival (mortality) are not well understood. In this study we quantified how stand and tree-level predisposing factors are associated with survival of black and scarlet oaks. Sixteen-year monitoring data from the Missouri Ozark Forest Ecosystem Project (MOFEP) indicated that overall annual mortality of black and scarlet oaks averaged 2.2 and 1.7%, respectively, three to five times higher than expected (around 0.5%) for white oak, a common associate. For the first 8 years of the study (1990-1998), survival rates of black and scarlet oaks were similar. Thereafter, the survival rate of black oak declined relative to scarlet oak. Using the classification and regression tree (CART) method we classified black oak and scarlet oak trees into seven and nine risk groups, respectively, that differed significantly in rates of tree mortality. Groups were distinguished based on tree diameter, crown class, and size relative to competitors. An oak decline and mortality hazard index was thus developed as the weighted means of risk group mortality, which can help managers prescribe species-specific silvicultural treatments to help mitigate oak decline and associated mortality.