Comparative Leaf Phenology of White Oak and Northern Red Oak

In the landscape, loss of interveinal tissue in developing leaves (leaf tatters) is common for white oak (Quercus alba L.), but not northern red oak (Quercus rubra L.). Previous research identified the cause of leaf tatters, as exposure of unfolding leaves, to low concentrations of chloroacetanilide herbicides. Both white oak and northern red oak were injured by these herbicides at the leaf unfolding stage. Reports from landowners suggest white oak is injured more often than red oak, leading us to theorize that white and northern red oak leaves emerge at different times, and white oaks were more likely to be at the leaf unfolding stage when chloroacetanilide herbicides are applied. A study of comparative leaf phenology of white and northern red oak was done at three sites in Urbana, IL. Identifying oak pairs was challenging, and at each location, four to six paired mature white oak and northern red oak trees were used to observe phenological events. Key development stages (swollen bud, leaf unfolding, or fully expanded leaf stages) were considered to have occurred when reached by greater than 50% of the canopy. Northern red oak expanded leaf stage occurred earlier when compared to white oak. Time between phenological events was similar for both species. Although northern red oak leaves emerged earlier, there was a range of emergence times within short distances. Difference between locations illustrates the problem in predicting tree phenology even among populations a few kilometers apart. Leaf phenology alone does not explain leaf tatters, and other factors including distribution differences in oak species must explain landowner observations of tree injury. There was a strong correlation between growth phases of the two oak species with cumulative growing degree days, cumulative rainfall, and day length.