Acorns are a very important food for many wild mammals, including deer, squirrels, rabbits, raccoons, opossums, and mice; birds, including wild turkeys, northern bobwhite, wood ducks, mallards, woodpeckers, crows, and blue jays; and insects, including acorn weevils. In late fall and early winter, acorns may make up more than three-quarters of a white-tailed deer’s diet.
Oaks are a principal component of our native woodland and forest natural communities, and there are countless animals whose lives depend on oaks’ presence to survive. Birds and squirrels nest in trees. Spiders and a variety of insects pupate, hide, or overwinter in cracks in the bark.
Many upland species in the red/black oak group, especially in the Ozarks, began to experience dramatic die-offs in the late 1990s. This phenomenon, called “oak decline,” is a complex situation with several contributing causes. The stage was set by many trees already being old (they mainly live only 70 to 90 years) and weakened by environmental, climate, weather, and herbivore stresses, then being challenged by fungal diseases and a few particular types of insects that either bore under the bark or potentially defoliate the leaves.
Until about 100 years ago, woodlands of shortleaf pine used to dominate much of the south and central Ozarks, but after nearly all of those pine woodlands were cleared for lumber between 1880 and 1920, black, scarlet, and northern red oaks regrew in those areas instead of pines. With few pines left for reseeding, oaks have remained dominant, and the characteristics of the landscape, including soil chemistry, amount of exposed rock, presence of other types of plants, and presence of birds and other animals, have changed. The history of red oaks supplanting pines in these parts of the Ozarks is one reason why oak decline is happening — many of the oaks in this region began life in the years soon after the pines were cut.
Butterflies whose larvae eat oak leaves include several species of hairstreaks and many open-winged and dusky wing skippers. Moths whose larvae eat oak leaves include the waved sphinx, Polyphemus moth, buck moth, spiny oakworm moth, rosy maple moth, beloved underwing, the laugher, white-dotted prominent, one-spotted variant, scalloped sack-bearer, several types of slug caterpillar moths, black-waved flannel moth, and carpenter moth.
Oak gall wasps (family Cynipidae) insert their eggs into the leaves, twigs, buds, and other parts of oak trees. The plant responds by forming galls — enlarged, often rounded or inflated portions of leaves, twigs, and so on. The wasp larva feeds on plant materials formed within the gall. The various species create differently shaped galls in different parts of oaks. The galls are sometimes called oak apples or oak potatoes.
In many oaks, especially in members of the black oak group, the leaves are not totally deciduous (they don’t all fall off in the fall). Instead, the plants often retain their dried, brown leaves through the winter, especially on the lower branches. Other types of trees exhibit this phenomenon (called marcescence), too. Retaining dead leaves through winter may have a number of different adaptive advantages. The dry, unappetizing, loudly rattling leaves may deter herbivores such as deer from foraging on the twigs. Or, perhaps, keeping the leaves on the tree longer may facilitate their ability to add their nutrients back into the soil at the time the tree most needs those nutrients. Another idea is that during the winter, the leaves may capture additional snow and cause it to land and melt at the base of the tree, increasing the amount of water available to the roots.