Most species of oak trees are deciduous -- that is, they shed their leaves in the autumn and grow new ones in the springtime. Some oaks, like certain species of live oak, are evergreen and keep their leaves year-round. The timing of oak-leaf emergence varies depending on species and location, as well as ambient environmental conditions.
Phenology is the study of cyclical ecological events and their natural timing. The leafing out of deciduous trees in the springtime is a classic phenological measure. Factors include the plants' responses to warming temperatures, day length and moisture levels in the soil.
Oaks tend to leaf out between early and late spring, depending on the latitude and species. For example, across its substantial range in North America, eastern white oak typically grows new leaves between late March and late May. California black oaks in the Pacific Northwest usually leaf out in May.
In some parts of the world, oaks appear to be leafing out earlier than their historical schedule, possibly due to a climatic warming trend. In some sites in Great Britain, for example, studies in the first decade of the 21st century suggested oaks were growing new leaves some two weeks earlier in the spring than they did in the 1950s.