Oak Tree Habitat, pt.1

Oak trees, genus Quercus, have over 400 species worldwide, but almost one-third are threatened with extinction.  They are a keystone species, playing a central role in supporting the entire habitat in which they live.  They maintain the structure and stability within their ecosystem, substantially affecting the types and abundance of other species.  Oaks make up only 2% of plant species, but they directly support more than 30% of all plants and animals.

Oak trees live from a few decades to several centuries.  The Great Oak, Quercus agrifola, a species of Coast Live oak in California, is estimated to be over 2,000 years old.  Oaks clean the air by absorbing pollution and sequestering carbon.  They provide shelter, food, shade, and help to reduce erosion.  Humans get food from the acorns, dyes from the tannin, and wood for construction, ships, and barrels, plus cork for stoppers and oak chips for smoking meat, fish, and cheese.

In spring, tender new leaves containing fresh sap are consumed by a variety of insects.  In fall, older leaves will fall to the ground, but are slow to decompose, building up many layers of leaves over time which provide shelter for small woodland ground dwellers and food for decomposers. 

Oak trees have both male and female flowers.  Male flowers grow in early spring in long clusters hanging from tree limbs.  The pollen is a favorite food for a large variety of insects, especially bees.  Pollen is released slowly, over several weeks, to be blown by the wind to the female flowers, which appear as tiny, red flowers near the tips of twigs and new shoots.  Female flowers are eaten by insects and are a favorite of red and gray squirrels.  Oak flowers are one of the earliest food sources for animals in spring, and must be abundant on every tree so that some are able to survive to produce acorns, the seeds of the oak tree.

^{Bur Oak acorns, Quercus macrocarpa; Northern Red Oak acorns, Quercus rubra; Live Oak acorns, Quercus virginiana by Steve Hurst, ARS Systematic Botany and Mycology Laboratory, USDA-NRCS PLANTS Database;}

In late summer, after pollination, the female flower will produce a large, green nut called an acorn, which will mature in autumn, turn brown, and fall from the tree.  Acorns are a nutritious food source for many animals, being high in energy, carbohydrates, and fat.  In the eastern U.S., many species are dependent on acorn production, and the amount of available acorns significantly influences population sizes.  Some acorns will find suitable ground for sending out a root and starting a new tree, but most will be eaten.  Masting, an over-abundance of acorns, occurs every few years.  It is thought that masting creates a better chance for new oak trees to grow by overwhelming the animal populations’ need for food.  As a secondary result, animal populations may expand immediately following a mast year, only to have higher than normal mortality when food resources return to normal for the following years.

Oak roots grow close to the ground’s surface and extend well beyond the circumference of the tree’s crown.  A complex network of fungi live on the roots and provide additional nutrients to the tree, keeping both species healthy.  As the trees grow and age, roots and bark develop crevices and holes in their surfaces, or chunks may become loose, providing niches for plants and animals to shelter and grow.  Exposed wood gradually dies and decays providing even more habitat and food for decomposers and the other wildlife that live on them. 

In our next post, we will take a look at some of the many species that oak trees directly and indirectly support with shelter, food, and breeding opportunities.