Plants – Trees & Shrubs – Oaks

Oak Tree Habitat, pt.2

In the last blog, we discussed oak trees as a keystone species, substantially affecting the types and abundance of other species in their habitat. There are 400-500 species of oaks worldwide, supporting an extraordinary 2,300 wildlife species not including fungi, bacteria, and other microorganisms. Oak trees in all stages of life and death have numerous biotic relationships with wildlife.

^{Badger in tree cavity by Byrdyak, Getty Images||Golden millipede in Lagos by efeghali, Getty Images||Neanura springtail by Henrik_L, Getty Images||Pileated woodpecker in oak tree by mtruchon, Getty Images||Slug & rollie pollies by Sonya Kate Wilson, Getty Images||Wood Mouse in tree cavity by byjohn, Getty Images}

Fungi are often identified as attacking oak trees, but they can have very beneficial relationships. Fungi growing in the ground assist an oak tree in getting nitrogen, phosphorous, and other nutrients out of the soil, and in return get carbon and energy from the sugars in the tree’s sap. Oaks provide a place to live for over 700 species of lichens, plus uncounted liverworts and mosses. Lichens covering the oak’s surface help to keep them free of fungal infections and invasive insects. Liverworts and mosses help oaks to retain water and release it into the soil over a long period of time. As oak limbs fall and decay, water retention softens the wood providing food and habitat for decomposers.

Green moss on oak tree by tntemerson, Getty Images

Lichen & peat moss on oak by Sean Gardner, Getty Images

Fallen oak leaves are slow to decompose due to their tannin and lignin content, both preservative compounds. Leaf litter, composed of several layers of fallen leaves, becomes habitat for many millions of species including millipedes, springtails, woodlice, slugs, beetles, ants, and other organisms that feed on decaying plants. Through their activities, the creatures of the leaf litter help to return nutrients to the trees.

Oak forest leaf litter by Tina_C_Olsen, Getty Images

Acorns, with high amounts of fats, proteins, and carbohydrates, are one of the healthiest and most sought after foods in the woodland. White oaks, living between 200 and 300 years, will produce about 3 million acorns during their lifetime. Many bird species eat acorns, but oak trees enjoy a special relationship with blue jays, whose beaks are strong enough to pry an acorn open. In autumn, blue jays enjoy feeding on acorns, but also cache them for winter, burying several at a time under a few inches of soil in various locations up to a mile from the originating tree. Although blue jays cache large quantities of acorns, they can only remember and retrieve about 25% of them, allowing many new oak trees to grow and spread into new areas.

Blue jay with red oak acorn by mirceaux, Getty Images

Oak trees support 31 mammal species who consume acorns for food, use the tree for shelter, or use the surrounding habitat for hunting. Gray squirrels and red squirrels favor acorns in the autumn and throughout the winter from caches hidden in nearby locations. They remember far more cache locations than blue jays, but there are still uneaten acorns that will germinate in spring to start new trees. Acorns are also a favorite food for badger, deer, wild board, and wood mouse.

^{Gray squirrel eating acorn by viktor2013, Getty Images}

^{Red squirrel with acorn by Neil_Burton, Getty Images}

Oak trees offer shelter for many species. Squirrels often build their nests high up among the leaves that provide protection from weather and predators. Dead branches, especially those with cavities, may remain on the tree for many years, providing excellent roost sites for bats. Open limbs, leafed areas, and cavities all provide nest sites for a variety of birds. In addition, the many insects drawn to oak trees provide a ready source of food for all of these nesters. Humans have been acorn consumers for millennia. Raw acorns, high in tannin content, can be toxic in larger quantities; but, properly prepared, acorns are high in calories, vitamin C, starch, magnesium, calcium, phosphorous, and antioxidants. They have been a food staple in many cultures and can be found in the records of ancient Greeks, Iberians, Japanese, and English.

Oak trees have many direct and indirect relationships with insects, which we will explore in the next blog.

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.

^{A great oak in Nottingham Forest by Peter Shaw, Getty Images}

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.

^{Red oak with new leaves, Quercus rubra, by aleroy4, Getty Images}

^{Oak forest leaf litter in winter by Whiteway, Getty Images Signature}

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.

^{Branches of red oak with catkins, Quercus rubra,}
^{by anmbph, Getty Images}

^{Texas Red Oak female flowers, Quercus buckleyi by Doug Goldman, USDA-NRCS National Plants Data Team}

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.

^{Bur Oak bark, Quercus macrocarpa by Herman, D.E., ND State Soil Conservation Committee, USDA-NRCS PLANTS Database}

^{Dead oak log in Bialowieza Forest by Nirian, Getty Images Signature}

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.

The Woodlands at the Turning, an Illinois oak savannah, by Justin Kern, Nov 4, 2013

Oak-Hickory Forest

Oak-hickory forests makeup one-fourth of all the woodlands in the eastern United States. At the end of the last ice age, oaks were only found in the Appalachian Mountains and the driftless section where glaciers did not form in southwestern Wisconsin. In the last 15,000 years they have repopulated the land from the Mississippi River to the Atlantic coastline.

Clockwise from top left: Quercus alba, white oak by Chris Andrei, Getty Images; Quercus palustris, northern pin oak by Michel VIARD, Getty Images; Quercus rubra, northern red oak by ValerijaP, Getty Images; Fraxinus pennsylvanica, green ash by Garsya, Getty Images; Carya ovata, shagbark hickory by Elmar Langle, Getty Images; Prunus serotina, black cherry tree by Ina Hensel, Getty Images

Oak-hickory forests are an open woodland community where trees make up less than 10% of the plant life. Species of both the red oak and the white oak groups dominate these wood including Northern red oak, Quercus rubra, black oak, Quercus velutina, white oak, Quercus alba, and bur oak, Quercus macrocarpa. Two members of the hickory family, shagbark hickory, Carya ovata, and bitternut hickory, Carya cordiformis, are also found in abundance. These forests are rich with several other tree species most notably red maple, Acer rubrum, sugar maple, Acer saccharum, black cherry, Prunus serotina, black walnut, Juglans nigra, and green ash, Fraxinus pennsylvanica.

^{Arisaema triphyllum berries by ES3N, Getty Images}

^{Arisaema triphyllum, jack-in-the-pulpit by Holcy, Getty Images}

The open structure of the forest supports a large variety of other plants that offer food, shelter from predators and weather, and places to raise a family. Food sources are plentiful including fruit-bearing plants of the genus Vaccinium, which includes blueberries and raspberries, and vines from the genus Vitis, which includes several species of wild grape. Bloodroot, Sanguinaria canadensis, and Jack-in-the-pulpit, Arisaema triphyllum, along with several clovers, attract birds and insects that attract larger predators, creating a rich forest community. It is estimated that oak-hickory forests support over 300 animal species including birds, mammals, insects, reptiles and amphibians.

^{Oak woodland by fotoVoyager, Getty Images Signature}

Oak trees provide a haven for wildlife. Acorns, the fruit of an oak tree, are large and heavy, so they do not spread far from the tree. They offer good nutrition and are a favorite of many wildlife species, whether eaten immediately or cached and eaten over time. Oak trees retain their dead leaves throughout the winter, not letting them fall until early spring. The leaves provide shelter from wind and rain, as well as hiding many smaller birds and mammals from airborne predators. Oak trees have craggy bark providing shelter for insects and other small animals including many amphibian species that overwinter under the bark.

After dying, oak leaves retain tannin, a chemical that slows down their decay. As leaves build up on the forest floor year after year, they create thick layers used by amphibians, reptiles, small mammals, and insects for shelter from predators and weather. In winter, the leaf layer provides insulation from the snow and cold, allowing small animals to move around under the snow, hunting for food, or moving to other sites. However, the leaves can be advantageous to some predators, as well. Owls, and other nighttime hunters, have excellent hearing and can easily detect the sounds from crackling leaves as prey attempts to hide and flee.

^{Gnarled tree with large cavity from wepix, Getty Images Signature}

^{Nature lives forever in fallen trunks by Oleksiy Brunets, Getty Images}

Many tree species are susceptible to dry rot of their interior wood, and none more-so than members of the red oak family. This provides natural cavities for woodland species. Woodpeckers, owls, chickadees, and nuthatches, plus squirrels, raccoons, bats, amphibians, and reptiles all use cavities for shelter and protection. Almost 40% of animal species inhabiting oak-hickory forests will use cavities at some point in their lifetimes. As trunks and branches decay and fall to the forest floor, they create opportunities for food and shelter. In addition, animals drawn to these spaces provide food for larger predators.

As you walk through forested areas over the summer, take note of the tree species and the overall structure you observe Can you spot holes in trees, logs laying on the forest floor, and thicker layers of leaf cover? Are the woods open where you can see possible lanes of movement? What plants have fruit or seeds? Keeping a journal of observations helps us to identify the changes and enjoy these rich habitats.