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The True Value of Oaks
Article by: Jessica Warren, ANR Agent, Camden County

If there’s one thing that Coastal Georgia is known for, it’s our majestic oaks draped in Spanish moss. We know our native oak trees are beautiful, but is there more than just what meets the eye? Oaks are long-lived, slow growing trees that support more other species of life than any other tree genus in North America. Georgia is home to over 30 species of oak trees.

Native oak trees are keystone species. Keystone species are so named because they act as the keystone in a Roman arch – they are the crucial piece that holds the ecosystem together and supports the other pieces. When the keystone is removed the arch, or ecosystem, collapses. Oaks are communities of hundreds of species, and provide food, nesting materials, nesting sites, and shelter among other resources.

One of the biggest contributions of oaks is the number of caterpillars that they host. Caterpillars fuel the food web. Each insect species has evolved over time to be able to overcome the defenses of one or two genera of plants (their host plants). Oak trees host 897 species of caterpillars in the US, while most plant genera only host a few caterpillar species. Seventy-five percent of the insects required by birds and other animals are produced by only a few plant genera with oaks playing the biggest role. Not everyone loves to see caterpillars (though many turn into beautiful butterflies and moths), but each nest of baby songbirds needs 3000-4000 caterpillars to fledge. A landscape without native insects and caterpillars cannot support songbirds. Hummingbirds also need caterpillars and small insects for protein. In addition, they need lichens and spiderwebs for nest construction, which oaks conveniently provide as well.

Another wildlife resource that oaks provide is acorns. One oak can produce 3 million acorns in its life. Acorns provide protein, fat, and carbohydrates when food resources are scarce. Acorns are critical winter food for dozens of species of birds (from songbirds to turkeys), squirrels, rodents, black bears, opossums, raccoons, white-tailed deer, and others.

In addition to food, oaks offer a lot of shelter to other organisms. As oaks age they lose much of their inner xylem tissue, creating hollows in their trunks. Hollowing is a normal part of aging and does not affect the living cambium just below the bark. Cavities in trunks are used by many species as dens, nests, or roosts. Species include squirrels, bears, bobcats, racoons, opossums, bats, and birds. Oak foliage offers a lot of cover and shelter as well. Foliage conceals bird and mammal nests in the canopy, and since many oak species drop their leaves later than other deciduous species, they provide cover for wildlife when little cover is available. Several bat species including the Seminole bat, tricolored bat, and northern yellow bat roost in the hanging Spanish moss that our coastal oaks are known for. Birds such as yellow-throated warblers and Northern parulas build nests in hanging Spanish moss, and other birds collect Spanish moss to use in nest construction.

Leaf litter is one of the most valuable resources that oak trees provide. Leaf litter provides habitat for literally millions of organisms. Many are detritivores – essential organisms that break down organic matter and convert it into soil and nutrients that the whole ecosystem depends upon. In addition to these decomposing arthropods and fungi, other creatures that depend on this litter include salamanders, frogs, insect larvae (including butterflies, moths, and fireflies), small mammals, birds, bees, and turtles. Leaf litter provides food, shelter, and warmth for overwintering pollinators and wildlife. Over 90% of the caterpillar species that use oaks for their host tree drop to the ground to pupate underground or spin a cocoon in the leaf litter. In fact, many species (such as fireflies) require undisturbed leaf litter to complete their life cycle and reproduce. In addition to its benefits to wildlife and pollinators, leaf litter builds soil nutrition, structure, and quality. Leaf litter is the original (and most effective) mulch and helps maintain appropriate moisture and temperature levels in the soil and protect tree roots. Oak leaf litter (and roots) also produces dozens of fungi species including several edible species. Oak leaves in a heavy layer have been shown to deter some invasive species including Japanese stiltgrass and Asian jumping worms, and leaves that fall into waterways feed aquatic macroinvertebrates and offer them cover.

Oaks, like all native trees, offer a lot of ecosystem services. Ecosystem services are life sustaining benefits that we receive from nature and often take for granted. Oaks sequester carbon, prevent erosion, produce oxygen, regulate soil temperatures, and contribute to nutrient cycling and soil formation. The canopy and root system help rain and stormwater infiltrate into the soil instead of running off, improving water quality and reducing flooding. Oaks prevent the scouring of streams and protect aquatic ecosystems. They also offer cooler summer temperatures to homes under or adjacent to their canopy, reducing cooling costs.

If all of this has you thinking about planting an oak in your landscape, here’s a little advice. Acorns are easy and free. Acorns will grow into healthier trees than transplants, and if you’re using acorns collected locally from healthy trees, they’re likely to be well adapted to this area. If using transplants, smaller is better. Smaller transplants are more likely to survive planting and will outgrow larger transplants that will take longer to establish and recover from transplant stress. Protect small seedlings or volunteers with wire cages. Plant several trees instead of just one. Trees planted roughly 10 ft apart will interlock roots forming more stability and resilience to weather events than solitary trees.

All native trees offer more ecosystem resources and host more native pollinators, insects, and wildlife than non-native trees, though oaks are host plants to the largest amount of biodiversity. If you’d like to add more diversity and interest to your landscape, consider adding native cherries, willows, birches, hickories, pines, and maples in addition to oaks, as these trees also rank high in biodiversity support as host trees. If you’d like to learn more on this topic, I highly recommend Doug Tallamy’s book The Nature of Oaks. In fact, all his books are wonderfully well researched resources for bringing more life to your landscape.