The Addicted Gardener: Oak trees a keystone to our environment

Donna Lane, Columnist

In last week’s column, I related entomologist Doug Tallamy’s view on how we could improve our landscapes with native plants that will help keep the diversity of nature in balance and preserve species that have been in decline due to climate change and human interventions.

This week I will focus on a keystone plant – the oak tree. What’s a keystone plant? A keystone plant is one in which it is so vital in the local food web that if it’s removed the food web will collapse.

Why is a native tree like the oak better for our environment? Because non-native trees are “food deserts” (e.g., Bradford/Callery pear, Ginko and Norway maple); that is, insects and pollinators do not feed on them.

According to Tallamy, our landscapes should do four things. First, support the food web so that other living things can exist there. No other plant genus supports the food web the way Quercus, or oaks, do.  Plants capture energy from the sun and turn it into food, but that food is locked up in the leaves unless something eats those leaves and passes it on to animals. Most vertebrates don’t eat leaves; they eat something that ate plants, primarily insects, and more specifically caterpillars, which are passing on that energy. More than 900 species of caterpillars are supported by the oak trees in this country.

Second, we must sequester carbon; that is, pull it out of the atmosphere and lock it up in plant tissues. More importantly, we need to lock it into the soil for the long term. Plants with the biggest root systems do that best, and the oaks have the biggest root systems.

Oaks also help to manage our watersheds. Again, because of their large root systems, they hold water on our property better than those with tiny root systems, such as lawns! And, when it rains hard, the huge canopy of an oak softens and filters the rain (25-100 gallons of water can cling to the leaves of a large oak tree) so it doesn’t compact the soil. Compacted soil doesn’t absorb water as well, creating runoff. An oak’s root system absorbs much of the water. Don’t we all want to keep as much water on our property as possible rather than have it run off into the sewer system or, worse yet, our cellars?

Last, our landscapes should help to sustain a complex, diverse community of pollinators, primarily native bees. Because oaks are wind pollinated, it was thought that they didn’t have much to offer here. But recent research shows that when oak catkins are mature and ready to drop in early spring, bees gather the pollen and use it, but don’t pollinate the female flowers of the oak.

The oak family has over 90 species in this country and over 600 species in the world, a third of which are endangered. And, they are producing much of the food that moves the energy from leaves to caterpillars.

There are other services oaks provide to the ecosystem. They are not only number one in supporting caterpillars, they are number one in supporting other wildlife and ecosystems, providing nuts for birds and mammals (deer, squirrels, turkeys, etc.), and providing cavities for dozens of species of birds. It is also an excellent shade tree, which helps to cool the air.

If you’re thinking that an oak tree would be too big for your yard, here’s the thing. There are many types of oaks – from shrubs to small trees to the giants. Many of these are not carried in our local nurseries because there is no demand for them. Let’s create a demand and vow to plant an oak on each of our properties.

Here are a few of the many types that are found in Massachusetts. For a more exhaustive list go to http://gobotany.nativeplanttrust.org/genus/quercus/

•         Quercus alba, the eastern white oak, is one of the slower-growing but longest-lived oak species. Over 100 species of birds and mammals feast on the acorns, which are sporadically produced in a mast year.

•         Quercus coccinea, the scarlet oak, is a common wildlife food item. Blue jays, red-headed woodpeckers and squirrels remove many acorns while they are still on the tree. Its dramatic scarlet fall foliage makes it a popular tree, particularly for sunny areas.

•         Quercus prinoides, the dwarf chestnut oak, grows as a small tree or rhizomatous shrub, typically seen growing in dry areas. It can produce acorns at a very young age, and as such is an important wildlife food source.

•         Quercus velutina, the black oak, produces reliable acorn crops every two-three years which are consumed by insects, squirrels, other small rodents and wild turkeys. It is bit of a scruffy tree found mostly in forests and woodlands.

Donna Lane owns Lane Interiors & Gardens, is a master gardener, past president of the Norwood Evening Garden Club, and an active member of many other horticultural organizations.