Keystone Plants Trees – Ecoregion Five

Author: Adair Heuchan, Master Gardeners of Ottawa-Carleto..

“Oaks support more forms of life and more fascinating interactions than any other tree genus in North America. They produce enormous root systems over their lifetimes, and these help make them champions when it comes to soil stabilization, carbon sequestration, and watershed management.”

Douglas Tallamy: The Nature of Oaks – The Rich Ecology of Our Most Essential Native Trees

Keystone Plant and Species

As we learned in the first of our 4-part series on “keystone plants,” Doug Tallamy and his team of researchers at the University of Delaware have systematically studied which trees, shrubs, and plants are keystone hosts for caterpillars and other species. Their findings reveal that Quercus (oaks) are, without doubt, the number one keystone tree species. Oaks are spectacular hosts for several reasons. They are dispersed very widely across the planet. Their size and spread above and below ground are vast. They live to a very old age. As a result, insects, birds, and other animals have had centuries to adapt to using oaks for their sustenance.

The Contribution of Oak Trees to Our Biodiversity

In North America, there are over 600 genera of oaks, which host 557 species of insects, birds, and animals. In our Ecoregion 5, Lake Simcoe-Rideau, we can readily grow varieties of white and red oak, including pin and bur oaks. Oaks, like other trees, take a very long time to grow to any great size and can therefore be planted in suburban and even urban lots. Ensure that there is enough room between your oak and any permanent structures, such as a house or shed, to accommodate at least half the width of a mature tree. For example, a red oak, which can grow to 18 m (60 feet) in width, should be planted at least 11–12 meters (35–40 feet) away from your house.

Your yard may be a bit small for an oak, but you can make a great contribution to our wildlife and nature by helping to protect oaks growing in public spaces and encouraging our municipalities to plant more oaks.

Several other trees, which are not as “mighty,” are also extremely valuable. Prunus trees, including plum, black cherry, and chokecherry, host 409 species of caterpillars. The genus Betula, including yellow birch and paper birch, hosts 385. Populus, including the American aspen and the balsam poplar, host 337 varieties. Malus, including crabapples and other apple species, host 285. Acer, including silver, red, and sugar maples, hosts 276. Salix, the willows, host 397. But you don’t need to be restricted to these top keystone plant genera. Many trees host more than 100 species of caterpillars. In our zone, these include: Alnus (alder), Pinus (pine), Ulmus (elm), Picea (spruce), Crataegus (hawthorn), Tilia (basswood), Fraxinus (ash), Juglans (walnut), Fagus (beech), Castanea (chestnut), Abies (fir), Cornus (dogwood), Larix (larch), Amelanchier (serviceberry), and Tsuga (hemlock).

Contribution of Exotic Trees

Exotic trees, such as Magnolia and Ginkgo, do not contribute much to the wildlife in our zone, hosting fewer than 5 species each, although they might well host more in their own native zones. Nevertheless, they are beautiful and do add some minimal value, so leave them if they are healthy specimens already in place. It is better, however, to choose a native keystone genus if replacing or planting new trees.

Take a look at your environment—your street, your neighborhood, and any parks or open spaces nearby—to see what trees are already present. Think about diversity and look at your own garden—front, back, side, or even a median in front of your house. See if there is room for a tree. If so, choose a keystone tree that adds to the biodiversity of what is already there and that suits the location, sun, and soil conditions. If your space is small, an Amelanchier, the hardy native serviceberry tree, or Cornus alternifolia, the pagoda dogwood, might work for you.

Canadian Serviceberry Example

The Canadian Serviceberry is a small tree (or shrub) in the rose family, native to temperate regions of the northern hemisphere. Although it is tolerant of a wide variety of conditions and largely disease-resistant, it is best to plant it in full sun or partial shade in well-drained soils. A serviceberry tree can live up to 50 years and grow to 3-6 meters (10–20 feet); however, annual pruning will keep it small. The roots grow slowly and are not invasive to foundations.

A serviceberry displays gorgeous white lacy blooms in early spring, making it a great source of nectar for pollinator insects early in the season. The blossoms resemble apple blossoms. In summer, it produces beautiful deep red and eventually purple edible berries, much loved by all kinds of birds. If you can harvest them before the birds eat them all, the berries can be eaten raw or made into jams, jellies, and pies. The serviceberry foliage structure is open and loose, allowing for dappled light to filter through to the semishade-loving plants below. In autumn, the leaves of the serviceberry turn vivid oranges and reds.

The Pagoda Dogwood

Pagoda dogwood is a deciduous shrub or small tree native to eastern North America. It prefers partial sun and a moist, well-drained location with a rich soil that is somewhat acidic. The pagoda dogwood is, however, adaptable to other conditions and is low-maintenance. It can live for up to 80 years, but more realistically, in our gardens, for around 30 years. Pagoda is the most cold-hardy dogwood in North America. Although this dogwood can grow tall, it does not have invasive roots and can be kept trimmed. Pagoda spreads sideways and looks spectacular by itself or as an understory to a larger tree and/or umbrella to shade-loving plants.

The pagoda dogwood’s fragrant white flower clusters in spring are followed by dark blue berries on red stems and spectacularly bright red leaves in autumn. Even in winter, the pagoda has interesting bark. And all year long, it provides food and shelter to a wide variety of insects, birds, and other creatures.

I conclude with a short verse, whose title is a misquotation from the movie “Field of Dreams.”

If You Plant it, They will Come!
A star you will be
If you plant a tree.
This is not a joke!
Consider the mighty oak,
Maple, willow or choke,
Apple, plum or cherry,
Or plant any bush of berry.
For hosting the young of our native species
Of butterflies, moths and specialist beesies.