By Elaine Mills, Extension Master Gardener
Photos by Elaine Mills
This April marks the 150th anniversary of Arbor Day, the national holiday dedicated to planting trees, and it is the perfect occasion to discuss the importance of maintaining the health of trees in our natural areas and adding trees to our home landscapes.
Benefits of Trees
The foliage of trees provides the oxygen we breathe as a by-product of photosynthesis, and it plays a critical role in the world’s water cycle by releasing water vapor into the atmosphere. Leaf cover also buffers noise and provides cooling shade as a relief from heat in the summer. In urban areas with lots of paved surfaces, trees can reduce the effect of heat islands, and, when appropriately sited near our homes, they can help reduce energy use. Trees shading houses can reduce cooling costs by 20 to 30 percent in the summer, and evergreen trees can block the wind from buildings in winter, reducing heating costs. Finally, the leaves of trees filter pollutants from our air, soak up greenhouse gases, and buffer the impact of rainfall on the ground.
The branches, trunks, and roots of trees sequester carbon, helping to offset sources of carbon dioxide in the atmosphere, such as deforestation, forest fires, and fossil fuel emissions. Roots also help to build good soil structure through their relationship with beneficial microorganisms. Additionally, root systems of trees reach well beyond the outside line of their branches, preventing erosion and absorbing and cleaning stormwater to prevent pollutants and sediment in runoff from reaching streams.
Trees release essential oils, called phytoncides, which have been shown to benefit human health. Studies by Roger Ulrich and Kathleen Wolf, measuring the effect of exposure to trees on stress, wellness, and physiology, have demonstrated improved immune response, decreased inflammation, and reduced cortisol levels. “Forest bathing” or walking in nature (shinrin-yoku) stimulates brain waves similar to those produced during deep meditation. Even having views of trees in hospital settings can speed patients’ recovery from illness and surgery.
Trees add beauty to our landscapes with their ornamental traits, such as spring flowers, unusual foliage, interesting bark, and fall color. Healthy, mature trees are green infrastructure, an amenity that can increase property value by 7 to 19 percent by providing privacy and energy savings.
Maintaining and Building Tree Canopy
Given all the vital ecosystem services and health and economic benefits that trees provide, many jurisdictions are making efforts to assess their tree cover and taking steps to sustain their tree canopy. Within communities, residential properties have the most space for building the tree canopy, and this can be done both by conserving existing trees and planting new trees.
Care of mature trees is essential because the larger the tree, the greater the benefits. Large trees are better at shading because of their stature, and their denser canopy will have more surface area to intercept water. According to Dr. Eric Wiseman, associate professor of urban forestry at Virginia Tech, a fundamental step in maintaining the health of older trees is cultivating soil health through the abundant use of compost and organic matter to replenish the soil food web. This can be done easily by retaining fallen leaves as leaf litter. He also emphasizes the importance of removing invasive vines, such as English ivy, wintercreeper, Oriental bittersweet, and porcelainberry, from the trunks of trees and protecting their root zones by minimizing construction disturbance. Tree Stewards of Arlington and Alexandria has helpful information on removing ivy from trees as well as other tips for care of mature trees. They also recommend consulting arborists, tree care professionals who can help homeowners maintain their investment through such services as the diagnosis and treatment of diseases, control of pests, pruning, and storm repair.
When adding trees to our home landscapes, there are excellent reasons for considering native tree species. These plants have evolved within a particular ecosystem, meaning they have adapted to our local soil and water patterns and, when properly sited for sun and moisture preferences, they are likely to do well without our resorting to fertilizers and pesticides. Most importantly, they have long-standing relationships with local wildlife, providing food, cover, and nesting sites for a wide range of animals from insects to birds and mammals. For example, a single oak tree can serve as the host for the caterpillar stage of over 500 butterfly and moth species which, in turn, are the primary food source for 96 percent of the young of our endangered bird populations.
While all tree cover is good, planting of native canopy-size trees (around 50 feet in height) is best, because those trees are taller and have perhaps 10 times more leaf area over the same space of ground. Dr. Jim McGlone, an urban forest conservationist with the Virginia Department of Forestry, advises to make sure there is room for the crown of a tree by checking for utility lines overhead and thinking about clearance for sidewalks and driveways. He also emphasizes allowing space for the root system. For example, the critical root zone for a tree with a trunk diameter of 10 inches extends to a 30-foot diameter under it.
Earth Sangha, an ecological restoration program in the Washington, DC, region, recommends the following high value native canopy trees: Quercus alba (white oak), Quercus rubra (red oak), Prunus serotina (black cherry), Betula nigra (river birch), Carya cordiformis (bitternut hickory), and Acer rubrum (red maple). These are “generalist” species that occur across a wide range of natural communities and should do well in any garden with enough space.
Note: Click on images to see enlarged photos, captions, and photo attributions.
On a mobile phone, click on the information symbol (circle with a letter ℹ︎ symbol).
Understory trees (around 15 to 30 feet tall) also play an integral role in home landscapes by providing habitat for our native songbirds which prefer to nest 5 to 15 feet off the ground. On larger properties, these can be planted beneath the canopy species but should be added as small specimens so as not to disturb the roots of established trees. In smaller gardens, understory trees can be combined with shrubs to create a dense layer comparable to that seen along forest edges.
Earth Sangha suggests the following native generalist understory species: Cornus florida (flowering dogwood), Carpinus caroliniana (American hornbeam), and Corylus americana (American hazelnut) along with shrubs such as Viburnum prunifolium (black haw) and Euonymus americanus (strawberry-bush).
Register for Celebrating Native Trees, an online public education presentation, on Friday, April 15, 10:00 am – 11:00 am, for more details on the characteristics, care, and landscape uses of 20 native tree species.
- McGlone, Jim. “Tree Benefits.” Presentation at Master Gardener College, June 2020.
- Native Trees: How to Select, Plant and Transplant, one of our recorded public education classes, has details on properly siting and planting trees.
- Plant NOVA Trees has additional information on planting trees and where to buy them.
- Tried and True Native Plant Selections for the Mid-Atlantic has helpful fact sheets on the wide variety of native trees for our region.