Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response.
By Elaine Mills, Extension Master Gardener
Photos by Elaine Mills, unless otherwise indicated
No discussion about how we can help cope with climate change would be complete without addressing the critical importance of trees. As mentioned in earlier articles in this series, trees offer habitat, nesting sites, and a variety of food to wildlife, while the “keystone” species in particular support the caterpillars that are a key component of the food web, sustaining populations of our native birds. In addition, the life functions of trees provide many other beneficial ecosystem services.
Through the process of photosynthesis, trees take in carbon dioxide (CO2) and convert this gas into a solid form, which can be stored as sugars in their tissues. A diagram in the Forest Carbon Primer prepared by the Congressional Research Service illustrates that 31% of the carbon in US forests is stored in the leaves, branches, trunks, and roots of trees. Around 5 to 6 percent each is stored in leaf litter, deadwood, and harvested wood products. The greatest amount is transferred by trees into the soil, where it is stored as solid carbon compounds.
This carbon sequestration service by trees is a critical factor in mitigating climate change as the build-up of CO2 in the atmosphere is a major contributor to global warming, and no technology has yet been devised that can economically replace the work of the trees. As an example, Urban Forest Conservationist Jim McGlone estimates that his sizeable red maple tree sequesters 1,000 pounds of carbon a year, about one-tenth of the CO2 output of his car or somewhat over one-half metric ton.
Reducing Energy Usage
Another function of trees is transpiration, the process by which water is drawn up through their roots and released through their leaves as water vapor in the atmosphere. This evaporative cooling reduces the ambient air temperature and results in lower cooling costs for homes and other buildings nearby. Studies have shown that street trees in cities, which experience the “heat island effect” of heat absorbed and re-emitted by large areas of impervious surfaces, can help reduce temperatures by over 5 degrees Fahrenheit.
Shade cast by trees can block solar radiation and reduce radiant heating, further cutting cooling costs by up to 30%. In addition, evergreen trees can block winter winds and reduce cooling at ground level, reducing heating bills by up to 10%.
Improving Air Quality
Importantly, trees act, in a sense, as the lungs of the earth. Through the gas exchange involved in photosynthesis, they release the oxygen that is essential for humans and animals to breathe. They also help to improve air quality by absorbing gaseous air pollutants, such as nitrogen oxides (NOx), sulfur oxides (SOx), and ozone (O3), that are emitted by cars, refineries, and power and chemical plants. Their leaves physically intercept and hold small particles on their surfaces like dust, ash, pollen, and smoke.
Trees help to manage water in our yards in multiple ways. Initially, their leaves and branches intercept rainwater from heavy storm events and prevent it from reaching the ground to become runoff, reducing the amount of surface pollutants and sediment carried into nearby streams. Secondly, trees interact with soil microbes to improve soil structure and improve water infiltration. Their extensive root systems prevent soil erosion and lessen flood damage.
Finally, trees beautify our neighborhoods, increasing property values and traffic to local business. Research by the US Forest Service appears to show that cities with a higher percentage of tree canopy and homes with large trees tend to have lower crime rates, perhaps because well cared for treed areas indicate attentive homeowners and government agencies. The presence of trees nearby also brings health benefits. Improved immune response, lowered stress indicators, and reduced depression are exhibited in people practicing Shinrin-yoko (forest bathing), and paths with trees have been shown to increase the duration of physical activity in walkers and runners. Projects such as the Green Road at Walter Reed National Military Medical Center illustrate how exposure to nature can play a role in treating conditions like PTSD and traumatic brain injury. Even having views of trees from hospital windows and workplaces can promote healing, reduce recovery time from surgery, improve concentration, and increase job satisfaction.
Projected increases in temperature and alternating patterns of heavy rain events and severe drought will put stresses on trees in the coming years, especially in developed areas where they experience the heat island effect, limitations of root zones by roads and sidewalks, and compaction of surrounding soil by foot traffic and mowing. Choosing trees that can acclimate to future climatic conditions will be essential to continue benefitting from trees’ many ecosystem services.
One tool that homeowners may consider using to assist in selection is the USDA Forest Service’s Climate Change Atlas. This online database can be used to assess current and projected suitable habitats for 125 eastern tree species. The site rates the adaptability of each species based on positive traits or negative traits. For example, red maple (Acer rubrum) scores highest of all eastern tree species for adaptability, while Eastern white pine (Pinus strobus) suffers from some negative traits, such as vulnerability to drought, fire, and insect pests. A variety of maps illustrate potential ranges, and charts with regional assessment summaries are also available.
Best Management Practices for Trees
Because trees constitute the foundation of home landscapes and many tend to be long-lived species, it is critical to follow good gardening practices for their planting and care.
- Match trees to the proper soil conditions for pH and moisture levels, selecting drought-tolerant species for dry areas, such as slopes, and moisture-loving species for low-lying areas and rain gardens, as well as stream and pond banks.
- Consider space needs at maturity, allowing room overhead for utility wires and lateral space for sufficient root growth.
- When planting, prepare a hole wider than the root ball, remove any girdling roots, be sure to plant the root flare above grade, do not add any soil amendments, and water thoroughly.
- Create a mulched area around the base of the tree, ideally to the drip line, so that any fertilization, watering, and mowing of surrounding lawn will not adversely affect the tree.
- Mulch in a doughnut shape rather than a “volcano” with a cover of only 2 to 3 inches over the roots, keeping the mulch from directly touching the trunk.
- Water regularly, one inch per week, for the first two years and during periods of drought after that.
See our Tried & True fact sheets on trees for assistance in choosing the best species for your site. The recording of our public education class on “Native Trees: How to Select, Plant and Transplant” with its accompanying resource list provides additional helpful information.
Programs and Sources of Native Trees
Plant NOVA Trees is a five-year native tree campaign that has just begun in Northern Virginia this fall. The website for the program lists weekly events to “Celebrate Trees!” and offers a wide variety of helpful information for homeowners, communities, and businesses, including planting tips; lists of shade trees, small trees, and shrubs; and fall color.
Tree Stewards of Arlington & Alexandria, a local group working to enhance a sustainable urban forest through volunteer activities and public education programs, will begin a new membership class in late January 2022. Applications for a hybrid program of at-home and hands-on outdoor training sessions are being accepted through December 31, 2021.
Small Trees Make Big Canopies is a program initiated by two interns in the local Extension Master Gardener program to help expand the tree canopy in Arlington. Tree saplings donated by neighbors are given away along with information cards in English and Spanish. Free trees of about a dozen species can be obtained by completing a Tree Request Form (Trees requested now will be available for spring planting).
In partnership with the Urban Forestry Commission, EcoAction Arlington administers the Tree Canopy Fund program through which owners of private property in Arlington County can apply to get a native tree. There are typically two opportunities offered each year to apply for free trees. Applications for Spring 2022 planting will be available this fall and will be due in January 2022.
Free native trees have already been distributed to Arlington County residents through the Department of Parks and Recreation for this fall, but this popular annual program should return next year.
Residents in the City of Alexandria can help grow the City’s tree canopy through the purchase of trees from the Fall Native Plant Sale beginning on November 15 and available for pick up on November 20. Through the Tree Planting Program, they also have the opportunity to request a tree to be planted on City property along the street in front of their homes.
Residents of Falls Church City can request a free street tree through the Neighborhood Tree Program.
- Climate Change Atlas, Northern Research Station, USDA Forest Service website.
- Forest Carbon Primer, Updated May 5, 2020, Congressional Research Service.
- McGlone, Jim. “Tree Benefits.” Presentation at Master Gardener College, June 2020.
- “Relationship between tree canopy and crime rates across an urban-rural gradient in the greater Baltimore region,” U. S. Forest Service.
- Suttie, Jill. “Why Trees Can Make You Happier,” April 26, 2019. Greater Good Magazine, University of California at Berkeley.
- “Using Trees and Vegetation to Reduce Heat Islands,” United States Environmental Protection Agency.