Selecting Community Trees in a Changing Climate

As our climate continues to change, with it getting warmer and wetter in the Northeastern United States, selecting a new tree to be planted in your community's park or along its streets might need to take into account whether it can handle conditions in twenty to thirty years. Remember, the young trees we plant today begin to provide us lots of benefits like shade, stormwater interception, carbon sequestration, and energy conservation as they reach twenty years of age and are healthy. If we decide to plant 50 new street trees of a given species and they begin to decline in health and die off in ten years, we have just wasted time and lost ten years of establishment.

The choices we make today, related to tree species selection, site preparation, proper planting technique, and providing good aftercare, are critical if our communities are to become healthy, livable, greener places that are prepared for the warmer temperatures that will affect us all.

Before communities start thinking about planting southern live oaks and crepe myrtles everywhere in Pennsylvania, we need to do a little research into species and cultivars (cultivated varieties of a given species) that are illustrating resilience in warmer climates, especially urban sites that tend to be warmer due to all the concrete, asphalt, buildings, and automobiles. It is not enough to think that we can just replace the older maples in town with new maples. We need to think about the increased heat load the trees will receive, the de-icing salts that are applied in larger quantities in our communities, the increased frequency of storms and heavy precipitation, and the compacted urban soils that are left to plant in following street reconstruction projects.

This is all new, uncharted waters, but some researchers like Dr. David Nowak, Leslie Brandt, and Chris Swanston of the USDA Forest Service are evaluating a list of 120 most common trees in the Chicago area to assess climate change impacts on individual tree species. That 2016 study, A framework for adapting urban forests to climate change, can be found in the journal Environmental Science & Policy.

Other studies assessed the impact of climate change on urban tree species selection in Philadelphia. Visiting scientist Jun Yang examined the impact of climate change on Philadelphia's urban trees and found that ten species will decline while two species will benefit. The study considered changes in precipitation levels, evapotranspiration, and impacts of major pests and diseases (many pests and diseases will benefit from the changes in climate). The USDA Forest Service produced a factsheet titled Philadelphia Region: Urban Forest Impacts and Vulnerabilities that contains a list of species that have low, moderate, and high vulnerability to climate change and other stressors. For example, species like eastern white pine, black cherry, hemlock, and American beech will likely experience severe declines, while blackgum, hackberry, honeylocust, zelkova, oaks, and red maple will adapt better to future climatic conditions in southeastern Pennsylvania. More resources on the USDA Forest Service's research can be found on the Climate Change Response Framework website.

Dr. Bert Cregg at Michigan State University's Department of Horticulture has been experimenting with growing and comparing several species and cultivars in greenhouses to simulate increased temperatures and several sites in downtown Detroit. The species are being evaluated for their tolerance to increased temperatures. Some species acclimated to increased temperatures. For example, swamp white oak (Quercus bicolor) increased its optimum temperature for photosynthesis, while red maple (Acer rubrum) and honeylocust (Gleditsia triacanthos) increased stomatal density to deal with heat. Callery pear (Pyrus calleryana) and swamp white oak decreased their leaf thickness in response to increased temperatures. Dr. Cregg explained his research in a recorded presentation titled Urban Tree Selection in a Changing Climate.

Even nurseries are researching species that will adapt well to our changing climate. J. Frank Schmidt and Sons Nursery has been evaluating cultivars for their adaptability and climate resilience for several years now, producing some materials that can help municipal shade tree commissions select some new species and cultivars. Trees such as Redpointe Maple, a drought and heat-tolerant red maple cultivar, or Emerald Sunshine Elm, a disease-resistant hybrid elm, and Urban Pinnacle Oak, a selection of bur oak, are just a few new species and cultivars to try planting in our communities. 

Before you order the same species of trees you have been planting in your community for years, or what's available and inexpensive, consider examining the planting locations thoroughly and selecting a few new species or cultivars that will tolerate and adapt to our changing climate and urban landscapes.