Standards for Selecting Climate Resilient Trees in the Northeast
Assisted migration requires two simultaneous projects: 1. preserving rare and at-risk plants endangered by climate change, and 2. expanding the range of resilient trees northward that will have the best likelihood of surviving a rapidly changing climate, and form the backbone of northern forests in the next century.
There is not, however, a well-defined set of criteria for selecting plants that will be most successful north of their current range in the near future. Urban foresters, communities, and homeowners are increasingly interested in improving climate resiliency at a local level. The following criteria can help anyone in the mid-Atlantic or Northeastern United States select the trees that will stand the best chance at thriving, even in a worst-case climate change scenario.
The more of these criteria that apply to a species, the better is the case for planting it today.
Note: all trees under consideration must be currently hardy in your location. Ideally, a species will be hardy to at least one hardiness zone level below yours, in order to allow for unusually harsh winters that are also becoming more prevalent. In other words, if you live in USDA hardiness zone 7, a plant you choose should be hardy to at least zone 7, and preferably zone 5 or 6.
1. Currently grows well in hardiness zones 8-10+ and heat zones 7-10+
In a high-emissions climate change scenario, Philadelphia could jump from hardiness zone 7 to 9b and heat zone 5 to 8 by 2050, becoming still hotter in the decades afterward. An urban forester or homeowner in Philadelphia will need to anticipate and plan for these changes, and select trees that are sure to survive in winters and summers that reflect those changes.
2. Resistance to extreme weather events
Hurricanes, floods, droughts, and fires could all become more common or exacerbated in the Northeast. Plants that have resistance to one or more of these potential stressors can be planted as a safeguard. For example, trees that exhibit good resistance to breakage or failure in hurricanes and high winds are good candidates for street trees.
3. Capacity to support biodiversity
Plants are the foundation of most terrestrial life in eastern North America - countless species of caterpillars and other insects rely on specific host plants, and are also food for life higher on the food chain like birds and mammals. The more insect species a plant can host, generally, the better able it is to support biodiversity in its ecosystem. The number of lepidoptera (moths and butterflies) a particular genus or species can host is easily searchable as well, though it is not the only measure of capacity to support wildlife.
4. Thrives in southern cities
In many ways cities already have many of the same conditions projected to be felt regionally in the near future. As such, they are excellent testing grounds for what may or may not be adaptive in a changing and challenging climate. Tree species that thrive in highly urbanized areas, particularly in the Deep South, are prime candidates for predicting future success.
5. Ability to replace species currently native to the Northeast that are threatened with extirpation in a warmer climate
Many native species in the Northeast will not survive the coming changes to our climate. Southern plants that are related, or perform similar ecological functions, are important to establish now in order to reach maturity and produce seed in time to replace what will be lost.
6. Meets aesthetic or subsistence needs of people
Ultimately, people define what the landscape and ecology of eastern North America will look like. Plants that people want, need, and enjoy will be favored - particularly as more land becomes developed. Trees that provide beauty, food, medicine, monetary value, or specific ecological services will be favored over those that do not.