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Tree Hardiness Zones

Hardiness zones, defined as the average annual minimum temperature in which a plant can grow, are the current standard for arborists and gardeners to guide tree selection. Urban trees provide a myriad of services, including the ability to effectively mitigate the urban heat island effect through shading and evapotranspiration. With anthropogenic warming, hardiness zones are shifting northward over time, causing tree species to lose adaptation to parts of their historical range. As temperatures rise, some species of tree will no longer remain adapted to their urban settings. For the 50 most populous metropolitan statistical areas (MSAs), we are determining decadal hardiness zone shifts and how the associated changes in tree species distribution influence urban tree selection. Our ultimate goal is to assist cities in selecting suitable trees for changing climatic conditions.

In the following interactive graphic, clicking on an MSA (orange circle) opens two identical lists of common tree species historically adapted to that particular MSA. In the left column, species highlighted in yellow signify those no longer adapted to an MSA because of shifting hardiness zones from 1961-70 to 2001-10. In the right column, species highlighted in red signify those that would no longer be adapted to an MSA following a future shift in hardiness zones equivalent to the historical shift observed for each MSA over the 1961 to 2010 period. In both columns, species without highlighting signify those that remain adapted to an MSA despite hardiness zones shifting over time.

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Dr. Brian Stone, Kevin Lanza