Considering Climate for a Healthy Urban Tree Canopy

By: Kate Sievers, Project Manager, Nature Based Solutions

Urban trees offer great promise to improve residents’ daily lives and make communities more resilient to the impacts of climate change. Benefits to residents include cleaning the air, improving mental and physical health, and mitigating the urban heat island impact during increasing summer temperatures. Choosing the right tree species can help ensure these benefits grow with us as our climate changes.

Selecting the right tree from the many available species may feel like an overwhelming decision. The Climate Tree Species Guide provides a quick three-page suitability reference for over 180 tree species and each tree suitability for the changing climate of the Puget Sound region. A more detailed assessment of species can be found here.

A tree planted today will see decades of growth as we experience the impact of climate change. In the Puget Sound region warmer temperatures and altered precipitation patterns are projected to increase. Historically the region has experienced an annual average of 1-7 days above 86F, but by 2070 this is expected to increase to 45-60 days. Summers and winters are expected to be drier, while spring and fall are expected to be wetter. This changing climate leaves our ecosystem vulnerable. So, what does this mean for our current trees and how do we make the best planting decisions today for a resilient tomorrow?

Volunteers plant trees on this annual day of service to support the health of Tacoma’s green spaces. ©Hannah Letinich.

Understanding where we are now:

Using the Seattle street tree inventory as a general guide, common trees in the area were assessed for vulnerability to shifts in temperature. Understanding every city’s current inventory helps develop succession plans to build a resilient future. For example, in the case of the species in the City of Seattle’s street tree inventory, 13% of species were considered low risk, 20% were low-moderate, 18% were moderate, 39% were moderate-high, and only 9% were considered high risk. If you have a current tree inventory, you can compare that to the vulnerability index to understand the resilience of your existing urban forest.

Choosing what to plant:

If you’re thinking about planting a tree yourself or as part of a larger program you can use the guide to help choose a climate-resilient species. The guide has three main evaluation criteria: urban adaptability, suitability, and vulnerability.

The Climate Tree Species Guide accounts for a variety of factors that impact the urban canopy.

  • Urban adaptability provides a regional survivability index based on ability to grow, resistance to changes in flood regimes, extreme weather events, insects and disease, and nonnative invasive species. It also assesses growth factors important for urban planting programs like shade tolerance, soil needs, required space for root growth, and ability to thrive in various urban land use conditions. These factors may be important when selecting trees for a narrow sidewalk or a larger boulevard.

  • Suitability is based on a species current and projected ability to grow in a geographic region’s temperature zones. Hardiness is based on the minimum winter temperatures and heat zones are based on the number of days above 86F. Some plants can withstand cold winters but wither in summer so it’s important to optimize for both temperature extremes.

  • Vulnerability takes into account a variety of climate-related stressors including intense heat, drought, flooding, and changing patterns in pests and disease

The guide evaluated 180 species of trees to determine their vulnerability to a changing climate now and into the future.

Some other factors to consider include ensuring high tree species diversity and allergenicity. In general, it’s better to have different types of trees instead of a single type of tree planted. This helps a system remain resilient to pests and varying environmental conditions. If you are planting in a high pedestrian area you may also want to consider if the tree you’re planting is a common allergen to humans. The full species guide identifies if a tree is known to cause allergies and how severe the response may be.

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