Selection in the City: Understanding Where City Trees Come From

Many city trees are bought at tree nurseries or big box stores. When these trees grow and mature, they may produce seeds that grow into new trees in the surrounding area.  So, it is important to understand the source of these trees, and how decisions are made about which tree species or cultivars (cultivated varieties) to plant.

People and organizations along the tree supply chain include breeders, nurseries of all sizes, landscape architects, contractors, and urban foresters. All of these people make decisions about which tree species and cultivars to grow, sell, select, and plant, and they are each responding to others along the supply chain when they make decisions.

While urban foresters may want to increase the diversity and climate resilience of their city’s tree population, there are limitations along the supply chain that may not allow them to buy all of the species or cultivars they desire. Over time, planting the same kinds of trees repeatedly in a city can lead to low species diversity and low genetic diversity, which can make the urban forest vulnerable to pests and diseases.
 

Key Findings

Early data collection and analysis have found that:

• Tree planting organizations and landscape architects rarely have specific goals for increasing tree species diversity, though many do aim to increase diversity in general.
• It can be hard to enforce tree diversity goals throughout the decision-making processes because many people and organizations are involved.
• People tend to grow and plant familiar species and cultivars.
• Tree form, aesthetics, and familiarity drive many tree planting decisions while genetic diversity and climate adaptation are rarely considered.
• Over time, nurseries have observed increasing interest in native trees and branded cultivars.
• There are logistical challenges to increasing production of some native species (like hickory trees).

Photo Credit

Photo courtesy of Baltimore City Department of Recreation & Parks.

Volunteers planting a tree in Baltimore, Maryland.

Approach 

This project is gathering information from people and organizations involved in the tree supply chain in Baltimore, Maryland in order to answer questions like:

• How do people decide which tree species and cultivars to breed, grow, purchase, or plant?
• What do urban foresters and landscape architects want most when picking trees for city projects?
• Are factors like tolerance to urban conditions, climate resilience, or genetic diversity being considered across different sectors in the tree supply chain?
• Are people aware of Maryland’s plan to plant 5 million native trees by 2030 (the Tree Solutions Now Act)?
  • How has this legislation changed the conversation around:
    • Tree nursery production?
    • Urban tree planting?
    • The climate resilience of cultivars?
    • The genetic diversity of planted tree populations?

Outcomes 

Early data analysis suggests:

• There is a need for local discussions about what “native” and ”non-native” mean for tree varieties.
• Policy support is needed to encourage breeding and planting a greater variety of native species.
• Goals for planting native species and increasing species diversity might conflict with goals for tree survival in cities.
• Communication between different sectors of the tree supply chain is crucial.