UrbanForestResilience

Urban Forest Resilience

By Laszlo Marton, Maya O’Brien McLeod, Rebecca Perez & Harrison Karr

Photo by Durham Executive Group

As cities and regions begin to prepare and implement climate action plans, urban forests are taking on a new value as they are increasingly seen as important tools for climate adaptation and mitigation.

Urban forests help to mitigate climate change by:

capturing and storing carbon dioxide,
decreasing energy needs,
reducing runoff of stormwater,
and reducing urban heat island effect.

However, many urban trees and forests are vulnerable to the effects of climate change as there are increases in extreme weather events, unpredictable weather patterns, and new pests and diseases. For cities to employ their urban forests as tools, they must first assess their forest’s vulnerability to future changes in their region.

A suburb of the Twin Cities, Woodbury has a population of around 70,000 people and is the ninth largest city in Minnesota. The following information will include recommendations for forest resiliency as well as community engagement strategies that will allow the city of Woodbury foster a strong relationship between community members, city staff, and their environment, laying the foundation for a resilient urban forest in Woodbury for generations to come.

Photo by Ashley Miller

Adapting to Climate Change

What do future climate projections for Woodbury look like?

Photo by Dr Nina Bassuk

Creating Diverse Urban Forest

What is the current tree diversity, and how might Woodbury develop a diversity framework?

Photo by Arbor Day Foundation

Community Engagement

How can we effectively engage residents of Woodbury in order to create a more resilient forest?

Planning for a Changing Climate in Woodbury, MN

Minnesota's climate is changing. Minnesota state climatologists have found evidence of changes in temperature and changes in precipitation. The overall trend is that Minnesota is getting warmer and wetter.

The state is expected to warm another 3 to 8 degrees Fahrenheit by the end of the century. Summer temperatures are expected to increase, with up to nine more days with maximum temperatures above 95 degrees Fahrenheit by 2050 (Minnesota Forest Resources Council, 2020). Winter temperatures are also expected to increase which will make it more likely that winter precipitation will change with an increase falling as rain rather than snow.

These changes to Woodbury’s climate could result in changes to forest composition as native species struggle to adapt to new climate norms. It is likely that the ecosystem will shift in response to changing temperatures and precipitation and trees and vegetation will gradually shift.

High Plains Regional Climate Center. (2021). City Data Explorer - Sister City Tool

Predicted Rise in Average Temperatures & Precipitation

A changing climate will result in a changing forest and increased opportunities for the spread of pests and pathogens. With the expectation of warmer temperatures in the winter, this may allow for increased spread of invasive species in areas where they have previously been restricted by cold temperatures.

Minnesota Department of Natural Resources. (2020). Best Native Yard Trees For Our Changing Climate

Resilient Tree Species

Although native trees are often preferred, some native tree species won't be as resilient to future shifts in the climate zone of the Twin Cities Metro Area.

This image from the Minnesota DNR provides a list of tree species that will be the most adaptable to the expected changes in temperature & rainfall.

All species except the Silver Maple are also not as susceptible to current invasive species issues.

What are these pests, and what do they look like?

In 2009, the introduction of the invasive species Emerald Ash Borer (EAB) put urban foresters on high alert as ash trees are abundant throughout Minnesota and in its urban landscapes (Hahn, 2020). Adult beetles are metallic green and about a half-inch long. Eggs are tiny, at about 1/25th of an inch. EAB targets ash trees by laying larvae which bore holes underneath the bark, compromising the integrity of the tree.

Considered to be a future threat to forests in the Midwest, Asian Longhorned Beetle (ALB) has potential to be even more severe in its damage to forests in Minnesota and across the nation by impacting an even wider variety of genera. As shown in the table below, approximately 31% of Woodbury’s tree population is susceptible to ALB. If ALB were to become prominent in Woodbury a total of 8,489 trees would be at risk of being lost.

Trees Susceptible to Asian Longhorn Beetle (ALB)

Approximately 31% of Woodbury’s tree population is susceptible to ALB, with the Maple (23%) making up the most of this number. If ALB were to become more prominent in Woodbury a total of 8,489 trees would be at risk of being lost.

This table was created using GIS data from the 2021 Inventory of the City of Woodbury and filtering for ALB susceptible species.

Moving forward, urban foresters must work to reduce the dominance of vulnerable species such as elm and ash, while prioritizing the maintenance and planting of species that are resilient to pests, diseases, and climate change as a whole.

Creating a Diverse Urban Forest

Literature review and synthesis of existing knowledge allowed us to analyze Woodbury’s 2009 Urban Tree Canopy Assessment against current urban forest resilience research and existing frameworks for forest adaptation in urban areas. We then used available GIS data to assess the current-day makeup and distribution of tree canopy and species across the city of Woodbury.

We determined that a diverse tree canopy can help Woodbury's forest become more resilient to pests as well as expand the forest's range of benefits. The current tree inventory is made up of 27,308 trees.

This graph was created using GIS data from the 2021 Inventory of the City of Woodbury and compares the abundance of the most common species.

Top 10 Species:

Green Ash
Silver Maple
American Linden
Swamp White Oak
Black Hills Spruce
Blue Spruce
Red Maple
Common Hackberry
Norway Maple
Crabapple

This graph was created using GIS data from the 2021 Inventory of the City of Woodbury and compares the abundance of the most common genera.

Top 10 Genera:

Maple
Spruce
Oak
Ash
Basswood
Elm
Apple
Pine
Hackberry
Honey Locust

One of the frameworks used in the 2011 Urban Forestry Plan for the City Of Woodbury was using the 10-20-30 rule to regulate the species diversity of the urban forest. The 10-20-30 rule sets guidelines that no more than 10% of the same species, 20% of the same genera, and 30% of the same family should be planted in a chosen area.

Although following this rule to help regulate species diversity has evidently helped create a more diverse urban forest in Woodbury, going forward as part of this report we are recommending further tailoring those percentage guidelines. Using the past trends in diversity and current recommendations from the City of Minneapolis, the benchmark for species diversity of trees in Woodbury should focus primarily on genus diversity going forward. Specifically, the city should aim for following the guideline of having no more than 10% of any one genus in a neighborhood.

Community Engagement

Collaborating through accessible channels with the community will help the City to work on the resiliency of Woodbury's tree population while increasing agency of residents and taking into consideration cultural relevance of trees in the city.

RLS Human Care. 2020. Two way flow of communication

Establish Two-Way Flows of Communication:

The city should avoid an engagement strategy that is too focused on a one-way flow of information.

While the City should set diversity goals and work to address unequal tree canopy distribution, they should be wary of a top-down approach that doesn’t consult with residents.

Tree Selection Form, City of Minneapolis

Event: Fall Tree Sale

The Fall Tree Sale in Woodbury could be used to help inform residents of their importance in helping establish a resilient tree canopy for the future.

An increase in tree sale diversity can translate into city wide diversity. By limiting the sale of overly abundant species and promoting less abundant genera and species that also are suitable for current and future climate conditions, Woodbury can improve the resiliency of its urban forest.

A graphic inspired by tree selection forms from the City Of Minneapolis (right) could be utilized to guide the selection of tree species while allowing residents to be involved in the decision making behind what trees are being planted.

Photo by Bloomington Urban Forestry Research Group

Event: Arbor Day Tree Reporting
An effective way to help community members relate to the trees around them is to encourage them to look at what’s already in their own backyard. This process can also give the city more information about the state of its urban forest across private lands, allowing for a fuller picture of the current state of the urban forest in order to effectively prepare it for a more resilient future.

Using the momentum from the Arbor Day holiday, we suggest launching an Arbor Day Resident Tree Count. This tasks Woodbury residents with reporting the genus and species of the existing trees on their property, serving two purposes: 1) Informing the City of Woodbury about its privately owned urban forest, and 2) Giving residents an opportunity to reflect on the trees in their environment, the purposes they serve, and the importance of future resilience.

This Tree Reporting event is an opportunity to show residents the importance of tree diversity in an urban canopy, allowing them to understand the problem in order to make future decisions that benefit the whole community. This type of citizen science has been proven successful in the Audubon Christmas Bird Count, which is the nation’s longest running community science project (National Audubon Society).