Climate Resilient Urban Greening Best Practices
The changing climate will impact Southern California in several ways, including more days with extreme heat, rising sea levels, more frequent wildfires and shifting precipitation rates. With this reality, urban greening, and urban trees in particular, can play a key role in improving community health, mobility, and overall quality of life, and achieving greenhouse gas emissions goals. SCAG outlines urban greening as the greening of developed areas within Southern California communities that can benefit from cooling strategies to reduce urban heat island effects and extreme heat.
Health Benefits and Urban Heat Reduction
Urban greening is an important tool to improve resiliency and equity in a changing climate. Urban greening is especially important in low-income communities where residents are more likely to depend on walking or biking to access public transportation and reach key destinations such as jobs, schools, healthcare and shopping but often lack trees that can lower urban heat. Urban trees are shown to reduce air pollution, improve water quality and boost mental health through lowered stress levels. They have also been shown to mitigate existing heat conditions by providing shade and lower temperatures making it more comfortable to walk or bike which leads to better health outcomes and lowered Vehicle Miles Traveled (VMT).
Resilience and Greenhouse Gas Emissions Benefits
Trees have a variety of resilience and greenhouse gas (GHG) emissions benefits, including:
Reduce stormwater runoff: Street trees support cleaner water by increasing the capture of urban runoff from rainfall. This rainfall would otherwise enter waterways such as the ocean, rivers and streams carrying pollutants that are unhealthy to plants, animals, and people.
Replenish groundwater: Tree roots increase the ability of soil to absorb rainwater, creating an estimated annual savings of $4.55 per tree in groundwater replenishment.
Reduce electricity use: Urban trees reduce overall electricity usage. It’s estimated that annual electricity savings from reductions in air conditioning use through urban heat mitigation can be equivalent to the amount of energy required to air condition over 210,000 households each year, translating to $11.08 annually per tree.
Reduce GHGs: Approximately 7.78 million tons of carbon dioxide are stored in California’s 9.1 million street trees. California street trees remove and avoid 567,748 tons of CO2 emissions annually, equivalent to removing 120,000 cars from the road.
Reduce VMT: Street trees improve comfort for people who walk, bike or roll making it easier to choose more sustainable modes of transportation.
With extreme heat projected to become more frequent and severe due to climate change, communities should prioritize tree planting and other strategies that reduce urban heat islands.
Urban Forestry Best Practices
Urban forestry programs should practice the right tree, right place rule. Planning for the right tree that will thrive in its location, combined with a plan for continuing care, is crucial for the tree to grow to full maturity. Without early planning and tree care implementation, trees are likely to have high mortality rates, undercutting the ability of tree-planting programs to provide the full range of potential community benefits. Giving trees the best chance of success includes the following recommendations:
- Conduct a site-specific evaluation
A site-specific evaluation can determine whether a particular tree is well suited to be planted in an area, the benefits that can be expected from this tree and whether it will be at risk for early removal. This evaluation should include checking to see if there are risks to infrastructure, such as underground or overhead utilities and sidewalks, that could result from a particular tree. Jurisdictions can partner with community organizations that provide ecological consultation to determine the expected benefits and plans of action for tree planting programs.
- Provide adequate soil
Trees that are planted in sites with sufficient soil volume will grow faster, develop larger canopies and outlive those grown in smaller volumes. The recommended soil volume for a healthy street tree is 600 cubic feet for a small tree (20-30 feet), 900 cubic feet for a medium tree (30-60 feet) and 1200 cubic feet for a large tree (60+ feet).
- Plan for tree care in the first 10 years
Mature, healthy trees provide more benefits than younger trees. Long-term tree maintenance is crucial for tree survival and growth. Many tree planting programs have high mortality rates due to a lack of long-term, planned care. Once planted, street trees are often left to survive by precipitation alone.
- Effective tree care includes:
- Planned irrigation: The overseeing organization should develop a plan for appropriate, continuous irrigation rather than relying only on precipitation.
- Adding mulch: As temperatures rise and communities become dryer, adding wood-chip mulch will keep the soil temperature cooler, thereby preserving warmth for the roots, hold moisture for longer and prevent weeds from growing.
- Prevention of early tree removal: Planting programs should coordinate with appropriate stakeholders to ensure that planted trees will not be removed prematurely.
- Effective pruning: Trees should be pruned by a certified arborist. Pruning should occur once a year and consists mainly of the removal of diseased or dead branches or causing a threat to public safety. Trimming throughout the year may be needed to remove branches along travel corridors. Arborists can refer to the ANSI A300 Pruning Standards and companion “Best Management Practices – Tree Pruning” for acceptable practices.
- Select the right species
Species selection has been shown to have a large impact on tree survival rates, even before changes environmental changes due to climate change. With projected changes in multiple factors such as air temperature, wind, and precipitation, that will affect tree survival rates by mid to end-of-century, communities should consider creating a list of trees that can withstand future environmental conditions.
- Ensure a diverse urban forest
Species diversity is important to protect the urban forest against catastrophic failure caused by a species- or genus-specific pathogen or disease.
- Engage and collaborate with local communities
Engagement with local communities where tree planting programs take place is crucial to the success of the program. Tree planting programs that target private planting should incentivize and educate about maintenance challenges, and identify the appropriate trees. Dig sites on public lands have failed due to a lack of engagement with residents or a lack of long-term tree maintenance from the municipality. Communities should consider using volunteers, with clear guidance and supervision to reduce the costs of tree planting programs.
- Plan for post-planning monitoring and evaluation
A plan for post-planning monitoring and evaluation can help to evaluate the effectiveness of tree-planting programs.
- Resources for urban forestry best practices
City Tree Ordinances
Because of the immense value of planting and caring for trees, communities should consider implementing tree ordinances that establish clear guidelines and regulations for the maintenance and removal of trees on public and private lands. Comprehensive tree preservation ordinances can play a central role in successfully protecting trees during and after residential development.
- SCAG Region Urban Greening Management Plan Examples
- Resources for creating a Tree Ordinance
Identifying Priority Areas for Tree Planting
Urban forestry programs can be used as one of the strategies to improve health, environmental and heat reduction outcomes in underresourced communities. which often lack parks and open spaces. The following resources may help you determine which areas to prioritize tree planting.
- Resources to consider when prioritizing areas for tree planting
- The USDA Forest Service Tree Canopy Data: This map overlays tree canopy coverage data at 60-cm spatial resolution, ozone and PM 2.5 pollution information, CalEPA’s urban heat island index, and other public health indicators. This map shows which areas lack tree canopy coverage, combined with low health indicators, to prioritize areas for tree planting.
- Multi-Resolution Land Characteristics (MRLC) Consortium: This resource provides land cover datasets at the national scale.
- California Healthy Places Index: This map shows data on social conditions that drive health such as education, economic opportunities, clean air and water, and other indicators.
- California Healthy Places Index: Extreme Heat Edition: This resource shows the number of days above 90 or 100 degrees at various time scales such as mid- and end-of-century down to cities and elementary school districts. It also shows other indicators such as tree canopy coverage (from the national land cover database), park acres, impervious surface cover, urban heat island index, PM 2.5, Diesel PM, ozone and more.
Tree Planting in a Changing Climate
Climate change is projected to cause changes in air temperatures, precipitation, disease frequencies and other factors that will affect the ability of trees in urban and natural forests to survive. The interactions of drought, heat stress and insect outbreaks due to climate change, as well as other factors, can lead to forest mortality in complex patterns. Drought-related tree mortality may even occur years or decades after the drought. Different sequences of climate events, such as a flood followed by a drought, can also affect tree growth and the risk of mortality.
Selecting trees that are appropriate for the site and able to withstand a variety of landscape characteristics will be key in planting resilient, climate-ready urban forests. Communities should consider forming a tree list that includes a variety of different tree species that show promise in withstanding the local projected climate hazards. Please note that research into climate-ready trees is still in its early stages and there are information gaps on how climate change will impact tree survival rates. Here are the factors to consider when forming your tree list. Communities should determine priority based on their projected climate hazards.
- Consider plants that have low water requirements or are drought tolerant
Planting programs may want to prioritize trees that use low amounts of water because climate models predict that drought and heat waves will increase water loss from tree surfaces (evaporative demands).
- Consider plants that can tolerate multiple water levels or soil moisture levels
In some regions, flooding will be an increased risk within this century. Selecting trees that have been known to be tolerant to multiple water levels may increase their ability to survive in projected flood-prone areas. There is limited information on tree species’ levels of tolerance to water levels. Communities should consider working with local arborists to identify trees that can tolerate high water levels if increased flooding is projected.
- Identify trees that can do well in new projected USDA hardiness zones
Select trees that can potentially do well in the projected future U.S. Department of Agriculture (USDA) Plant Hardiness Zones, minimum and maximum temperatures.
- Identify trees that can do well in new projected air temperature
Warmer temperatures alone can increase forest water stress independent of precipitation amount and can accelerate drought-induced mortality. Selecting species that have been known to thrive in multiple temperatures can potentially increase their chances of survival in a rapidly changing climate.
- Select trees with more tolerance for salinity levels
The SCAG region has been increasingly using recycled water, which tends to have higher salinity due to the water treatment process. Selecting species that have higher salinity tolerance may increase these trees’ chances of survival.
- Consider pest resilience in the tree palette
Climate change is projected to induce uncertain changes to pest environments. Due to warmer climates, some communities may experience new and longer pest seasons. Diverse species will decrease the chances that an urban forest made up of a singular species will be wiped out due to pests targeting that species.
- Consider native trees that can thrive and support biodiversity
Species diversity should be an important factor for communities when forming tree lists to account for both the need for the urban forest to provide habitat for native species and climate resiliency.
- Create a plan for managing non-native and invasive species
Non-native plants are species that have not historically lived in one area but have been introduced by humans. Invasive species are also non-native but their introduction causes harm to the surrounding ecosystem. Communities should avoid the use of known invasive species and undertake continuous monitoring to ensure that any non-native species that are introduced do not cause harm to the local ecosystem. There is no source of information that comprehensively identifies which species will be invasive in a local area. Communities should utilize the best available resources and work with local arborists to select trees that have not been known to be invasive and plan for continuous monitoring.
Resources for Creating a Tree List
- Projected risk factors and future climate
Risk Factor: This tool by the First Street Foundation allows you to see the projected risk factors of your city, including flood, extreme heat, fire and wind factors.
Cal-Adapt: Cal-Adapt allows you to see projected mid-century (2035-2064) and end-century (2070-2099) annual average maximum and minimum temperatures, and annual average precipitation, broken down by cities, counties, congressional districts and other boundaries.
Climate Assessment Tool: This tool allows you to see how suitable certain taxa, or groups of trees are to the future climate of a botanic garden’s location or by longitude and latitude coordinates. Not all species of plants and botanical gardens are included. The analysis of the climate suitability of species is based on mean annual temperature.
Future Plant USDA Hardiness Zone: This interactive web tool by Davey Institute and Arbor Day Foundation allows you to see the projected new USDA Hardiness Zone of your area based on multiple emissions scenarios.
USDA Hardiness Zone Map: The USDA Hardiness Zone Map allows you to identify your current USDA hardiness zone based on zip code. The map is based on the average annual minimum winter temperature. Although plants are often noted to show that they can live in certain USDA zones. A plant may still thrive in an area not indicated to match its USDA zone.
- Tree characteristics
Climate Ready Trees: Climate Ready Trees is a resource compiled by a coalition of university, USDA forest service researchers, and ecological non-profits that highlights a number of trees that may be particularly suited to tolerate the stressors of a changing climate.
SelecTree: SelecTree allows you to search California trees by tree characteristics such as whether they are native, height, tree shape, salinity tolerance, water use rating, sun exposure and landscape information such as USDA hardiness zone, sunset climate zone, utility precautions, available planting size, and more.
The Water Use Classification of Landscape Species: This database by the UC Davis California Center for Urban Horticulture assigns ratings for irrigation needs from very low to high.
CalScape: This resource by the California Native Plant Society can help you identify where, on a map, different native species are found in California. This resource also provides basic information about plant characteristics and how to care for them.
Waterwise Garden Planner: This tool allows you to search for different California plants based on whether they’re native, plant type (trees or shrubs), evergreen or deciduous, water needs, sun exposure, aesthetics and functions.
- Where trees historically lived and invasiveness
CalScape: This resource by the California Native Plant Society can help you identify where, on a map, different native species are found in California. This resource also provides basic information about plant characteristics and how to care for them.
Inventory of Invasive Plants: This list by the California Invasive Plant Council ranks the invasiveness of species.
- Guides on how to create your tree palette
California Tree Palette for Schoolyard Forests: This resource by the Green Schoolyards of America is intended to help you identify trees that are climate-resilient and are appropriate for a schoolyard setting by sunset climate zones in California.
Planning for resilient urban forests: A datadriven approach to assessing urban tree species suitability in California: This presentation by Natalie Love, Ph.D. describes a process by which communities can use to select a tree list for their city.
Climate Change and Urban Forests: This presentation by Igor Lacan, UC Agriculture and Natural Resources, (UCANR) describes the space-for-time substitution process that can be used to identify tree species for a community’s future climate.
Tree Species Selection Guidelines for the Albuquerque Metro Area: A study on identifying climate-ready trees by the Nature Conservancy for the Albuquerque area.
- List of locally specific climate-resilient trees considered by other organizations
California Tree Palette for Schoolyard Forests: This resource by the Green Schoolyards of America is intended to help identify trees that are climate-resilient and are appropriate for a schoolyard setting by sunset climate zones in California.
Planting Resilience: Identifying Climate-Resilient Tree Species and Increasing Their Presence in Los Angeles’ Urban Forest: This study by the UCLA Luskin School of Public Affairs and TreePeople identified 28 climate-resilient tree species that show promise for Los Angeles County.
Climate Ready Trees: Climate Ready Trees is a resource compiled by a coalition of university, USDA forest service researchers, and ecological non-profits that highlights a number of trees that may be particularly suited to tolerate the stressors of a changing climate.
Tree Species Selection Guidelines for the Albuquerque Metro Area: A study on identifying climate-ready trees by the Nature Conservancy for the Albuquerque area.
References
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