Collaborator Findings:

Collaborators are concerned with tree conflicts where safety concerns arise from blocked signage and infrastructure (e.g., business and road signage, solar, safety cameras, lighting park attributes). There is also concern for considering the diversity of soil types within the community when planting. Some ideas include planting trees in parks farther away from sidewalks, irrigation techniques that suit each soil type, and proper tree selection.

Climate Ready Trees

Increasingly, cities are recognizing that climate change and the associated shifts in weather patterns are likely to alter species habitat ranges and render some species less adapted to the region. Research on climate change in complex urban ecosystems is challenging and still evolving. Although there is no clear consensus on the future outcomes, it is thought that extraordinary weather events are likely to increase in years to come. In recent years, Davis has experienced increased storm intensity which can lead to higher wind speeds and flood conditions. They have also experienced prolonged droughts. As temperatures rise and precipitation patterns fluctuate from historical norms, existing trees must adapt or succumb to the changes in climate. Impacts on urban forestry programs may include:

• Health and structural impacts on tree species that are not adapted to new and changing conditions
• Increase in pests and disease as a result of changes in temperature, precipitation, and tree stress
• Additional costs for mitigation and tree removal for marginal species
• Canopy loss, especially where key species (e.g., predominant species) become marginalized

With a potential for an increase in maladapted species in the tree population and for urban areas to exacerbate the stresses, the City is evaluating tree performance of established species in the community and selecting high performing species for future tree planting. They are also looking toward alternative climate-ready trees. Climate-ready trees are trees that are likely better suited to the new climate trends in a particular region with traits that allow them to be drought tolerant, resist pests and disease, and withstand other stressors like increased salt and wind.

The City of Davis and the greater community are leaders in the climate-ready trees initiative. The City is collaborating with various groups like CAL FIRE, United States Forest Service, University of California Davis, Tree Davis, nursery growers, and nursery brokers to experimentally plant trees that are best suited to the new challenges brought up by climate change. The University of California Arboretum and Public Gardens are testing 45 species in their Texas Trees Trial (UC Davis, 2022). Twelve tree species that are climate-ready candidates are being evaluated in 4 Sacramento city park plantings (McPherson et al. 2015). The trees are being monitored annually for the first ten years because mortality is most likely during this establishment period. Data from similar trials started in the 1990s has allowed urban forest collaborators to discourage species that did not fare well (e.g., Persian ironwood) and promoted species that are likely to succeed in the future conditions (e.g., red push pistache and mesquite). In collaboration with the City of Davis, Tree Davis recently planted 1,000 trees in Davis as part of the Community Canopy Grant Program. The species palette included several climate-ready species and potential candidates that will be monitored for performance. The City is also planting native and near native trees which are important to protect the biodiversity they support in California.

The availability of climate-ready trees and potential candidates is going to be important for Davis to maintain canopy cover. The City is part of a regional working group to identify appropriate species for the region as well as test species to plant and monitor performance. In addition, the City is in conversations with CAL FIRE and other collaborators with the intent to provide easier access to climate ready species, whether it be through opening a historic nursery to grow hard to find species or coordination efforts to obtain the desired species. 

Prioritization of climate-ready trees is a step the City and their collaborators are  actively taking to prepare Davis for the new climate challenges. Climate-ready trees are essential to the prosperity of Davis’ urban forest and the ongoing benefits provided by trees.

Collaborator Findings:

Collaborators expressed concern about water usage and security. Some ideas include utilizing reclaimed water and expanded irrigation systems so that all trees have access to water. Collaborators observed that mature trees suffer from lack of water after turf has been converted to xeric landscaping.

Xeriscaping and Trees

California experiences periodical droughts and one of the most recent droughts began in 2012. This drought was characterized by the lowest calendar-year and 12 month precipitation, the highest annual temperature, and the most extreme drought indicators on record (Diffenbaugh et al. 2015). Changes in the region's climate could lead to more drought-like conditions which can have lasting negative impacts on the urban forest. The Southwestern United States drought persistence is increasing competition among various sectors of water use, affecting economic security (Lynam & Piechota, 2021). In response to these conditions, California has adopted emergency drought rules that prohibit the watering of grass that is purely decorative at businesses and in common areas of subdivisions and homeowners associations. It will be important to consider how current and future trees will deal with changes in watering frequency to prevent canopy loss. 

Droughts cause changes in soil composition and watering frequency which can stress large, well-established trees and can lead to increased tree morality (Anderegg et al. 2013). Economic consequences of drought include increased maintenance and removal costs, and greater susceptibility to pests and disease. The ongoing drought has also caused issues by shortening the planting season. 

In Davis, community members recognize the impacts droughts have on their water security. Some community members are not in favor of planting trees in their yards and others are modifying their landscaping to be more drought tolerant through careful species selection and xeriscaping. Xeriscaping is the practice of landscaping with slow-growing, drought tolerant plants and trees to conserve water and reduce yard trimmings. Xeriscaping is most effective when considering the region's specific needs. This targeted approach maximizes benefits and allows the chosen plants to thrive. Xeriscaping with plants that use less water can reduce outdoor water use by as much as 50 percent (DWP, 2022). The City has implemented xeriscapes in some planting strips and understands that trees may not always be incorporated due to the lack of water. Though this landscaping technique could be a great step in the right direction, care should be given when transitioning landscapes with already established trees. Minimizing changes in irrigation and disturbance to roots of already established trees will help preserve canopy while moving toward a more xeric landscape overall. Increased public education and strategic partnerships may help guide the community residents to a more water conscious culture without sacrificing the urban forest.