In total, there are 30,692 trees in the 2018 City tree inventory. Street trees make up the largest proportion of City trees (34.7%), followed by City trees in the green belts (27.1%). Green belt trees are trees along bike paths that connect to the many parks around Davis. Trees in park areas make up 20.5% of the overall inventory and are followed by rights-of-way trees and “City trees”. 

Location of City Tree Resource

 

Stocking Level 

The stocking level is the total number of available sites that are planted with a tree. The City asset management system was not originally implemented to track stumps or vacant sites as assets, rather when a tree is removed it becomes a work request for tree planting in the asset management system. Because of this, it is difficult to track available planting sites and an accurate stocking level could not be determined, but will be revisited during the next inventory.

There are 207 unique species in Davis’ City tree inventory, which is higher than the mean of 185 species reported from 18 California communities (Muller and Bornstein, 2010). The three most common species are Platanus x acerifolia (London planetree, 7.8%), Pistacia chinensis (Chinese pistache, 6.6%), Lagerstroemia indica (common crapemyrtle, 6.2%) and make up 21% of the population. 

Most Prevalent Species in the Davis City Tree Resource

Maintaining a variety of species in a City tree inventory is important. If any single species or genus is more common, storm events, drought, pests and disease, or other stressors could result in losses of hundreds or even thousands of trees. This loss of trees could result in high costs to pay for removals or other treatments. Severe losses could even result in significant losses of benefits to the community. Invasive shot hole borers (Euwallacea spp. and Fusarium euwallaceae), spongy moth (Lymantria dispar), sudden oak death (Phytophthora ramorum), and emerald ash borer (Agrilus planipennis) are a few examples of unexpected pests that have resulted in the death of thousands of trees in other communities and are direct threats to Davis’ tree resource. In cities affected by these pests, the costs to remove dead trees or treat infested trees were extremely high. These examples highlight the importance of planting different species of trees from a variety of genera to increase the strength of the forest against pests, disease, and other pressures. 

Many cities have used the rule of thumb:

• No species should represent more than 10% of a population

• No one genus should represent more than 20% of a population

• No family should make up more than 30% of a population (Clark et al. 1997; Santamour, 1990) 

Although Davis’ City tree resource is within these diversity thresholds, the Urban Forestry Division and local collaborators like Tree Davis are focusing on new and replacement tree plantings to increase diversity. With climate change, urban forests are at more risk to storm events, long periods of drought, and/or pests and disease. Because of these dangers, Davis is working to further increase species diversity. 

To increase the strength of the urban forest, future tree planting should focus on including as many species of trees as possible while also phasing out species that are not performing well. Plantings should also limit species that already make up significant portions of the population (e.g., Platanus x acerifolia). New species that are expected to do well in the local climate and that are less in danger of being attacked by pests and disease should be planted. Additionally, new species should be better able to survive higher temperatures and periods of drought. 

Species Fitness

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Selecting trees that are well-adapted to current and expected future conditions is an important aspect of maintaining a healthy urban forest. Trees that are well-matched to climate conditions are less stressed than poorly adapted trees and thus better able to resist pests and diseases. This reduces long-term maintenance and replacement costs.

The fitness of the most common 24 species, all of which compose at least 1% and total approximately 69% of the city tree resource, was evaluated and ranked as low, medium, or high-risk. The evaluation considered factors such as water requirements in Davis (WUCOLS 2023), susceptibility to potential pests, and suitability for future climate conditions, which are assumed to be warmer and drier than current conditions. A description of the rankings follows:

• High-risk species have high water needs, serious known threats from pests or disease, or are not tolerant of high temperatures. A phasing out of high risk trees should be considered through replacement with lower risk trees over time rather than replanting of the same species.

• Moderate-risk species require moderate water or have a known pest of significance, and thus may be more likely to become stressed by future increases in drought and heat. Moderate risk species are suitable for planting after consideration of site-specific factors.

• Low-risk species require low or very low water and have no other significant identified pests.

It should be noted that new introductions of invasive pests may occur at any time, putting new species at moderate to high risk of failure. Risk evaluations should be updated as new information on pests becomes available.

Species identified as high-risk are coast redwood (Sequoia sempervirens), three species of ash (Fraxinus spp.), both groups make up approximately 5% of the city tree resource. Coast redwood requires high levels of irrigation to survive and can become stressed by heat as they mature. Ash trees are threatened by the emerald ash borer (Agrilus planipennis), an invasive insect that has been steadily spreading westward across the United States. Although not currently known to be in California, it has been found in Oregon and is likely to spread into California in time (CDFA 2022). 

Eleven prevalent city tree species were identified as moderate risk. Ten species, totaling approximately 29% of city tree resources, require moderate amounts of water making them potentially more susceptible to drought and heat stress. Trees requiring moderate water include trident maple (Acer buergerianum), European hackberry (Celtis australis), Chinese hackberry (Celtis sinensis), ginkgo (Ginkgo biloba), goldenrain tree (Koelreuteria paniculata), California sycamore (Platanus racemosa), London plane tree (Platanus x acerifolia), Callery pear (Pyrus calleryana), Chinese tallow tree (Triadica sebifera), and Chinese elm (Ulmus parvifolia).

Valley oak (Quercus lobata), which accounts for approximately 6% of city tree resources, is identified as a moderate risk tree. Although it requires low water and is adapted to drought, it is susceptible to an emerging disease complex along with trident maple, California sycamore, London plane tree. Over the past ten years, several species of invasive shot hole borer beetles and a symbiotic Fusarium fungus (ISHB-FD) have emerged as a significant threat to trees in southern California. While the current population is centered around Los Angeles, the wide range of host plants and lack of preventative treatment allows for further spread (UCANR 2023, Eskalen et al. 2013).

The remaining nine prevalent city tree species, totaling approximately 24% of total city tree resource, are identified as low risk due to their low water requirements, drought tolerance, and lack of known significant pests. This includes river she-oak (Casuarina cunninghamiana), honeylocust (Gleditsia triacanthos), common crapemyrtle (Lagerstroemia indica), Turkish pine (Pinus brutia), Canary Island pine (Pinus canariensis), Chinese pistache (Pistacia chinensis), blue oak (Quercus douglasii), cork oak (Quercus suber), and chaste tree (Vitex agnus-castus). It should be noted that although honeylocust and cork oak are both host species for ISHB-FD, health impacts from the disease are generally limited to smaller localized areas of the plant in these species (UCANR 2023). 

Knowing the percentage of trees that are younger, middle-aged, and mature is useful for anticipating and planning for maintenance (Richards, 1982/83). The diameter of a tree can be used to estimate its age. Trees with smaller diameters are generally younger and trees with larger diameters are older. 

Ideally, 40% of a population of trees should be young (less than 8 inches in diameter) and 10% should be mature (greater than 24 inches in diameter). The age distribution of Davis’ City tree resource shows a well-established population. Nearly 36% of all trees are less than 8 inches in diameter and 9.6% are greater than 24 inches. The data indicates that a number of recent tree plantings have been directed towards both large and small statured trees. 

In general, trees greater than 24 inches in diameter require more regular inspections and routine maintenance as they mature. Davis’ City tree inventory has 2,959 mature trees (9.6%). The Urban Forestry Manager can gain a better understanding of the specific risks that individual mature trees pose with regular inspection and risk assessment.

Trees between 8 and 18 inches in diameter are generally established and are a mixture of young, large- and medium-stature tree species and mature small-stature species. This age group is a positive indicator of future benefits from the City trees since large shade trees typically provide more shade, pollutant uptake, carbon sequestration, and rainfall interception than small trees. The 12,762 trees between 8 and 18 inches in diameter represent 41.5% of the City tree resource.

Trees below 8 inches in diameter indicate young trees and new tree plantings. Of the City trees, 11,012 trees are below 8 inches and represent 35.8% of the population. This figure reflects tree planting over the last two decades from both City projects and conditions of approval for development projects. 

Age Distribution of the Davis City Tree Resource

Tree condition is an indication of how well trees are managed and how well they are performing in the region and in each site-specific environment (e.g., street, median, parking lot, etc.). Condition ratings can help the Urban Forestry Manager anticipate maintenance and funding needs. In addition, tree condition is an important factor for the calculation of resource benef­its. A condition rating of “good” assumes that a tree has no major structural problems, no significant mechanical damage, and may have only minor aesthetic, insect, disease, or structural problems, and is in good health. When trees are performing at their peak, as those rated as good or better, the benefits they provide are maximized. Improvements in condition are possible if young trees receive selective or “training pruning”, established trees receive regular pruning on a minimum of a 5 to 7 year cycle, and during maintenance trees receive inspections for structural concerns, pests and disease, and changes in condition.

Based on the inventory data (2018), City trees in Davis are in overall fair or better condition (93.3%). Approximately 5.7% of trees are in poor condition and 1.0% are dead.

Condition of the City Tree Resource 

Annual Benefits from City Trees to Air Quality, Carbon Sequestration, and Stormwater Runoff Reduction

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City trees provide numerous tangible and intangible benefits to the community (see the Story Map for more information on the benefits of trees). While ongoing research demonstrates the relationship of trees and canopy to many environmental and socioeconomic benefits, the majority of these benefits cannot currently be quantified in direct monetary terms, including aesthetic contributions and positive effects on mental health. A few benefits, including those to air quality, energy savings, carbon sequestration, and reductions in stormwater runoff, can be measured in dollars. Regardless, City trees are worth far more than what can currently be quantified.

Where benefits can be quantified, the value of benefits is dependent upon the location, species, size, and condition of the trees in the population. Currently, City trees are annually providing $213,857 in quantifiable benefits (excluding energy⁶),  for an average of $6.97 per tree. These annual benefits have the potential to increase over time with proactive maintenance and as young trees mature. The greatest benefits come from large-stature, mature trees. Promoting tree health and longevity can maximize these benefits. To accurately account for annual benefits, the tree inventory data requires regular updates to record changes in size, condition, removals, and new trees. 

Annual Quantifiable Benefits from the City Tree Resource

Cleaning the air

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City trees remove 10.4 tons of air particulates and provide $117,423 in annual savings from fewer health conditions like asthma and respiratory diseases, which is an average of $3.83 per tree.

Taking carbon out of the air

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City trees take 421.5 tons of carbon dioxide (CO₂) out of the air every year, a value of $71,882 or an average of $2.34 per tree. 

Slowing stormwater

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City trees absorb and slow the flow of 2.7 million gallons of water from entering streams, creeks, and rivers every year. With less water entering stormwater control and treatment systems, there is an annual savings of $24,552, an average of $0.80 per tree. 

Reducing energy use

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City trees help save money on electricity and natural gas. Less electricity is used during high temperatures, because trees shade and cool buildings. In cooler temperatures, trees can block cold winds from hitting buildings, which lessens the need to heat the building with natural gas. To calculate this benefit, data on the distance and direction of trees from buildings is needed. The 2018 inventory does not include this data, therefore the dollar savings from this benefit are not known. 

⁶i-Tree Eco requires information on the distance and aspect of individual trees from homes and other conditioned structures to calculate energy benefits. This information is currently unavailable for Davis’ City tree resource.