Tumwater Tree Board’s latest tree inventory shows that trees in Tumwater city facilities store 1,968 tons of carbon dioxide.
Combined with the carbon stored in sample trees from large city properties where the city did not manually count every tree, the inventory determined that the trees in city properties store a total amount of 5,971 tons of carbon dioxide.
The benefits of the city trees also generate an annual value of $18,010 or $3.68 per tree based on various environmental and health functions.
The Tree Board met on Monday, March 11, to discuss the report completed on February 28.
The scope of the inventory includes trees in the city-owned facilities. For properties where it would cost too much to manually count every tree, a sample size of 589 trees was also studied.
Such places, called “natural areas” in the report, include stormwater sites and large parks such as Pioneer Park, Palermo Pocket Park, and Tumwater Hill Park, as well as other areas. Despite this distinction, Sustainability Coordinator Alysa Jones Wood told the board that the team had counted all trees in Tumwater Historical Park.
Tumwater has 7,345 city tree sites in its database, which was determined through data collection by community volunteers and additional data by Davey Resource Group, which the city contracted to conduct the tree inventory.
Out of those trees, only 4,890 trees had sufficient data to be used for analysis in i-Tree Eco, a software that can provide a quantified evaluation of the structure, functions, and benefits of trees.
In terms of benefits, the city trees reduced stormwater runoff by intercepting 839,871 gallons of stormwater, removed 1.1 tons of air pollutants from the atmosphere and sequestered 26.7 tons of carbon. All measurements were taken at an annual value.
The environmental benefits for trees in natural areas were greater as the trees removed 4.4 tons of air pollutants annually and sequestered 159 tons of carbon dioxide.
These benefits were quantified to understand their monetary annual value. Reduction in stormwater run water was valued at $7,505 or $1.53 per tree; air pollution removal was valued at $5,957 or $1.22 per tree, while carbon sequestration was valued at $4,548 or $0.93 per tree.
The health benefit of the trees was also valued at $3,275 in terms of reduced medical costs and hospital visits per year.
Jones Wood noted that the report did not account for a lot more other types of benefits such as beautification, comfort, shading, privacy, wildlife habitat, and historical value.
The inventory shows that city trees comprise 110 unique tree species, with only 19% of the species considered native to Washington.
The Norway maple (Acer platanoides) makes up 15.3% of the city and which has the largest share in the composition, followed by Callery pear (Pyrus calleryana) and red maple (Acer rubrum) at 9.5% and 9.2%, respectively.
The natural areas consist of 16 species with big leaf maple (Acer macrophyllum), Douglas-fir (Pseudotsuga menziesii), and red alder (Alnus rubra) as the top three most abundant species at 28.2%, 21.9%, and 17%, respectively.
Jones Wood noted that the big-leaf maple often provided the most benefits among all tree species. If quantified, the species generate a yearly value of $18 per tree.
“One running theme in here is that the big leaf maple gives us are the biggest bang for our buck in terms of ecosystem services,” Jones Wood said, adding, “They sequester the most carbon. They remove the most air pollution. They were the most functional tree in terms of ecosystem service benefits, which is not how I normally hear them characterized by others.”
In terms of condition, 65% of the city trees are in “very good” condition, but 95% of them are found to be susceptible to 44 emerging pests and diseases. Threats include the Asian longhorned beetle, defoliating moths, and pine shoot beetle.
Within the natural areas, 87% of the sample trees rated “fair” or better in terms on condition.
Regarding size, 44.5% of the trees are less than 6 inches in diameter at breast height while only 9.8% are larger than 24 inches in diameter at breast height.
The city trees have a combined value of $11.9 million such that it would cost this much to replace all of them and achieve the same sizes, composition and condition.
The inventory also identified how much it would cost and how much work would be required to maintain the city trees.
Based on Davey Resource Group's rates, maintaining the trees will cost $850,839 every four years.
The inventory anticipates 1,759 hours of inspection work at an annual cost of $41,770.
In terms of pruning work, 267 trees require maintenance pruning. Two-hundred and eight of these trees are considered large and routinely prune them will cost $114,800 per year. Fifty-nine trees are considered small, so the pruning costs are lower at $11,800 annually.
The inventory also identified that 56 trees will need to be removed at a cost $313,600. Such trees flagged for removal include those considered hazardous while some were non-native.
Trees in city streets are not included in the inventory as the city has a separate inventory for them.
The $60,000 inventory project was two-thirds funded by USDA’s urban and community forestry program administered through Washington Department of Natural Resources.
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