About sequestration –
The iTree project estimated that 235,851 acres, or 51% of the actual land, was canopy cover in 2010, and that the canopy was sequestering 648,534 tons (590,539 metric tons) of CO2/year, or about 2.5 tonnes/acre. Our total inventoried 2016 emissions were 2,965,754 metric tons/year, so that annual sequestration would be 20% of it.
If we added another 11,793 acres of mature trees to the canopy, expanding it by 5%, the additional annual sequestration would be about 29,500 metric tons CO2e/year, or about 1% of our annual inventoried emissions. (By way of comparison, an average US car emits about 4.6 metric tons a year, so this would be the equivalent of taking 6,400 cars off the road.) If we planted a new acre of trees, the saplings would not be sequestering carbon at that rate for quite a few years, though. (A few years ago, Olympia estimated it had roughly 9,000 spaces available for additional street trees – that’s the rough equivalent of thirty acres of forest at 300 trees to an acre.)
About storage –
iTree County estimated that the county canopy in 2010 was holding the equivalent of 47,654,275 tons of accumulated CO2e, or 42,888,847 metric tons. Since our annual inventoried 2016 emissions were 2,965,754 metric tons, that’s roughly 14 years worth. (With these numbers, each acre of trees in the 235,851 acres of canopy is storing roughly the equivalent of 182 metric tons of CO2e, so if all the stored carbon went back to the atmosphere when you logged an acre it would roughly equal the emissions of 40 cars for a year. The Sierra Club estimates that when a tree’s converted to lumber roughly a third of the carbon continues to be stored for a fairly long time, though. That would mean about 120 tonnes went into the atmosphere when we logged an acre, and in addition, we’d lose the 2.5 tonnes of annual sequestration in each year after that.) Clearly, it’s a lot more useful to avoid cutting an acre of trees than to plant a new one.
TRPC does an estimate of the farmland and forest lands lost each year as part of its Sustainable Thurston Report Card on conserving rural lands. Unfortunately, I don’t think it’s possible to use that as an estimate of canopy loss. Those estimates don’t include whatever’s happening to the canopy in Capitol Forest and other working forests or in the area of the county that’s part of JBLM. In addition, when rural land is developed, it’s generally on parcels of five acres or more, and we don’t know how often those new homeowners actually keep most of the trees on their property…
iTree Methods
The iTree estimates are available through an interactive webpage. (Their figures for canopy coverage are based on 2010 National Land Cover Database estimates, and may well be low, since the resources for the project include an article which compares the NLCD estimates with ones from higher resolution photointerpretation, and concludes that the NLCD results underestimate canopy cover by an average of 9.7% nationally, and by 11.6% for Washington State overall.) According to the discussion of their methods, they use a national average to estimate the carbon storage by that much urban canopy, and a state average for the sequestration from that. Their sequestration estimates for that amount of forest canopy are based on county level Forest Service Inventory data. (Note that they use 495,000 acres as the area of the county, though only 462,000 acres of that is land; I’ve corrected the percentage of coverage in the summary above to take account of that.)
There is a 2016 version of the NLCD database; unfortunately, it hasn’t been used by iTree, and as far as I can see, we’d need to load the available data into a GIS system and repeat iTree’s land coverage analysis ourselves to get comparable estimates.