Overview – County Forests
The climate plan uses a value based on local projects, 10.18 tonnes CO2 per acre-yr averaged over the first thirty years of growth, as the basis for its estimates of potential sequestration from adding forested land in the county. (This is almost four times the US Forest Service’s iTree Canopy project’s estimates for carbon sequestration by the county canopy in 2010, and implies an estimate for carbon storage by an acre of thirty year old trees that’s 168% of iTree’s estimate of storage for the whole varied canopy, but those are based on a number of approximations and assumptions.
The Thurston Regional Planning Council estimates we’ve lost about 525 acres of forest lands a year in Thurston County during the years between 2000 and 2014.
About sequestration –
The current draft of the plan proposes adding another 37,000 acres of forest in the county to sequester an average of 379,000 tonnes of CO2 per year over the thirty years between now and 2050. Our total inventoried 2016 emissions were 2,965,754 metric tons/year, so this added annual forest sequestration would have been 13% of that.
37,000 acres is about 8% of the land in the county, or 168% of the current 22,000 acres in cropland. This new forest would provide 99% of the sequestration included in the plan, while shifting to regenerative agricultural practices on 6,600 acres of the cropland would provide the last 1%. (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, so street trees can’t contribute much additional sequestration.)
By way of comparison, an average US car emits about 4.6 metric tons a year, so this additional forest sequestration would be the equivalent of taking 82,391 cars off the road.
About storage –
An average sequestration of 10.18 tonnes CO2/acre-yr over thirty years implies that an acre of thirty year old trees would be storing the equivalent of 305 tonnes of CO2. (iTree County estimated that the county canopy in 2010 was holding the equivalent of 47.65 million tons of accumulated CO2e, or 42.89 million metric tons. Since our annual inventoried 2016 emissions were 2.96 million metric tons, that would be roughly 14 years worth.) If the canopy were all thirty year old trees,the draft’s estimate would mean it was storing the equivalent of 71.94 million tonnes, or roughly 24 years’ worth, and if all the stored carbon went back to the atmosphere when you logged an acre it would roughly equal the emissions of 66 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 205 tonnes went into the atmosphere when we logged an acre, and in addition, we’d lose the 10.18 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…