Sat Jan 08 17:00:15 CST 2022. Forest Inventory and Analysis Database, St. Paul, MN: U.S. Department of Agriculture, Forest Service, Northern Research Station. [Available only on internet: http://apps.fs.fed.us/fiadb-downloads/datamart.html]

This map was built using data from an annual forest survey which records data from plots once every 10 years. This is best viewed on a larger screen which will give the user greater filtering and visualization capabilities. For instance on desktop, one may choose a combination of tree species which will show up as different colors.

A few interesting things you might notice

Douglas fir and western hemlock are found all over the PNW
Pines are found in greater concentration on the east side and the coastal areas
Oaks and sugar pine are rare in Washington but more common in Oregon

The size of the dots corresponds to total above ground carbon which is estimated using a range of factors including measured size, species, growth type, etc. If you are interested in learning more about the methodology or viewing the raw data, you can find more information here.

I believe the most accurate information comes from the national forests as there is a greater interest by foresters in understanding the value of timber in these areas. Private land, national parks, and state/local land also likely has different survey data which might not be included here.

Known limitations include:
Time since last survey – each plot is only sampled every 10 years
Fire and timber harvest – Recent changes will not be reflected until land is surveyed
Visualization accurately reflecting biomass – The size of the dots are not linearly correlated with the amount of carbon