Study area. Northwest Oregon counties, forest land, vegetation zones, and Forest Service inventory plots. The Picea sitchensis zone boundary was derived from Franklin and Dyrness ; the Willamette Valley boundary was obtained from Level III Ecoregions of Oregon  and forest land coverage came from the Oregon Gap Analysis Program . Inventory plots  are shown somewhat offset from their true locations to protect landowner privacy and to comply with applicable laws and regulations. Plot concentration on the Siskiyou National Forest (southern Tillamook and western Yamhill Counties) is greater because of greater sampling intensity on National Forest Land.