Historic Extent of Puget Sound Prairies as Determined by An Examination of Soil Charcoal.

It is widely known that the prairies in the Puget Sound region of the Pacific Northwest were maintained by tribes using fire to suppress the growth of trees and promote a prairie ecosystem.  Over the last two centuries of fire suppression (since colonization by Europeans), forest encroachment has reduced regional prairies to less than 10% of their former extent.  Before land managers can take steps to restore these endangered prairie ecosystems they must first determine their historic extent.  The focus of this study is to determine the historic extent of three remaining Puget Sound prairies and adjacent forests by examining soil and soil charcoal properties that are indicative of past prairie or forest ecosystems.  Soil samples were collected along transects extending from the current prairie directly into encroaching forest and were examined for total black carbon (charcoal), relative size class of charcoal, and charcoal structure.  The presence of prairie soil characteristics as far as 300m into the current forest supports the reports of significant forest encroachment.  The forest surrounding one prairie has encroached approximately 65m from the north and 275m from the East.  Soil samples taken at another prairie indicate that the entire transects (up to 215m into the forest) was historically prairie. This suggests a much greater forest encroachment than initially thought.