Historical Colonization of South Puget Sound Prairies by Douglas-fir at Joint Base Lewis-McChord, Washington

Native Americans used fire to maintain prairies in western Washington, but few prairies remain due to land-use conversion and forest encroachment. We describe the process of forest encroachment on former western Washington prairies. We used 1853-1871 General Land Office Survey data to describe the historical prairie and associated timbered area conditions at six Joint Base Lewis-McChord sites (JBLM). We described modern stands growing in these same areas with stem density, Reineke stand density index (Reineke 1933), and site index. After harvesting selected trees we measured decadal tree-ring increments and analyzed trauma rings on 242 Douglas-fir stumps to reconstruct stand development. We plotted individual tree stem diameter growth curves to illustrate patterns of establishment. Douglas-fir density increased in waves from 1878-1938 associated with low-intensity fires with fire return intervals of 10 to 91 years. Historical tree density, which ranged from < 1 to 49 trees ha(-1) has increased to 122 to 207 trees ha(-1). Stand development was driven by fires, patterns of establishment, and site potential. Longer fire-free intervals resulted in few larger establishment waves while frequent shorter intervals resulted in multiple smaller cohorts. Woodlands, not prairies, resulted from ten or more year fire frequencies; however, mature forests grew in < 50 years suggesting fire frequencies appropriate for restoration of prairies and woodlands. Modern productive sites produced larger trees that experienced inter-tree competition, suggesting faster forest succession occurred on those sites. Light to moderately thinned Douglas-fir reproduced poorly on our sites. Higher intensity thinning or gap creation may be required for successful regeneration.