RELATIONSHIPS BETWEEN SOIL ORGANIC-MATTER AND FOREST PRODUCTIVITY IN WESTERN OREGON AND WASHINGTON

Mineral soil and forest floor C and N contents were determined in 154 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) stands in western Oregon and Washin gton ranging in age from 16 to 64 years. Relationships between site in dex and mineral soil and forest floor C, N, and C/N ratios were examin ed. Douglas-fir data were analyzed by geographic province (Puget Sound , Washington Cascades, Oregon Cascades, coastal Washington, coastal Or egon, and southwest Oregon). Average mineral soil C in Douglas-fir sta nds ranged from 102 Mg/ha to 177 Mg/ha in Puget Sound and Washington C ascades provinces, respectively. Soil N ranged from 3708 kg/ha in Puge t Sound province to 9268 kg/ha in the Washington Cascade province. Wes tern hemlock data were analyzed in three provinces (Washington Cascade s, coastal Washington, and coastal Oregon). Average mineral soil C in western hemlock stands ranged from 241 Mg/ha in the Washington Cascade s to 309 Mg/ha in coastal Washington and was higher than Douglas-fir m ineral soil C. Western hemlock mineral soil N was also higher than Dou glas-fir mineral soil N ranging from 10 495 kg/ha in the Washington Ca scades to 15 216 kg/ha in coastal Oregon. Forest floor C and N content s were also higher in western hemlock than Douglas-fir stands. Nonline ar regression analysis revealed a weak positive relationship between s ite index and total mineral soil C in Douglas-fir (r2 = 0.19). A simil ar relationship was observed between Douglas-fir site index and total soil N (r2 = 0.19). Relationships were weak because of the large varia bility in mineral soil C and N within as well as across provinces. Max imum Douglas-fir site indexes occurred across a broad plateau of miner al soil and forest floor C/N ratios ranging from 15-25 and 35-45, resp ectively. Minimum site indexes also occurred in these C/N ranges. No i ncrease in Douglas-fir productivity occurred above mineral soil C leve ls of 125 Mg/ha. There were no relationships between site index and mi neral soil C and N or C/N ratios in western hemlock stands.