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 Washington ranging in age from 16 to 64 years. Relationships between site index and mineral soil and forest floor C, N, and C/N ratios were examined. Douglas-fir data were analyzed by geographic province (Puget Sound, Washington Cascades, Oregon Cascades, coastal Washington, coastal Oregon, and southwest Oregon). Average mineral soil C in Douglas-fir stands ranged from 102 Mg/ha to 177 Mg/ha in Puget Sound and Washington Cascades provinces, respectively. Soil N ranged from 3708 kg/ha in Puget Sound province to 9268 kg/ha in the Washington Cascade province. Western hemlock data were analyzed in three provinces (Washington Cascades, coastal Washington, and coastal Oregon). Average mineral soil C in western hemlock stands ranged from 241 Mg/ha in the Washington Cascades to 309 Mg/ha in coastal Washington and was higher than Douglas-fir mineral soil C. Western hemlock mineral soil N was also higher than Douglas-fir mineral soil N ranging from 10 495 kg/ha in the Washington Cascades to 15 216 kg/ha in coastal Oregon. Forest floor C and N contents were also higher in western hemlock than Douglas-fir stands. Nonlinear regression analysis revealed a weak positive relationship between site index and total mineral soil C in Douglas-fir (r2 = 0.19). A similar relationship was observed between Douglas-fir site index and total soil N (r2=0.19). Relationships were weak because of the large variability in mineral soil C and N within as well as across provinces. Maximum Douglas-fir site indexes occurred across a broad plateau of mineral soil and forest floor C/N ratios ranging from 15-25 and 35-45, respectively. Minimum site indexes also occurred in these C/N ranges. No increase in Douglas-fir productivity occurred above mineral soil C levels of 125 Mg/ha