What the soil reveals: potential total ecosystem C stores of the Pacific Northwest region, USA

How much organic C can a region naturally store in its ecosystems? How can this be determined, when land management has altered the vegetation of the landscape substantially? The answers may lie in the soil: this study synthesized the spatial distribution of soil properties derived from the state soils geographic database with empirical measurements of old-growth forest ecosystem C to yield a regional distribution of potential maximum total-ecosystem organic C stores. The region under consideration is 179,000 square kilometers extending from the southern Oregon border to the northern Washington border, and from the Pacific Ocean to the east side of the Cascade Mountains. Total ecosystem organic C (TEC) was measured in 16 diverse old-growth forests encompassing 35 stands and 79 pedons to a depth of 100 cm. The TEC ranged between 185 and 1200 Mg C ha-1. On an average, 63% of TEC was in the vegetation, 13% in woody detritus, 3% in the forest floor, 7% in the 0-20 cm mineral soil, and 13% in 20-100 cm mineral soil. The TEC was strongly related to soil organic C (SOC) in the 0-20 cm mineral soil, yielding a monotonically increasing, curvilinear relation. To apply this relation to estimate the TEC distribution throughout the region, 211 map units of the state soils geographic database (STATSGO) were used. The SOC in the 0-20 cm mineral soil of the map units was consistent with values from previously measured pedons distributed throughout the region. Resampling of 13 second-growth forests 25 years after initial sampling indicated no regional change in mineral SOC, and supported the use of a static state soils map. The SOC spatial distribution combined with the quantitative old-growth TEC-SOC relation yielded an estimate of potential TEC storage throughout the region under the hypothetical condition of old-growth forest coverage. The area-weighted TEC was 760 Mg C ha-1. This is approximately 100 Mg C ha-1 more than a previous estimate based on a coarser resolution of six physiographic provinces, and 400 Mg C ha-1 more than current regional stores. The map of potential TEC may be useful in forecasting regional C dynamics and in land-management decisions related to C sequestration.