Soils of temperate rainforests of the North American Pacific Coast

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Temperate rainforests have high conservation and natural resource value, but the soils of this bioregion have not previously been studied as a unit. Here we examine the soils of North America's Pacific coastal temperate rainforests, utilizing databases from the United States Natural Resources Conservation Service and the Canadian Centre for Land and Biological Resources Research to (i) identify the soil taxa, (ii) evaluate the soil properties, and (iii) compare soils in temperate and tropical rainforests. There are strong climate gradients within these temperate rainforests, with the mean temperature declining from 11.7°C to 6.1°C and the mean annual precipitation increasing from 1500mm to around 3000mm from northern California (CA) to northwestern British Columbia (BC) and southeastern Alaska (AK). There is also high pedodiversity in this region, with soils representing 8 orders and 31 suborders, and, in the US portion, 65 great-groups, 142 subgroups, and 482 soil series. Twenty-six percent of described soil series are endemic to temperate rainforests in the US portion of the region, with the proportion declining with latitude. Dominant soil suborders vary along the latitudinal gradient from Humults–Udalfs/Ustalfs–Udepts–Udults in CA, to Udands–Udepts–Udands Humults in western Oregon (OR) and Washington (WA), to Orthods–Folists in BC and Cryods–Saprists in AK. The dominant diagnostic horizons are ochric/argillic (CA), umbric/cambic (OR, WA), and albic–histic/spodic (BC, AK). Whereas soils in CA, BC, and AK tend to have a mixed mineralogy, those in northern OR and WA commonly are derived from volcaniclastic materials and have a ferrihydritic or isotic mineralogy. Soils in this region are generally deep, hold abundant moisture, are not subject to deep-freezing, and are enriched in extractable Fe and Al. Organic C and total N contents are high overall, but also variable, with right-skewed distributions. Compared to tropical rainforest soils in the Pacific Basin, Pacific temperate rainforest have greater weatherable minerals, cation-exchange capacities, soil organic C, and total exchangeable base cations. However, soils of both bioregions tend to be deep, acidic Al-saturated, and can have large N reservoirs. This investigation provides a foundation for a more unified understanding of the soils of a globally significant bioregion.