Organic topsoil thickness as a proxy to assess spatial variability of carbon stocks in boreal pine and spruce forest floors

Boreal forests are estimated to store between 20–40% of the terrestrial carbon (C) stock. Large, but variable proportions of the boreal forest C stock appear in the form of soil organic carbon (SOC) in the forest floor topsoil, i.e. the O or FLH soil horizons. This proportion of the soil C stock is the most dynamic part of the soil C because it responds rapidly on both climate change and forest disturbance via e.g. wildfire or silvicultural management practices. Boreal forest floor SOC stocks are characterized by a large degree of size variability across a broad range of spatial scales, which implies that large numbers of samples are needed to obtain representative stock estimates. However, high costs and labor efforts are associated with such estimates because of soil sampling in the field and laboratory C analyses. Here, we show that a simple measure of topsoil thickness is a useful and effort saving proxy for SOC, and by using thickness as proxy, we give spatially explicit estimates of SOC stocks and show that they are highly variable in size across forest floors of mature boreal Scots pine- and Norway spruce forest landscapes. Soil sampling strategies are also briefly discussed.