Western red cedar’s natural durability is attributed to a unique set of heartwood extractives. These can be lost in service due to leaching and biological activity. The microorganisms responsible for extractive depletion from wood in service are not well understood, and it is not known whether such loss of extractives has a substantial impact in reduced durability. The present work exposed western red cedar blocks in ground contact and above ground in a high humidity environment for one year. Trichoderma spp., Penicillium spp. and zygomycetes were found to initially colonise the blocks. This coincided with significant losses of extractives. Biological degradation is the most likely cause of extractive depletion in the ground contact and above-ground exposures in this experiment. This degradation corresponds to reduced decay resistance to at least one brown rot fungus.