While devastating to a mature closed canopy forest, the Spruce Budworm actually plays an important role in carbon cycling and nutrient availability in forest soils. The closed canopy of the forest blocks most of the sunlight from reaching the forest floor. This creates cool and moist conditions that favor the growth of moss in the understorey. The forest floor of a mature Balsam Fir stand is often completely covered by a dense carpet of moss. The soil temperature is kept low, both by the shady conditions under the canopy and the thermal insulating properties of the moss cover The low soil temperatures combined with the acidic needle litter from the coniferous trees suppress decomposition by soil microbes. Carbon based litter accumulates on the forest floor faster than it can be broken down. The result is the net build-up of carbon rich soil over time. The Boreal forest is considered one of the world's major carbon sinks for this reason. Net primary production is severely limited by the slow rate of decomposition.

The build-up of litter on the forest floor further insulates the underlying soil and further prevents warming and decomposition. This situation is known as a self-perpetuating cycle, and could continue for hundreds of years in the absence of disturbance.