Do Boreal Forests Need Fire Disturbance to Maintain Productivity?

Fire is considered as a major driver of ecosystem processes of the boreal forest with important effects on soil and forest productivity. When the interval between successive forest fires is long, a thick organic layer can develop and eventually interfere with processes involved in tree nutrient uptake. We thus hypothesized that the organic layer of well-drained boreal stands increases with time since last fire and that thick organic layers are associated with low values of soil temperature, nutrient availability, and site productivity. This was tested on a chronosequence composed of 90 boreal stands ranging from 1 to more than 2000 years after fire within which we measured organic layer thickness (OLT), mineral soil and foliage nutrient concentrations, soil temperature, ground cover of Sphagnum sp. and Ericaceae sp., leaf area index, aboveground biomass production, and growth efficiency index (GEI). The OLT increased during the first 64 years after fire but stayed statistically constant thereafter. This initial increase in OLT was accompanied by an increase in the C/N ratio and decreases in soil temperature, foliar N, and GEI. The absence of a significant decrease in productivity from 80 to 2000 years post-fire suggests that these characteristics reach a steady state early in the chronosequence that persists in the absence of major disturbances or changes in site conditions. These results imply that management practices may not be necessary to maintain boreal forest productivity in the absence of fire on well-drained sites.