Recently,in many areas of Russia, the frequency and extent of forest fires originatingfrom anthropogenic causes has increased at a higher rate than those originatingfrom natural causes.

Renewal ofpine forests is ecologically dependent on fires, but if fires become too frequentthey can disrupt the equilibrium and sustainability of these ecosystems as weknow them. Field studies of the effects of fire are challenging because of the inherentheterogeneity of forest ecosystems and because of the heterogeneous effect offire on recovery of vegetation. As an alternative to costly and complex fieldstudies mathematical models can be used as a tool to assessthe complex dynamics of natural ecosystems as they recover after fire. Soils are a fundamental part of terrestrial ecosystemsand the accumulationor dynamics of soil organic matter (SOM) is often used as a criterion to assess sustainabilityin forest ecosystems. The aim of this study was to apply the ecosystem model EFIMODto analyse the effect of surface fires on soil degradation and its feedback ontree productivity in Scots pine forests on different soil types in Russia: HaplicPodzols (Mor humus form) in the Leningrad region, and Psamment Entisols (Moderhumus form) of the fragmented steppe in the Samara region. Simulation of the cumulativeeffects of repeated fire cycles over 140-years showed that one firedid not affect growing stock but decreased SOM by about 10% at both sites, andthat three fires reduced the growing stock by 30% on the Haplic Podzols and 9%on the Psamment Entisols and decreased SOM by about 30% on both sites. Forest firesled to the significant loss of soil carbon (C), as well as nitrogen (N) whichis a principal limiting factor in forest ecosystems of boreal and temperate ecozones.The effect of repeated fire cycles on land degradation is similar to that ofsoil erosion, through the loss of soil C and N. From a silviculturalperspective, the effect of fires is generally considered positive (e.g.,promotion of seedlings establishment, removing of fuel from soil surface),however, this study concluded that repeated fire cycles can have a negativeeffect on forest ecosystems by reducing long-term soil productivity.