Fire and site type effects on the long-term carbon and nitrogen balance in pristine Siberian Scots pine forests

Effects of fire and site type on carbon (C) and nitrogen (N) balances were determined by following the change of total and component C and N pools along four chronosequences of fire- prone Siberian Scots pine ecosystems. These differed in the mean return interval of surface fires (unburned - moderately burned, 40 years - heavily burned, 25 years) and site quality (lichen versus Vaccinium site type). Of the Vaccinium site type (higher site quality) only a moderately burned chronosequence was studied. A total of 22 even-aged stands were investigated with stand ages ranging from 2 to 383 years. The C balance was dominated by the opposing dynamics of coarse woody debris (CWD) and biomass and could be divided into three phases: (1) Young stands (up to 40 years) acted as a net source for C of 6-10 mol C m(-2) year(-1) because the previous generation CWD pool originating from stand-replacing crown fires decayed much faster than biomass increased. During this period the C pool in the unburned lichen type chronosequence decreased from 807 to 480 mol C m(-2). (2) Middle aged stands (40-100 years) being in a stage of maximum biomass accumulation were a net sink of 8-10 mol C m(-2) year(-1). (3)Mature stands (100 to > 350 years) continued to sequester C at a lower rate (0.8-2.5mol C m(-2) year(-1)). Differences in the rates of C sequestration during the two later phases could be explained by the complex interaction between surface fire regime and site type. Recurrent surface fires resulted in enhanced mortality and regularly redistributed C from the living to the CWD pool thereby lowering the rate of C sequestration. Site quality determined the potential to recover from disturbance by fire events. Differences in site type did not correlate with soil and total ecosystem N pool size. However, the N status of needles as well as the N pool of physiologically active tissue was highest in the stands of the Vaccinium type. The 'woody' C pool (biomass + CWD) was sensitive to differences in surface fire regime and site type. It was lowest in the heavily burned lichen type chronosequence (297 +/- 108 mol C m(-2)), intermediate in the unburned and moderately burned lichen type chronosequence (571 +/- 179 mol C m(-2)) and highest in the moderately burned Vaccinium type chronosequence (810 +/- 334 mol C m(-2)). In contrast, the total soil C pool (organic plus mineral layer down to a depth of 25 cm) was independent of stand age, surface fire regime and site type and fluctuated around a value of 250 mol C m(-2). The organic layer C pool oscillated in response to recurring surface fires and its C pool was dependent on time since fire increasing at a rate of about 1.5 mol C m(-2) year(-1) during the first 40 years and then reaching a plateau of 170 mol C m(-2). The total ecosystem N pool was 7.4 +/- 1.5 mol N m(-2) on average of which only 25 % were stored in biomass or coarse woody debris. Total ecosystem N was independent of stand age, surface fire regime and site type. No correlation was found between total ecosystem C and N pools. Average total ecosystem C:N ratio was 114 +/- 35 mol C mol N-1. A conceptual model illustrating how changes in the regime of stand-replacing crown fires and recurrent surface fires and changes in site quality interact in determining the long-term C balance in Siberian Scots pine forests is presented