Bonanza Creek LTER

Fire Has Long-term Affects on Boreal Forest
Productivity

Severe, stand-replacing fires affect large areas of northern temperate and boreal forests, potentially modifying ecosystem function for decades after their occurrence. Because these fires occur over large extents, and in areas where plant production is limited by nitrogen (N) availability, the effect of fire on N cycling may be important for long-term ecosystem productivity. Here, we review what is known about postfire N cycling in northern temperate and boreal forests experiencing stand-replacing fires. We then build upon existing literature to identify the most important mechanisms that control postfire N availability in systems experiencing severe, stand-replacing fires compared to fires of lower severity. These mechanisms include changes in abiotic conditions caused by the opening of the canopy (e.g., decreased LAI, increased solar radiation), changes in ground layer
quantity and quality (e.g., nutrient release, permafrost levels), and post-fire plant and microbial adaptations affecting N fixation and N uptake (e.g., serotiny, germination cues). Based on the available literature, these mechanisms appear to affect N inputs, internal N cycling, and N outputs in various ways, indicating that severe fire systems are variable across time and space due to complex interactions between post-fire abiotic and biotic factors. Future experimental work should be focused on understanding these mechanisms and their variability across the landscape. This research will be published in the journal Ecosystems: Smithwick, E., M. Turner, M. Mack and F.S. Chapin III. 2004. Post-fire soil N cycling in northern conifer forests affected by severe, stand-replacing wildfires (In Press).

Soil Carbon Loss Greater in Disturbed Boreal Forests

The ability to store carbon is an important ecosystem function-especially when humans produce excess carbon by burning fossil fuels. At the same time, climate warming is more pronounced at high latitudes than in other regions of the globe, which could make northern regions more attractive for agriculture and increase rates of forest clearing in coming decades. Using paired forest-field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Mean C stocks in agricultural soils were 44 or 8.3 kg m-2 lower than C stocks in original forests. This suggests that soil in boreal regions are more vulnerable to C losses from land-use change than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences suggested sometimes large soil C losses during the first decade following deforestation with net C storage during later decades. Net ecosystem C gain of 2.1 kg m-2 was observed in a 60 years old grassland, which probably resulted from a combination of large C inputs from belowground biomass and low C losses due to a small original forest C stock and low tillage frequency. Reductions in soil N stocks caused by land-use change were smaller than reductions in C stocks (34% or 0.31 kg m-2), resulting in lower C/N ratios in field compared to forest mineral soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which indicates that decomposition rates are more sensitive in forest than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes widespread in the boreal region, the resulting C losses (51% or 11.2 kg m-2 at the ecosystem level in this study) will positively feedback to climatic warming and additional land-use change. However, by selecting relatively C poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.This research will be published in the journal Global Change Biology: Gruenzweig, J.M., S.D. Sparrow and F.S. Chapin III. 2004. The impact of agricultural land-use change on carbon storage in boreal forests (In Press).