Impacts of a changing fire frequency on soil carbon stocks in interior Alaskan boreal forests Elizabeth E. Hoy
Fire Frequency
Patterns of Burned Area and Fire Frequency
Soil Organic Layer and Fire Frequency
North American boreal forests have seen increases in the frequency of large fire years and late season burning, as well as an increase in the total area burned each year. These fire-regime changes are leading to a shorter fire return interval (FRI) in the Alaskan interior, and shorter Fire-free interval (FFI) between individual fire events. While some studies have analyzed ecosystem changes due to fire frequency through field studies, more research can be done using remote sensing and GIS to analyze the patterns of burned area relating to fire frequency in interior Alaska.
Current FFI Across Interior Alaska
Much of the carbon stored in North American boreal forests is in the deep organic soils of black spruce (Picea mariana) forests. In Alaska alone, these forests cover 26 x 106 hectares and the deep organic soil layers located here store greater than eighty percent of the region’s non-peatland terrestrial carbon (> 1 Gt). Recent studies have shown that an average of greater than 30 t C ha-1 is released from the burning of surface organic matter in black spruce forests, which is ten times the amount released from the burning of above ground biomass in this ecosystem. Immature Unburned Stand
Immature Burned Stand
Vegetation Based on the National Land Cover Dataset 2001 across the interior of Alaska, much of this region is considered to be a form of shrub or scrub vegetation (37%) while only 28% is considered to be evergreen. This is surprising as black spruce forests represent >70% of the forested areas that burn in Alaska. Further analysis at the ecoregion scale could alter the percent composition, leading to a higher evergreen percentage.
Fire-Free Percentage Interval (yr) of Interior < 10 12.6 10-19 5.6 20-29 2.2 30-39 3.4 40-49 3.2 50-59 6.1 ≥ 60
The percentage of the evergreen cover type at FRI 80 years old) from Kasischke et al. (2008) showed (see chart above): 1) mature stands had deeper organic soils prior to burning than immature black spruce stands; 2) more frequent burning resulted in the loss of less organic material during burning (8.4 cm of soil reduction in immature burned stands compared with 12.7 cm reduced in mature stands) and 3) less organic matter remained after the fire in an immature burned stand (2.5 cm in immature burned stands compared with 10.1 cm in mature burned stands).
North facing slopes are cooler and wetter than their south facing counterparts. These north facing slopes can burn less completely during fire, allowing for an increased buildup of soil organic carbon. In warm and dry summer conditions, these sites can become more vulnerable to deep burning. The analysis of aspect in evergreen forests did not yield large differences across different FRIs, however North facing slopes do represent a considerable portion of the forested area. And based on the field measurements, frequent reburning could lead to less build-up of soil carbon on some of these slopes. Site drainage can also be an important factor controlling the depth of burning in the soil organic layer. However, results from the current study do not show strong differences in the percent of poorly and moderately drained areas as compared with well drained areas across different FFIs.
Sources Kasischke et al. (2008) Evaluation of the composite burn index for assessing fire severity in Alaskan black spruce forests. International Journal of Wildland Fire 17, 515-526.
Acknowledgements The research for this study was supported through NASA Grant number NNX09AO09H.
Contact Information E.Hoy:
[email protected]
