Boreal Forest Regrowth Dynamics

   

Donnelly Flats Fire, near Delta Junction, Alaska. June 13-20th 1999. Photo courtesy of Tom Lucas, Delta News Web.

    

Introduction

One of the major issues in understanding how climate change will influence carbon storage in high northern hemisphere terrestrial ecosystems is the role of disturbance. Natural disturbance, through its effects on species composition, diversity, and landscape characteristics can initiate dramatic changes in ecosystem function, including carbon cycling. In boreal North America, fire is the dominant type of disturbance.

Fire is an important and natural component of boreal regions, and is a fundamental control on many ecosystem processes. Over the past 30+ years, however, a gradual increase in both the frequency and intensity of fires has been observed (see figure). From the 1960's to the 1990's, the annual area burned has increased from an average of 1.4 to 3.1 million hectares per year. Continued warming and drying of the boreal region will very likely exacerbate this trend.

One of the primary goals of the WHRC boreal North America program is to determine the impact of fire on boreal carbon cycling through a broader understanding of how changes in the boreal fire system alter the fate of stored carbon and sequestration through forest regrowth.

Patterns of annual area burned in the North American boreal forest illustrating a continuous rise in fire activity since the early 1970s (Kasischke 1999) - Select image for larger version.

The Effects of Fire

Fire influences carbon cycling both directly and indirectly. Stand replacement fires, a common type of fire within boreal regions, result in the death and partial burning of most overstory trees, complete burning of understory vegetation, and partial burning of mosses, lichens, litter, and organic soil. Fires with a longer or more intense smoldering phase are capable of burning the carbon-rich organic layer to significant depths, often down to the mineral soil. The result of this burning is a large, almost instantaneous flux of carbon to the atmosphere. It is estimated that anywhere from 10 to 200 tonnes of carbon per hectare can be released to the atmosphere, depending on the length and intensity of the burn.

Boreal Fire Locations between 1980 - 1994

Indirectly, fire disturbance alters the carbon cycle and influences climate through changes in albedo (surface reflectivity), succession, and CO2 sequestration during regrowth. Directly following a burn, the radiative energy balance of the region changes due to reduced forest cover and decreases in albedo.

 

1999 Burn illustrating post-fire condition and charcoal covered sediments.

   

In simple terms, biomass burning produces a large amount of charcoal which darkens the ground surface, and significantly increases the amount of solar radiation absorbed. Increased absorption of incident radiation warms the surface and promotes higher rates of decomposition and increased sensible heat fluxes to the overlying atmosphere.

The elevated rates of decomposition typically persist for a number of years, resulting in a net source of carbon to the atmosphere. As burn scars age and succession and forest regrowth ensue, rates of net primary production (NPP) rise, balancing the flux of carbon to the atmosphere from decomposition. Ultimately boreal carbon source/sink dynamics are a balance of NPP and decomposition (microbial respiration) rates, and the age structure of the forest, which is determined largely by fire disturbance regimes. Our work focuses on the use of satellite remote sensing and carbon modeling to map and monitor the balance of these ecosystem processes.