How the function of carbon sequestration in boreal forests changes by permafrost table subside and re-rising process after forest fire

(USA side)

International Arctic Research Center, University of Alaska, Fairbanks.

(Canadian side)

Wood Buffalo National Park, Parks Canada

Aurora Research Institute, Northwest Territories, Canada

(Japan side)

Niigata University

Shinshu University

FY 2022-2024 Grant-in-Aid for Scientific Research (KAKENHI)

MATSUURA, Yojiro

Permafrost exists under ca. 70 percent of boreal forest region. Most of former researches were performed within permafrost-free boreal forests. Recent years, frequent large forest fires in boreal region reduce not only biomass storage in forests but affect stand structure of regenerated forests due to permafrost table subside and re-rising process. That basic knowledge of specific in permafrost-affected forests is necessary to predict carbon storage of boreal forests under changing climate.

We clarify the forest regeneration process with measurements of biomass accumulation, stand structure, and permafrost table depth. We also clarify the inflection point with growth shifting events and speculate the trajectory of carbon sequestration function.

We set up a couple of intensive research sites in Caribou-Poker Creek Research Watershed and Poker Flat Research Range, near Fairbanks, Interior Alaska. Additional sites will be set up in Wood Buffalo National Park, NWT, Canada. Permafrost condition of these two regions are different. Stand ages of those research sites vary from 4-year to ca. 100-yr after fires. Biomass accumulation, T/R ratio, permafrost depth will be measured. Stress-affected tree growth stage will be detected using stable isotope measurements.

We measured permafrost table depth and aboveground biomass approximately 20 years after a forest fire on a north-facing slope in interior Alaska, where black spruce had previously been dominant. The results showed that the permafrost table depth, which was about 160 cm one year after the fire, remained at 150 ± 35 cm even after 20 years, indicating that there was no significant permafrost recovery. In contrast, aboveground biomass had reached 33 ton/ha (approximately 16 ton carbon equivalent), corresponding to about 60% of that in old-growth stands that have developed over roughly 100 years since the forest fire. Based on census data from 2022 and 2024, the annual rate of increase in aboveground biomass was estimated as 4 ton/ha (approximately 2 ton carbon equivalent), suggesting a rapid recovery in recent years.

In old-growth stands approximately 100 years post-fire, we collected tree-ring cores from black spruce stems to estimate radial growth rates and assess potential water stress over the past 80 to 170 years. No significant variations were observed in the stable carbon and oxygen isotope ratios in the rings, suggesting that no severe water stress had occurred. Nevertheless, radial growth rates showed clear long-term variations (see figure below). Near the bottom of the slope where the permafrost table is shallow, growth began to decline around 40 years after the forest fire, and similarly, growth was poor from that time onward in the lower slope area; in contrast, on the upper slope where the permafrost table is deep, relatively good growth has been maintained even 100 years after the fire.

Fig. Long-term variation in radial growth increment (stem cross-sectional area increase) of black spruce growing on permafrost