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March 21, 2003
Fire Frequency Determines Forest Carbon Storage
Scientists studying trees ranging from saplings to 130 years old in
Canada's northern forests have discovered that the period since a fire
last swept through an area determines how much carbon the forest can
store. Twenty to forty year old stands absorb more carbon than those
70 years old and older, despite being smaller and having less biomass
or plant material.
Boreal or northern forests account for close to 25 percent of total
carbon stored in vegetation and soils in the Earth's biosphere. Wildfires
burn down individual areas every 40 to 250 years and are an important
part of this ecosystem. Whether or not these forests are likely to lower
or raise levels of carbon dioxide in the atmosphere depends on how these
carbon reserves respond to, and recover from, both climate change and
disturbances such as wildfires.
NASA funded part of this study under its Earth Science Enterprise (ESE),
whose mission is to understand and protect our home planet. Earth Science
in NASA seeks to understand trends in land cover and land use, such as
forest fires, that drive global climate. Another Earth Science program
objective is to understand the Earth system's response to natural and
human-induced changes, and effects on global carbon cycle.
Marcy Litvak, plant ecologist at the University of Texas at Austin and
lead author of the study that appeared in a recent issue of the Journal
of Geophysical Research - Atmospheres, said that the ability of tree stands to store carbon changes
as they regenerate from fire. Forests will store more or less carbon
depending on the dominant tree species, the amount of moss cover, and
changes in forest structure due to fire. Those factors determine how
much total carbon is exchanged with the atmosphere.
Carbon is transferred from the atmosphere to the forest through the
process of photosynthesis. Carbon is returned to the atmosphere through
the process of respiration as soil microorganisms decompose dead organic
matter, and trees and mosses metabolize the products of photosynthesis.
It is the balance between these two processes, taking in carbon during
photosynthesis and "exhaling" carbon through respiration, that determines
how much carbon is stored in the forest.
Between 1999-2000, Litvak and her colleagues, Scott Miller and Michael
Goulden of the University of California, Irvine, and Steve Wofsy of Harvard
University, used solar-powered anemometers and infrared gas analyzers
mounted on towers to monitor carbon emissions over five black spruce
stands in Manitoba, Canada. These stands ranged in age from 11 to 130
years old. Results indicate that the ability to store carbon is almost
zero in the 11 year-old stand, increases to a maximum in the 36 year-old
stand, then gradually falls back down to zero in the 130-year old stand.
They concluded that most of the net carbon absorption appears to take
place from 20-50 years after a fire.
"Seedlings of Aspen, Jack Pine, and Black Spruce all regenerate simultaneously
following wildfire in areas once dominated by mature black spruce forests
in this region of Manitoba. Aspen and Jack Pine tend to dominate in
young stands where light is not limited.�"Black Spruce grow the slowest,
but eventually out-compete the Aspen and Jack Pine by blocking the sunlight
available to these species. By 70 years following a burn, these forests
are dominated by Black Spruce once again," Litvak said.
Stands less than 20 years old store less carbon than older trees because
they lack sufficient leaf area for rapid carbon accumulation. Carbon
storage is highest in stands 20-50 years old that are dominated by rapidly
growing aspen trees that take up carbon at higher rates than black spruce
and jack pine trees.
"Stands [of trees] older than 70 years are dominated by black spruce
trees and thick moss cover that 'exhale and inhale' equal amounts of
carbon. That means stands older than 70 years are in near carbon balance
with the atmosphere," she said.
Knowing the rate at which trees respire will help scientists to better
estimate the trees' contributions to the global carbon cycle. This is
especially important because of the changing climate. "Increased fire
frequency, as predicted from global warming scenarios, has the potential
to significantly impact the contribution boreal forests make to the global
carbon cycle," Miller said.
NASA data from the Boreal Ecosystem-Atmosphere Study (BOREAS) was also
used in the study. BOREAS was a large-scale international experiment
in the northern forests of Canada between 1993 and 1996, whose goal was
to improve understanding of interactions between the boreal forest and
the atmosphere, and clarify their roles in global change.
This work was supported by NASA, the National Science Foundation, and
U.S. Department of Energy.
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Contacts:
Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
Barbra Rodriguez
The University of Texas at Austin
(Phone: 512/232-0675
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 Canadian Land Cover Map
/ Study Areas
The BOREAS-follow on Project to determine how carbon
storage in boreal forests change in response to wildfire was set in the
northern edge of the Canadian boreal forest in Manitoba, Canada, the
former Northern Study Area (NSA) in Thompson Manitoba from BOREAS. CREDIT:
Canadian Model Forest Program
 Instrument Towers in Manitoba,
Canada
Litvak and her colleagues used instruments positioned
at various heights on towers like this one to measure how the transfer
of carbon dioxide between the forest and the atmosphere changes with
forest age. The towers also measured the transfer of heat and moisture
and radiation. CREDIT: Marcy Litvak, University of Texas at Austin; Michael
Goulden, University of California, Irvine
 Measuring Carbon from Boreal
Forest Trees
Scientists during the BOREAS follow-up project used
probes like the one pictured above to measure the amount of carbon released
from the forest tree trunks into the atmosphere. They also monitored
air temperature and stem temperature to better understand factors that
influence carbon exchange. Once the processes are understood, computer
models can be developed that can predict future change. CREDIT: Marcy
Litvak, University of Texas at Austin
 The Boreal Forest Area
The Boreal forest, rich in diversity of vegetation and
wildlife, covers over one-tenth of the Earth's northern land surface,
including one-third of Canada. Boreal forest (depicted in green) covers
approximately 28% of the North American continent, north of Mexico. Today's
boreal forests comprise 8% of global forestland.CREDIT: BOREAS project
(From Hare and Ritchie (1972))
 Northern Study Area
This satellite image of the Northern Study Area, Manitoba,
Canada was taken by the Landsat satellite on July 25, 1990. The bodies
of water can be seen in blue. The landscape is a mosaic of different
aged stands that are in various stages of recovery from wildfire. Each
of the colored patches corresponds to a different aged stand. The years
corresponding to the areas indicate the last year of forest fire. CREDIT:
NASA/USGS
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