Research
Carbon Balance of Arctic Plants and
Ecosystems(and the relation with global warming)
“Because of the large carbon stocks present
in northern soils and the presumed sensitivity of soil carbon
accumulation or loss to climate change, northern ecosystems may be
particularly important to global carbon balance in the future. Between 250 and 455 petagrams(1 Pg = 10^15 g)
of carbon are present in the permafrost and seasonally thawed soil
layers. This amount is about one-third the
total world pool of soil carbon. Warmer
soils could deepen the active layer and lead to thermokarst erosion and
the eventual loss of permafrost over much of the Arctic and the boreal
forest. These changes could in turn alter
arctic hydrology, drying the upper soil layers and increasing
decomposition rates. As a result, much of
the carbon now stored in the active soil layer and permafrost could be
released to the atmosphere, thereby increasing CO2 emissions and
exacerbating CO2-induced warming. Alternatively,
elevated atmospheric CO2 and changed nutrient availabilities could
change plant communities and vegetation. New
communities might be taller and have higher rates of primary
productivity than does extant vegetation. The
net result could be higher primary productivity, increased carbon
storage in plant biomass, and a negative feedback on global atmospheric
CO2.
Arctic and boreal forest ecosystems are
unique in their potentially positive and negative response to elevated
CO2 and associated climate change. Few
other ecosystems have the capacity for massive continuing, long-term
carbon accumulation that permafrost-dominated northern ecosystems do,
and few systems are as sensitive to global warming.
The uncertain effects of global change on arctic and
boreal forest carbon balance make the study of northern ecosystem
response a key to understanding and predicting future global
atmospheric CO2 patterns. Focusing on
processes likely to change with a doubling of CO2 over the next 50-60
years, it is important to discuss the major controls on carbon cycling
in arctic ecosystems, and the likely effects of elevated atmospheric
CO2 and concomitant climate change on carbon storage.”
(Source:
CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic
Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991.
P. 139-140)
Current Net ecosystem carbon storage and flux
Carbon pools and rates of carbon accumulation
vary, depending on vegetation type and environmental conditions.
More than 90% of the carbon in arctic
ecosystems is located in soils, with even higher percentages (98%) in
soils of northern peatlands. In upland
boreal forest, in contrast, only about 55% of the ecosystem carbon is
found in the soil. Not only is the
proportion of soil carbon substantial, but so are the absolute amounts. Arctic tundra has 55 Pg of carbon stored
as soil organic matter in the A horizon, compared with 87.5 Pg in
non-peatland boreal forest and 122 Pg in forest peatlands and
Gorham’s(1991) estimates of 455 Pg C are considerably higher. Tussock and wet sedge tundra soils account for
the bulk of circumpolar tundra carbon stores because they have large
amounts of carbon per square meter and cover large areas.
Although per unit area, carbon storage in polar
semideserts is only about one-half that in the wet sedge tundra, the
greater extent of these semideserts results in carbon stores
approaching those of wet sedge tundra.
For these large stores of soil organic matter
to have accumulated in northern ecosystems, production must have
exceeded decomposition at some time in the past. Recent
estimates (Post, 1990; Gorham, 1991) indicate that northern ecosystems
still constitute a small net sink for atmospheric carbon; current
accumulation rates, however, are difficult to assess.
Because the rates vary with conditions and ecosystem
type, soil carbon accumulation is positive in some areas and negative
in others. The overall balance is still
uncertain.
(Source:
CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic
Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991)
Global warming, vegetation changes, and the nutrient cycles
Under global warming, temperature and
precipitation changes in arctic regions are occurring already. In much of Alaska, approximately 1C per decade
of warming has been observed. Vegetation
changes have been recorded in Alaskan tussock tundra over the past
decade(Chapin et al. 1995), and these changes are expected to have
important feedbacks on the region’s biogeochemical cycles through
altered rates of C exchange between biosphere and atmosphere, and
changes in the region’s energy balance.
The arctic tundra ecosystem consists of both
C sinks and sources. Detailed modeling
analysis of arctic biogeochemistry (McKane et al. 1997) suggest that
the source/sink strength of tundra depends on changes in photosynthesis
that result from the partitioning of nitrogen(N0 between vegetation and
soils, and on changes in soil moisture, which affect soil respiration
rates.
“Shaver et al. (1996) from their study on a
toposequence in arctic Alaska suggested that leaf area might be a
reasonably accurate predictor of productivity at the landscape level in
the arctic.”
“The limitations of photosynthesis by low
temperatures and low solar radiation has been clearly demonstrated and
simulated for arctic vascular plants(Miller et al. 1976; Limbach et al.
1982; Tenhunen et al. 1994) But, because of the saturating response of
photosynthesis to light, day length is an additional important limiting
factor to productivity.” “While moisture stress has no impact on
productivity at the mean climate conditions selected, photosynthesis is
vulnerable to changes in soil water potential and hydraulic constraints
on water transport.” A reduction in soil
water potential can cause stomatal closure, to balance transpiration
against reduced soil water intake. Lower
hydraulic conductance can lead to stomatal closure and reduce gross
primary productivity as rates of water supply are constrained by the
characteristics of the vascular system. Photosynthesis
in arctic ecosystems is linked closely to the hydrological cycle. C loss from ecosystems is also linked to soil
moisture (Oechel et al. 1993).
In order to understand the ANWR ecosystem, it is also
necessary to investigate the energy and nutrient cycles.
The carbon balance of the ecosystem have been highly
influenced by global climate changes and CO2 content changes.
The arctic contains 11% of the world’s organic matter pool,
and within the arctic tundra ecosystems, there are both carbon sinks
and carbon sources. Vegetation
changes in the Alaskan tussock tundra over the past decade has
brought about important feedbacks on the region’s biogeochemical
cycles, through altered rates of carbon and energy exchange between
biosphere and atmosphere. Modeling
analysis suggests that the source/sink strength of tundra depends on
changes in photosynthesis that result from the partitioning of
nitrogen between vegetation and soils, and on changes in soil
moisture, which affect soil respiration rates.
All of these factors may be affected by machine and human
activity in the region and disturbances in the permafrost.
Nutrient cycling and fertilization studies in arctic
ecosystems show that plant growth is strongly limited by nutrient
availability. Such
activity depends highly on decomposition, nitrogen mineralization,
phosphorous availability, and controls on carbon and nutrient
cycles, which in turn depend on temperature, moisture,
decomposability of litter inputs, depth of thaw etc.