Carbon Balance of Arctic Plants and Ecosystems

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.

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.

A. Wet Coastal Tundra

Many studies have indicated that wet and moist tundra and northern bogs may be net carbon sinks for atmospheric CO2 under present conditions. AT current accumulation rates for wet coastal tundra, this soil carbon pool of 14 kg m^-2 would have required about 500 years to develop. Rates of soil decomposition and accumulation change as the soil organic layer develops, however, and decomposition slows considerable as organic matter is buried more deeply. These factors, combined with the high degree of spatial heterogeneity in the content of soil organic matter and the small database, make the available estimates only rough approximations at best

¡K Given the time spans of the thaw-lake cycle, 300-500 years for carbon accumulation seems realistic.(at Barrow, Alaska)

Wet coastal tundra ecosystems have the potential for long-term carbon sequestering in the soil. As the permafrost table moves upward because of the insulating effects of the graminoid-moss vegetation and its accumulation peat, decomposer activity decreases dramatically below depths of 25-30 cm, conserving soil carbon indefinitely at depth and in the permafrost. Buried peat horizons are commonly found embedded deep in the permafrost in the coastal tundra at Barrow. Such burial is due either to a combination of thaw-lake cycle activity, wind deposition, solifluction, and frost heaving or to the independent influence of any one of these factors.

Although terrestrial surfaces seem to be accumulating carbon, carbon is nevertheless being exported to aquatic ecosystems. Tundra ponds and streams release CO2 to the atmosphere, and peat is exported to the oceans. The net carbon balance of the land alone- including export of organic matter to streams, ponds and marine ecosystems ¡V is thus uncertain. If one accounts for the material exported to aquatic systems, which may be substantial, then the overall carbon budget of the wet coastal tundra may be roughly in balance.

B. Tussock Tundra

The current carbon balance of tussock tundra is more problematic; recent changes in climate may have significantly altered carbon storage in this ecosystem type. Alaska¡¦s tussock tundra is better drained than wet coastal tundra and thus has improved soil aeration, higher overall decomposition rates, and less carbon storage.

Recent information indicates that historic and long-term carbon accumulation rates may have decreased in the last few decades at Toolik Lake and possibly elsewhere.

Temperature profiles of bore holes in the permafrost on Alaska¡¦s north slope indicate that air temperatures have increased 2-4 C in the last century, possibly within the last few decades. Thus warming of surface layers over the last few decades may be related to the apparently recent loss of carbon from tussock tundra ecosystems. At this time, neither the regional extent of carbon loss, nor the reduction in carbon accumulation rates is the existence or magnitude of recent corresponding changes at the coast.

C. Taiga Little information exists on whole-system carbon budgets of taiga vegetation. Some data are available on soil respiration, rates of peat accumulation, and primary productivity. Flux rates of CO2 form boreal forest soils average about 165 g C m-2 yr-1. The seasonal pattern of atmospheric CO2 concentrations in the Northern Hemisphere may partially reflect seasonal patterns of taiga photosynthesis and respiration. Increased productivity in the taiga, resulting form atmospheric increases in CO2, may help explain the higher seasonal amplitude of atmospheric CO2 recently recorded at Barrow and elsewhere. Soil CO2 loss from microbial activity and root respiration was also measured through an entire season. The results show a positive carbon balance for the Carex mat that was 13 times that of the old black spruce bog forest. This disparity reflects the wet, nutrient-rich habitat at the edge of a thaw pond, in contrast to the cold, relatively nutrient-poor and drier conditions in the forest, which permits high productivity and carbon capture by Sphagnum and the graminoids. Carbon flux in the ¡§climax¡¨ bog forest is near a steady state, whereas the successional sedge community has a relatively high rate of net carbon capture. After reviewing the paleontological, macrofossil, and carbon balance evidence for central Alaska, in particular for the Fairbanks bog, we conclude that this region has bee a carbon sink throughout the last 5000 years. It remains a carbon sink but with different carbon balances for the various vegetational communities resulting from the thermokarst and fire cycles inherent in the subarctic environment

Source: CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991

Last Updated: 10/22/2003