The effects of climate charge on land—atmosphere feedbacks in arctic tundra regions

REVIEWS Stewart, C.S., eds), Academic Press (in press) 37 Sogin, M.L. (1991) Curr Opin. Gener. Deu. 1,457-463 38 Johnson, P.J., D’Oliveira, C.E., Gorrell, T.E. and Miiller, M. (1990) froc. NatlAcad. SC;.USA 87,6097-6101 39 Lahti, C.J., D’Oliveira, C.E. and Johnson, P.J. (1992)J. Bucteriol. 174, 6822-6830 40 Brul, S., Veltman, R.H., Lombardo, M.C.P. and Vogels, G.D. (1994) Biochim. Biophys. Acta 1183,544-546

41 Cavalier-Smith, T. (1987) Ann. N. Y. Acad. Sci. 503,55-71 42 Keller, G-A.et al. (1991) J. Cell Biol. 114,893-904 43 Marvin-Sikkema, F.D., Kraak, M.N.,Veenhuis, M.. Gottschal, J.C. and Prins, R.A.(1993) Eur. J. Cell Biol. 61,86-91 44 Cavalier-Smith, T. (1993) Micro&o/. Rev. 5i, 953-993 45 Hasegawa, M., Hashimoto, T., Adachi, J., Naoyuki, I. and Miyata, T. (1993) J. Mol. Euol. 36,380-388 46 Fenchel, T. and Ramsing, N.B. (1992) Arch. Microbial. 158,394-397

The effects of climate changeon land-atmos ks in arctic tundra regions Walter C. Oechel and George L. Vourlitis

A

Recently reported high-latitude warming of the global soil C pool, even tmospheric COZis increasthough they only make up about ing at the rate of about has the potential to affect arctic ecosystem structure and function in the 6% of the total land areaiOJiJ3. 1.5% per year from a preHigher temperatures could inshort and long term. Arctic ecosystems industrial level of 250are known sources of atmospheric CH,, crease the depth of the soil active 280 ppm to a current ambient level layer and’lower the water table, and recent CO, flux measurements of about 360 ppmi. Atmospheric resulting in greater soil aeration indicate that these ecosystems are now, CH,, which is about 20 times and higher rates of soil decompoat least regionally, net sources of more reactive than CO, as a greensitioni*JJ-18. If soil decomposition atmospheric CO,. It appears that over the house gas*, is increasing at a rate increases more rapidly than net short term (decades to centuries), arctic of 0.8% to 2.0% per year3. The primary production, the system ecosystems may represent a positive increase in the concentration of could represent a significant feedback on global atmospheric CO, these gases has the potential to source of global atmospheric concentrations and associated increase surface temperature and Wi9. Alternatively, nutrient mingreenhouse gas-induced climate change. effect climate on a global scale’. eralization rates should increase In addition, short-term feedbacks may be High-latitude warming has due to enhanced soil decompolarge enough to affect both local and global been recently reported in arctic sition17X1g,resulting in greater surface temperatures. Over the long term, Alaska, Canada and the Former changes in the structure, function and plant growth20and ecosystem proSoviet Union (FSU). Thermal productivityi4Jg. Under this scenario, composition of arctic ecosystems may files of permafrost indicate a temecosystem productivity could increase C accumulation relatively more perature rise of 2-4°C across the increase more rapidly than soil than the amount lost, thus restoring the north slope of Alaska and throughdecomposition, with the system sink status of arctic ecosystems. out northern Canada within the being a net sink for atmospheric past century, or possibly even C02’4J9. during the past few decades4g5. Walter Oechel and George Vourlitis are at the Here, we review recent re Northern latitude weather reDept of Biology, San Diego State University, search on the possible effects of cords indicate a similar increase San Diego, CA 92182, USA. global change on net ecosystem C in annual surface temperature6,’ (CO, and CH,) balance in arctic (Fig. 1). Near Barrow, Alaska, surface temperature has increased by approximately 1.5”C tundra ecosystems. Because net CO, flux is the sum of gross ecosystem productivity minus whole ecosystem resover the 70year average during the past decade alone (Fig. 2). However, the relatively short weather records, in- piration, it is an integrative measure of ecosystem function, and the processes that regulate its function. Also, becreasing heat island effects around urban weather station@, cause of the large soil C stocks and the potential positive and the effect of increasing winter snow depth on bore hole temperatures9 creates uncertainty about the magni- or negative feedback of arctic ecosystems on the regional and possibly global atmosphere, the effects of elevated tude and significance of the temperature variations. CO, and climate change on net C flux are acutely importHigh-latitude ecosystems are particularly vulnerable to climate change due to the large C stocks in northern lati- ant in understanding the future effects of arctic ecosystems on the global climate. tude soils and the predominance of permafrost. Northern ecosystems (arctic, boreal forest and northern bogs) repRecent change in net ecosystem CO, flux resent approximately 14% of the total global land area; In the past, net primary productivity often exceeded however, they contain approximately 25% of the total world heterotrophic respiration in northern ecosystems, due to soil C pool in the permafrost and seasonally thawed soil the predominance of cold, wet soilsi2. As a result, arctic eco active layerlO-* (Table 1). Tundra ecosystems alone contain approximately lgOxlOi5g of soil C (Table l), or 12% systems were estimated to be net sinks for atmospheric C

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REVIEWS of about O.l-0.3x1015gC per yeaWl. Recent measurements of CO, flux in arctic Alaska indicate that tussock and wet sedge tundra are now sources of atmospheric CO,7.22 (Fig. 3). Tussock tundra ecosystems are now losing approximately 112 gC m-zyr-1, corresponding to a net change in flux of 135gCm-2yrl (Ref. 7). Similarly, wet sedge ecosystems now appear to be slight sources of atmospheric CO, (Fig. 3). Compared to earlier estimates of C balance using harvest, cuvette and aerodynamic techniqueW4, the current sink strength of these wet sedge ecosystems has diminished by approximately 73 gC m-2yr-1 over the past two decades (Fig. 3). An additional 24 gC m-*yr-l may be lost from lakes and stream@, thus increasing the actual amount of C lost from arctic ecosystems annually. Because of the large C stores in arctic soils, if the Fig. 1. Trends in annual temperature Adapted from Ref. 6. current efflux of CO, from the arctic continues, these ecosystems could represent a strong positive feedback on global atmospheric CO, concentration and concomitant climate change7. These estimates may underestimate the annual loss of CO, in tundra ecosystems by excluding emissions from plant and soil respiration during the early spring, late autumn and winter periods26. Recent measurements made in the Kolyma Lowlands of the Russian arctic indicate that approximately 14gC m-2 (0.15 gC m-2d-1) is lost to the atmosphere between December and February26. This loss is attributed to biological activity in a relatively warm, moist layer underneath the frozen surface soP. This unfrozen layer is ephemeral, and as the winter season progresses, the layer freezes and rates of soil respiration diminish26. The change in net C balance of arctic ecosystems is thought to be due to a reduction in soil moisture and increased water table depth associated with the recently reported increase in high-latitude surface temperature7. Organic matter accumulation in arctic soils is due primarily to low soil temperature and high soil moisturel4. A decrease in the water table allows for greater soil aeration and higher rates of decomposition16 which could lead to the current situation where soil decomposition exceeds plant primary productivity7. The change in water balance may be because of a number of factors, including changes in the timing and amount of runoff during snow melt and an increase in evapotranspiration resulting in surface soil desiccatiorP. The importance of soil moisture in controlling net CO, flux of northern ecosystems and soils is well established. For example, draining of northern peatlands has been shown to convert these ecosystems from a CO, sink of approximately 25 gC m-2y-1 to a source of about 150gC m-*y-l to the atmospherels. A similar change in C balance is ob-

increase (“C per decade; see color bar) between 1961-1990

for northern regions.

served in laboratory manipulations of soil moisture and temperature. Using intact cores from wet sedge tundra ecosystems exposed to both elevated and ambient levels of CO,, a 1Ocm decrease in the height of the water table reduced net ecosystem CO, incorporation by 212 gC m-2, resulting in a net CO, efflux of approximately 84 gC m-2 (Ref. 16) (Fig. 4). With the water table at the soil surface, wet sedge ecosystems were significant sinks for atmospheric CO, (Ref. 16) (Fig. 4). The effect of temperature on net CO, flux is complex, owing to the interaction between soil moisture and temperature. Soil decomposition is affected by temperature” while photosynthesis is only weakly influenced by

I

“3

1 1930

1950

1970

1990

Year Fig. 2. Average summer (June through August) temperature increase over the past 70 years in Barrow, Alaska (71”18’N, 156”4O’W). Data were calculated using a three-year running mean. Dashed line depicts 70 year average. Data provided by the National Weather Service, and adapted from Ref. 7.

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Table 1. Distribution of global soil organic C pools by ecosystem type”

Ecosystem type

World area (ha ~10~)

Total world soil organic C pool (Pg)b,C

8 12 20 24.5 12 8.5 15 9 18 24 14 2 147

177.0 203.0 380.0 258.6 156.5 61.4 57.5 174.8 101.2 3.0 178.7 139.5 1511.2

Tundra and alpine Boreal forest Subtotal Tropical forest Temperate forest Woodland and shrubland Tropical savanna Temperate grassland Desert scrub Extreme desert, rock and ice Cultivated Swamp and marsh

Total

Percent of global soil c pool 11.7 13.4 25.1 17.1 10.4 4.1 3.8 11.6 6.7 0.2 11.8 9.2 100.0

aAdapted from Ref. 13. bLitter and soil C fractions combined. CPg = petagrams = gx1015.

160-r

Source _T_

80f % “i 0 9

O-

. . . . ...........I . +*A*.* -2...*.* .-.-.-.*I -.*.-.a.* .*.-2.s. *.*.-*-.. . . . . .-.-2.-s .... .*.*...*1 ..*.*.*. ; .*...*. . . .a..... . ... .. ... . ... .. . .. . .. . ...I. . . .......... . .. ... . .. . ... .. . .. . .. . ...I. . . . . . . . . . . . ..I. . . .. ... . .. . ... .. . .. . ....* ... . . .. . . . .. . . . . . . . . . . . ..*....... . ... .. . .. . ... .. . .. . .. . ...a .. . . . . . ...*. . . . . . . . .. .. . . . . . . ..a ..

T

?! ;F g z?

-80 -

1

Sink -160

I Tussock

I Wet sedge

Fig. 3. Estimates of previous and current CO, flux in arctic tundra ecosystems on the North Slope of Alaska. Means * 1 SE are shown. Positive flux values indicate CO, loss to the atmosphere. Previous (black bars) tussock tundra flux is from Ref. 10, and previous wet sedge flux is averaged from Refs 10. 23 and 24. Current fluxes (stippled bars) are from Refs 7 and 35.

temperature*g. Increases in soil temperature are likely to increase growth and respiration of soil microorganisms; however, the majority of arctic soil bacteria and fungi are adapted to cold temperature9. Typical Qlo values computed from arctic soil microbes are low compared to temperate soilsl8, suggesting that the direct effects of elevated soil temperature may not be as important as the indirect effects of temperature on soil moistureIT. In general, decomposition rates are slow and independent of temperature under conditions of low soil-moisture content (120% of dry mass), while temperature sensitivity increases until soil moisture content reaches approximately 200%17.Under saturated conditions, however, the temperature sensitivity of soil microbial populations is low, and respiration rates appear to be limited more by poor soil aeration16J7. 326

Increased nutrient mineralization is expected as arctic soils become warmer and drier”. In laboratory incubations of tussock and wet sedge tundra soils, N mineralization increased by approximately 45% with a 12°C increase in soil temperature (3-15”C)lT. Fertilization of arctic plants acts to stimulate tissue production20 rather than increasing leaf photosynthesis, and because essentially all of the N taken up by arctic plants is supplied by mineralization of organic matter*O,increased N availability through climate change is likely to stimulate whole ecosystem productivity and C accumulation (Fig. 4). In the long term, however, soil moisture conditions are likely to control organic matter accumulation in arctic soils18. The future C balance of arctic ecosystems will undoubtedly be a function of the amount of additional nutrients available for plant uptake (mineralized minus leaching), changes in soil C loss per unit of N mineralized, and changes in plant use of mineralized N20.If thaw depth increases as predicted with high latitude warming*T,leaching losses of mineralized N may be substantial, and the ecosystem could remain a net C source*O. In contrast, if plant response to increase N availability is greater relative to the amount of C lost per unit N mineralized, the system will once again become a C sink*O.In wet sedge tundra microcosms that were subjected to elevated CO, and nutrient levels, increased nutrients resulted in a significant increase in C accumulation (Fig. 4). Eventually, the total additional carbon stored in the vegetation could be greater than that lost from arctic soils. However, there are genetic constraints on plant size and productivity2”, and the full extent of this transformation will require changes in species composition and vegetation migration. Effects of global change on CH, flux Because of the vast belowground C storage and the predominantly waterlogged, anaerobic conditions of tundra soils, arctic ecosystems are important sources of CH,“O. Rates of CH, flux in arctic ecosystems can range between 0.1 and 200mgC m-‘day-1 (Table 2), and are extremely variable on both temporal and spatial scales31. Methane efflux in tundra ecosystems is positively correlated with soil moisture, with both linear and curvilinear relationships having been reported30-35. Other site-specific variables (e.g. substrate quality, vegetation cover) may also be important. For example, soil moisture directly influences redox potential by limiting 0, in the soil profile; however, soil redox potential may be highly variable over small spatial scales in arctic ecosystems, and may be influenced by the plant residues that make up a majority of the peat in arctic soils32. This physical and biological heterogeneity may be important in controlling the degree of spatial variability in CH, flux observed in arctic ecosystems. Soil temperature may also represent an important environmental control on CH, flux36.An increase in temperature from 2” to 12°C was found to increase CH, formation by a factor of 6.7, indicating that small changes in soil temperature may result in significantly enhanced CH, flUX36. Plant stems may be important conduits for soil CH, release to the atmosphere, as CH, efflux through plant stems may bypass oxidation at the soil surface37. The amount of CH, efflux through hydrophytes is a function of plant surface area and density37, suggesting that spatial variation in plant density and species composition may reinforce within-site variability in CH, flux. In addition, ecosystem productivity may represent an important control on CH, emissions, as a strong positive correlation TREE

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0

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Table 2. The range of net CH, flux in arctic ecosystems on the North Slope of Alaska during the summer growing season (June-August)” Wet sedge tundra

Maximum

Mrnrmum

Maxrmum

Refs

9.4 119.3 3.8 4.0

25.3 0.1 11.3 1.3

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30 31 34 35

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Fig. 4. Past, present and estimated future net CO, flux of arctic ecosystems in response to changes in CO, concentration, water table depth and nutrient availability. Time scales are inferred from the IPCC (1992) ‘business as usual’ scenario for CO, emissionG. Previous flux estrmates are from Refs 10, 23 and 24. Current fluxes are from Refs 7 and 35. The effects of elevated CO,, water table depth and nutrient availabilrty are from Ref. 16. Changes in vegetation composition are not included. Filled squares: no change in nutrients or water table. Filled circles: water table - 10 cm. Open circles: water table at surface. Open squares: added nutrients.

1

between CH, efflux and ecosystem productivity has recently been reportedJ8. This stimulation in methanogenesis may be because of increased substrate associated with increased productivity, root exudation, root turnover and litter productior9. Increased soil temperature and decreased soil moisture may be a possible outcome of global warming in wet tundra ecosystems+. This scenario may result in lower CH, flux, but higher CO, emissionsl4. Laboratory studies indicate that a 10 to 20cm lowering of the water table resulted in the cessation of CH, effluxsg. These studies, coupled with in situ observations of CH, flux over soil moisture gradients, suggest that drying of the upper soil layers may lead to lower net efflux and/or increased oxidation of CH,, resulting in a greater negative feedback on atmospheric CH, concentratiorGs3. Under a warmer and wetter climate change scenario, CH, emissions from tundra ecosystems could increase substantially while CO, emissions could decrease, resulting in a greater positive feedback on atmospheric CH, and radiative forcing of climate. Importance of time scales in arctic ecosystem responses to global change It is important to consider the widely varying response times of microbial populations, individuals, ecosystems and communities when considering the effects of climate change on arctic ecosystem structure and function14 (Fig. 5). For example, microbial metabolic rates change on much shorter time scales than do rates of ecosystem productivity and plant populations’“. Because of this, the recent change in net ecosystem CO, flux from a sink to a source may be a transient result of warming and drying of the tundra?. It is therefore likely that the net efflux of CO, from the arctic

will continue before it is reincorporated into aboveground biomass, as peak CO, sequestering could occur a century or more after the peak release of carbon from the soils (Fig. 5). On an individual leaf level, the response of the dominant tussock tundra sedge Eriophorum vaginaturn to elevated CO, appears to be transient, with acclimation to elevated CO, occurring on the order of days or weeks”0. Prolonged enhancement of photosynthesis by elevated CO, generally requires that nutrients are abundanttr; however, even with adequate nutrient supply some arctic plants generally show little photosynthetic response to elevated CO,Q.Without adequate sinks, accumulated carbohydrates may feed back to reduce the amount of RuBP carboxylase with a concomitant reduction in photosynthesis+“. Whole ecosystem response to elevated CO,, temperature and soil moisture may occur over weeks, years, even centuries14 (Fig. 5). Elevated CO, significantly increased net CO, accumulation in a tussock tundra ecosystem at Toolik Lake, Alaska during the first year of exposure, while tundra under ambient conditions experienced net CO, loss. However, photosynthetic capacity was reduced during the second year of exposure to elevated CO,, and decreased entirely during the last year of exposure14. Reciprocal CO, exposure experiments of tussock tundra at Toolik Lake suggest that acclimation to elevated CO? may occur over Soil microbial activitv

PooulationKIomDosition

1 IO 100 Time (years, log plot)

1000

Fig. 5. Hypothetical relative response trmes for adjustment to a new CO, environment and climate change, for changes at the level of leaf photosynthesrs and so11 microbial activrty, ecosystem CO, flux, populabon structure, community composition, vegetatron and evolution. Adapted from Ref. 14.

REVIEWS even shorter time scales (days to weeks)z*. In contrast, tussock tundra at Toolik Lake exposed to both elevated CO, and temperature were net sinks throughout the three years of experimentationi4. Elevated temperature may increase plant internal sink activity43, enhance mineralization and nutrient availabilityir, and/or increase nutrient up takelg thus allowing plants to use the greater amounts of carbohydrate produced under elevated CO, (e.g. see Fig. 4). Over time scales of years to decades, high-latitude warming is expected to result in changes in species composition@. Plants species respond differently to elevated CO, and temperature, which could result in differential growth40 and significant changes in competitive relationships44. To determine the long-term response of tundra ecosystems to elevated CO,, photosynthesis rates are being measured at a CO, emitting spring near Olafsvic, Iceland (A. Cook and W.C. Oechel, unpublished data). Preliminary results indicate that plants exposed to elevated CO2levels after a century or more exhibit significantly higher rates of photosynthesis per unit leaf area compared to ambientgrown plants. However, these plants appear to be compensating for higher photosynthetic rates per unit leaf area by producing less photosynthetic tissue per unit ground area. It is therefore expected that net CO, flux will be similar for both ambient and high CO, grown ecosystems after long-term exposure to elevated CO,. Paleobotanical studies indicate that vegetation boundaries and distributions may shift northward as high-latitude temperatures increase”. The vegetation in central and northern Alaska during the warmer mid-Holocene was composed primarily of a Picea-Beth-Alnus forest, while during the cooler late-Holocene, the vegetation became more similar to the modern boreal forest florali. Recent analysis of soil cores taken from the north slope of Alaska indicates that C accumulation of wet sedge tundra ecosystems during the mid-Holocene was significantly greater compared to the cooler late-Holoceneii.45, indicating that changes in vegetation distribution should lead to greater C sequestering. Spatial scales, scaling and regional climate feedback Although cuvette and plot level measurements contribute significantly to the determination of abiotic controls on net C balance, the spatial heterogeneity observed in arctic ecosystems make the extrapolation of plot scale measurements (square meter) to the landscape @er hectare) and regional scales (square kilometer) problematic. In addition, other variables that are important in controlling fluxes may be scale specific. For example, although aerodynamic resistance probably exerts a negligible influence on CO, or energy flux at the plot level, it is undoubtedly important at the landscape or regional leve146.Flux measurements must therefore be expanded from the plot to the regional and landscape scales before the effect of climate change on arctic ecosystems, and potential positive and negative feedback of arctic ecosystems on regional and perhaps global climate, can be determined. Micrometeorological techniques show promise in the characterization of landscape level fluxes of CO,, H,O vapor and energy. Using tower and aircraft platforms, eddy correlation techniques allow the quantification of material fluxes on hectare and square kilometer scales, respectively4r74s. Eddy correlation methodology was used extensively to quantify CO, and CH, fluxes in Alaskan sub-arctic tundra@. Although measurements were made only during the season peak (mid-July to mid-August), landscape level estimates of CO, and CH, flux are comparable to plot-scale measurements49. Eddy correlation methods also appear to be sen328

sitive to landscape level variations in soil moisture and vegetation type, thus allowing the determination of largescale controls on material and energy fluxes@. Correlation of spectral reflectance with aboveground vegetation biomass and wetness, coupled with Geographic Information Systems (GIS) databases of vegetation and abiotic variables, may represent a means for regional extrapolationso. Strong positive correlation between the Normalized Difference Vegetation Index (NDVI) and net CO, exchange have been observed for sub-arctic tussock tundra ecosystemsso. Remote sensing and eddy correlation techniques therefore represent powerful tools for regional and circumpolar estimation of trace gas fluxes. Future research goals and initiatives Given the possible feedbacks of arctic ecosystems to global CO, and CH, atmospheric content and regional and global weather patterns, additional study of the response of these ecosystems to global change is warranted. These research areas are being addressed at the international level by the Global Change in Terrestrial Ecosystems (GCTE) action of the International Geosphere-Biosphere Program (IGBP) initiatives. At the US national level, several programs are attempting to better understand large scale CO, and CH, flux dynamics (National Science Foundation, Arctic System Science, Land-Atmosphere Ice Interactions trace gas flux program) and energy and water balance feedbacks (US Dept of Energy, Atmospheric Radiation Monitoring). Other programs that are addressing largescale fluxes and atmospheric monitoring are those of the US Environmental Protection Agency (EPA), National Oceanic and Atmospheric Agency (NOAA), National Aeronautics and Space Administration (NASA),and the National Park Service. Despite these initiatives, additional coordinated research and new approaches are required to understand the complex relationships which may help to shape future atmospheric composition and climate. Newer techniques, such as the use of synthetic aperture radar (SAR) and unmanned aerospace vehicles for flux measurement, may further help us to understand the biophysical controls of trace gas flux. We feel that the technology and infrastructure are adequate to accomplish these goals provided that the scientific community develops the initiative. Conclusion The effects of future high-latitude warming and other aspects of climate change on arctic ecosystem structure and function are linked to associated changes in ecosystem water balance and the response times of the various ecosystem components to climate forcing. Recent field and laboratory investigations indicate that soil moisture status is the predominant abiotic variable controlling net CO, flux in arctic ecosystems. Over the area investigated, tussock tundra ecosystems are now strong sources of atmospheric CO,, while wet sedge tundra is either a weak source or approximately in balance. Arctic ecosystems are also important sources of CH,, however, the controls on CH, emissions are complex and involve soil moisture, temperature, organic matter quality, and vegetation cover and productivity. Regionally, arctic ecosystems appear to represent a currently positive feedback on the rise of global atmospheric CO,. Over the intermediate term, the effects of high-latitude warming and elevated CO, may result in changes in ecosystem productivity and changes in plant competitive status. Recent in situ experiments indicate that elevated CO, alone is insufficient to enhance plant or ecosystem level TREE vol.

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REVIEWS productivity indefinitely. Because arctic plants are primarily limited by nutrients, nutrient availability and/or uptake must increase before plants will be able to benefit markedly from CO, fertilization. Recent laboratory experiments indicate that nutrient mineralization will increase if the warmer, drier global change scenario ensues. As a result, ecosystem level productivity is expected to increase, although it is unknown whether productivity will increase more than decomposition, thereby causing arctic ecosystems to become net sinks for atmospheric CO, as in the past. Over the long term, changes in plant species composition are likely to occur because of changes in plant competitive relationships and/or changes in species distributions. Paleobotanical evidence indicates that northward shifts in species composition and a concomitant increase in C accumulation have historically occurred in at least some areas following elevated temperature. These changes, however, occur over decades to centuries, and it is likely that significantly more C will be lost from these ecosystems before changes in species composition counteract this loss. New approaches for estimating regional C, H,O and energy fluxes are needed before we can adequately assess the large-scale abiotic and biotic controls on these fluxes, and the overall response of arctic ecosystems to climate change. Exciting new methods include eddy correlation techniques and remote sensing for the extrapolation of point and landscape level measurements to the regional or circumpolar scales. Through a coordinated program using remote sensing and eddy correlation methodology, it should be possible to accurately estimate the current carbon flux in the circumarctic tundra, and to ,discover the relationships between flux and landscape surface type, topographic and climatic conditions. This is an important challenge facing scientists interested in understanding the role of the arctic in global atmospheric dynamics, and the associated effects on future regional and global climate. References 1 Houghton, J.T., Callander, B.A. and Varney, SK., eds (1992) in Climate Change 1992: The Supplemental Report to the IPCCScientific Assessment, pp. l-22, Cambridge University Press 2 Lashof, D.A. and Ahuja, D.R. (1990) Nature 344,529-531 3 Khalil,M.A.K.and Rasmussen, R.A.(1987)Atmos. Enuiton. 21,2445-2452 4 Lachenbruch, A.H. and Marshall, B.V. (1986) Science 234,689-696 5 Beltrami, H. and Mareschal, J.C. (1991) Geophys. Res. Lett 18,605-608 6 Chapman, W.L. and Walsh, J.E. (1993) Am. Met. Sot. 74,33-47 7 Oechel, WC. et a/. (1993) Nature 361,520-523 8 Dutton, E.G. and Endres, D.J. (1991)Arcr. Alp. Res. 23,115-119 9 Karl, T.R. (1993) Res. Explor. 9,234-249 10 Miller, PC., Kendall, R. and Oechel, WC. (1983)Simulation 40, 119-131 11 Billings, W.D. (1987) Quart. Sci. Reu. 6, 165-177 12 Corham, E. (1991) Ecol. Appt. 1, 182-195 13 Schlesinger, W.H. (1991) Biogeochemistry An Analysis of Global Change, Academic Press 14 Oechel, W.C. and Billings, W.D.(1992) in ArcticPhysiological Processes in a Changing Climate (Chapin, F.S., 111, Jefferies, R.L.,Reynolds, J.F.,

Shaver, G.R.and Svoboda, J., eds), pp. 139-168, Academic Press 15 Silvola, J. (1986) Ann. Bot. Fenn. 23,59-67 16 Billings, W.D., Luken, J.O.. Mortense, D.A. and Peterson, K.M.(1983) Oecotogia 58,286-289 17 Nadelhoffer, K.J., Giblin, A.E., Shaver, G.R. and Linkins, A.E. (1992) in Arctic Physiological Processes in a Changing Climate (Chapin, F.S., 111, Jefferies, R.L., Reynolds, J.F., Shaver, G.R. and Svoboda, J., eds), pp. 281-300, Academic Press 18 Flannagan. P.W. and Bunnel, F.L. (1980) in An Arctic Ecosystem: The Coastal Tundra at Barrow, Alaska (Brown, J., Miller, P.C., Tieszen, L.L. and Bunnel, F.L., eds), pp. 291-334, Dowden, Hutchinson and Ross 19 Shaver, G.R.ef al. (1992) BioScience 42,433-441 20 Chapin, F.S.. 111 and Shaver, G. (1985) Ecology 66,564-576

21 Schell, D.M.(1983) Science 219,1068 22 Grulke, N.E., Riechers, G.H.,Oechel, WC., Hjelm. U. and Jaeger. C. (1990) Oecotogia 83,485-494

Miller, P.C., Billings. W.D. and Coyne. P.I. (1980) in 23 Chapin, F.S.. 111, An Arctic Ecosystem: The Coastal Tundra at Barrow, Aluska

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(Brown, J., Miller, P.C., Tieszen, L.L.and Bunnel, F.L..eds), pp. 458-484, Dowden, Hutchinson and Ross Coyne, P.I. and Kelley, J.J. (1975) J. Appl. Ecol. 12. 587-611 Kfing,G.W.,Kipphut, G.W.and Miller, MC. (1991)Science 251. 298301 Zimov, S.A.et at. (1993) Geophys. Res. 98,5017-5023 Hinzman, L.D.and Kane, D.L. (1992) Ceophys. Res. 97,281 l-2820 Tieszen, L.L. (1973)Arctic andAlpine Res. 5,239~251 Chapin, F.S., III(1980) Annu. Rev. Ecol. Sys. 1I, 233-260 Sebacher, D.I., Harriss, R.C., Bartlett, K.B.,Sebacher. S.M. and Grice, S.S. (1986) Tellus 38B, l-10 Whalen, S.C. and Reeburgh, W.S. (1992) Glob. Biogeochem. Cycles 6. 139-159 Svensson, B.H. and Rosswall, T. (1984) Oikos 43,341-350 Vourlitis, G.L.,Oechel, WC., Hastings, S.J. and Jenkins, M.A.(1993)

34 35 36 37

Torn, MS. and Chapin, F.S., III (1993) Chemosphere 26,357-368 Oechel, WC. and Vourlitis, G.L.Adu. Soil Sci. (in press) Svensson, B.H. (1984) Appl. Enuiron. Microbial. 48,389-394 Sebacher, D.I., Harriss, R.C. and Bartlett, K.B.(1985) J Enuiron. Qua/

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