Walter C. Oechel

OBSERVATIONAL EVIDENCE OF RECENT CHANGE IN THE NORTHERN HIGH-LATITUDE ENVIRONMENT

Studies from a variety of disciplines documentrecentchange in the northern high-latitude environment.Prompted by predictions of an amplified response oftheArctic to enhanced greenhouse forcing, we present asynthesis of these observations. Pronounced winter andspring warming over northern continents since about 1970ispartly compensated by cooling over the northern NorthAtlantic. Warming is also evident over the centralArcticOcean. There is a downward tendency in sea ice extent,attended by warming and increased areal extent of theArctic Oceans Atlantic layer. Negative snow coveranomalies have dominated over both continents sincethelate 1980s and terrestrial precipitation has increasedsince 1900. Small Arctic glaciers have exhibitedgenerally negative mass balances. While permafrost haswarmed in Alaska and Russia, it has cooled in easternCanada. There is evidence of increased plant growth,attended by greater shrub abundance and northwardmigration of the tree line. Evidence also suggeststhatthe tundra has changed from a net sink to a net sourceofatmospheric carbon dioxide.Taken together, these results paint a reasonablycoherent picture of change, but their interpretationassignals of enhanced greenhouse warming is open todebate.Many of the environmental records are either short,areof uncertain quality, or provide limited spatialcoverage. The recent high-latitude warming is also nolarger than the interdecadal temperature range duringthis century. Nevertheless, the general patterns ofchange broadly agree with model predictions. Roughlyhalfof the pronounced recent rise in Northern Hemispherewinter temperatures reflects shifts in atmosphericcirculation. However, such changes are notinconsistentwith anthropogenic forcing and include generallypositive phases of the North Atlantic and ArcticOscillations and extratropical responses to theEl-NiñoSouthern Oscillation. An anthropogenic effect is alsosuggested from interpretation of the paleoclimaterecord,which indicates that the 20th century Arctic is thewarmest of the past 400 years.

Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response to decadal climate warming

Long-term sequestration of carbon in Alaskan Arctic tundra ecosystems was reversed by warming and drying of the climate in the early 1980s, resulting in substantial losses of terrestrial carbon. But recent measurements suggest that continued warming and drying has resulted in diminished CO2 efflux, and in some cases, summer CO2 sink activity. Here we compile summer CO2 flux data for two Arctic ecosystems from 1960 to the end of 1998. The results show that a return to summer sink activity has come during the warmest and driest period observed over the past four decades, and indicates a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate. The mechanisms involved are likely to include changes in nutrient cycling, physiological acclimation, and population and community reorganization. Nevertheless, despite the observed acclimation, the Arctic ecosystems studied are still annual net sources of CO2 to the atmosphere of at least 40 g C m-2 yr-1, due to winter release of CO2, implying that further climate change may still exacerbate CO2 emissions from Arctic ecosystems.

Global Change and the Carbon Balance of Arctic EcosystemsCarbon/nutrient interactions should act as major constraints on changes in global terrestrial carbon cycling

n the cold, arctic climate, global warming due to greenhouse gas accumulation in the atmosphere might be expected to increase both primary production and heterotrophic (mainly soil) respiration. Which of these processes will increase more or more rapidly? The answer to this question is critical in understanding the effects of warming on the net carbon balance of arctic ecosystems and of the earth itself. If primary production increases faster than heterotrophic respiration, carbon will be removed from the atmosphere and will accumulate on land. If the reverse happens, carbon will be lost to the atmosphere. Eventually, a new equilibrium may be reached, but this equilibration could take decades or even centuries. Meanwhile, dramatic losses or gains of carbon may occur over the entire arctic region (5.7 x 106 km2; Oechel 1989), with potentially important feedbacks on the global atmospheric concentration of

Predicting ecosystem responses to elevated CO2 concentrations

One of the many changes occurring in the biosphere due to human activities is the increase in the carbon dioxide concentration in the atmosphere. This change is due both to the burning of fossil fuels and to deforestation. We do not know how these changes are affecting terrestrial ecosystems. This ignorance is partly because we have relatively poor records of the functional and structural response of any ecosystem through time. More studies are required to be able to accurately assess the effects of carbon dioxide.

Cold season CO2 emission from Arctic soils

Recent evidence indicates that significant amounts of C may be lost as CO2 to the atmosphere from tundra ecosystems during the fall, winter and spring months. Because high latitude ecosystems are particularly vulnerable to climate change, and contain large soil C stocks, the annual C budget is of particular interest. Significant amounts of CO2 loss were observed in arctic tundra ecosystems of the North Slope of Alaska during the 1993–1994 cold season. Moist tussock tundra ecosystems lost approximately 0.3 gC m−2 d−1 between mid-October and late May while coastal wet sedge ecosystems lost on average 0.08 gC m−2 d−1. Efflux rates were greatest during the months of October and May when soil temperatures were at a maximum, and portions of the soil profile were unfrozen. These daily loss rates equate to a seasonal loss of roughly 70 and 20 gC m−2 season−1 for tussock and wet sedge tundra ecosystems, respectively. Combined with warm season estimates of net CO2 efflux, tussock tundra ecosystems were net sources of approximately 112 gC m−2 yr−1 (0.1 PgC yr−1 worldwide) over the 1993–1994 calendar year, while wet sedge ecosystems lost approximately 25 gC m−2 yr−1 (0.02 PgC yr−1 worldwide). This study indicates that estimates of annual net CO2 exchange, based on warm season measurements alone, underestimate the actual magnitude of CO2 efflux.

The Role of fire in Mediterranean-type ecosystems

Fire has been recognized as a vital agent influencing the diversity and vigor of landscapes. It is particularly important in Mediterranean ecosystems, such as those of California. This book is of interest to ecologists, policy makers, and land managers.

On the use of MODIS EVI to assess gross primary productivity of North American ecosystems

[1] Carbon flux models based on light use efficiency (LUE), such as the MOD17 algorithm, have proved difficult to parameterize because of uncertainties in the LUE term, which is usually estimated from meteorological variables available only at large spatial scales. In search of simpler models based entirely on remote-sensing data, we examined direct relationships between the enhanced vegetation index (EVI) and gross primary productivity (GPP) measured at nine eddy covariance flux tower sites across North America. When data from the winter period of inactive photosynthesis were excluded, the overall relationship between EVI and tower GPP was better than that between MOD17 GPP and tower GPP. However, the EVI/GPP relationships vary between sites. Correlations between EVI and GPP were generally greater for deciduous than for evergreen sites. However, this correlation declined substantially only for sites with the smallest seasonal variation in EVI, suggesting that this relationship can be used for all but the most evergreen sites. Within sites dominated by either evergreen or deciduous species, seasonal variation in EVI was best explained by the severity of summer drought. Our results demonstrate that EVI alone can provide estimates of GPP that are as good as, if not better than, current versions of the MOD17 algorithm for many sites during the active period of photosynthesis. Preliminary data suggest that inclusion of other remote-sensing products in addition to EVI, such as the MODIS land surface temperature (LST), may result in more robust models of carbon balance based entirely on remote-sensing data.

The Role of Bryophytes in Nutrient Cycling in the Taiga

The bryophytes of the boreal forest are interesting in that they may form a minor element of the community in terms of biomass, while simultaneously being a major element in terms of cover and primary productivity. Even more importantly, the mosses may control ecosystem function through rapid nutrient uptake and through their effects on both the thermal environment of the soil and associated development of permafrost. Consequently, mosses can have major effects on vascular plant productivity and nutrient cycling in the ecosystem.

Taiga Ecosystems in Interior Alaska

For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the black spruce type. Black spruce forests are widespread in interior Alaska and are the most fire-prone forest type. They are also the most nutrient-limited and least productive forest type, especially in the late stages of succession. Ecosystem differences in productivity and degree of nutrient limitation are controlled mainly by soil and forest-floor temperatures. (Accepted for publication 3 August 1982)

Global Change and Mediterranean-Type Ecosystems

1. Relevance of Regional Models for Analyzing Future Climate Change in the Iberian Peninsula.- 2. Modeling Leaf Level Effects of Elevated CO2 on Mediterranean Sclerophylls.- 3. Direct Effects of Elevated CO2 in Chaparral and Mediterranean-Type Ecosystems.- 4. Biomass Partitioning and Resource Allocation of Plants from Mediterranean-Type Ecosystems: Possible Responses to Elevated Atmospheric CO2.- 5. Preliminary Studies of the Long-Term CO2 Response of Mediterranean Vegetation Around Natural CO2 Vents.- 6. Anticipated Effects of Elevated CO2 and Climate Change on Plants from Mediterranean-Type Ecosystems Utilizing Results of Studies in Other Ecosystems.- 7. Plant Responses to Drought Under Climate Change in Mediterranean-Type Ecosystems.- 8. Functional Attributes of Angiosperm Hemiparasites and Their Hosts and Predictions of Possible Effects of Global Climate Change on Such Relationships.- 9. Effects of Global Change on Plant-Animal Synchrony: Implications for Pollination and Seed Dispersal in Mediterranean Habitats.- 10. The Coastal Sage Scrub-Chaparral Boundary and Response to Global Climatic Change.- 11. Vegetation Response to Climatic Change in a Dune Ecosystem in Southern Spain.- 12. Monitoring Ecosystem Response to Global Change: High Spatial Resolution Digital Imagery.- 13. Monitoring Ecosystem Response to Global Change: Multitemporal Remote Sensing Analyses.- 14.Microbe-Plant Interactions in Mediterranean-Type Habitats: Shifts in Fungal Symbiotic and Saprophytic Functioning in Response to Global Change.- 15. Soil Organic Matter in Mediterranean-Type Ecosystems and Global Climatic Changes: A Case Study-The Soils of the Mediterranean Basin.- 16. Ecosystem Response to Elevated CO2: Nutrient Availability and Nutrient Cycling.- 17. Volatile Organics in Mediterranean Shrubs and Their Potential Role in a Changing Environment.- 18. Using Catchments of Contrasting Hydrological Conditions to Explore Climate Change Effects on Water and Nutrient Flows in Mediterranean Forests.- 19. Water Balance of Mediterranean Ecosystems Under a Changing Climate.- 20. Patterns of Fire Occurrence Across a Climatic Gradient and Its Relationship to Meteorological Variables in Spain.- 21. Sensitivity of Fire Regime in Chaparral Ecosystems to Climate Change.- 22. Global Environmental Change to and the Future of Mediterranean Forest Avifauna.- 23. Monitoring Environmental Change Through Amphibian Populations.- 24. Conservation, Restoration, and Research Priorities for Mediterranean Uplands Threatened by Global Climate Change.