Steven J. Hastings

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.

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.

PHYSIOLOGICAL MODELS FOR SCALING PLOT MEASUREMENTS OF CO2 FLUX ACROSS AN ARCTIC TUNDRA LANDSCAPE

Regional estimates of arctic ecosystem CO2 exchange are required because of the large soil carbon stocks located in arctic regions, the potentially large global-scale feedbacks associated with climate-change-induced alterations in arctic ecosystem C sequestration, and the substantial small-scale (1–10 m2) heterogeneity of arctic vegetation and hydrology. Because the majority of CO2 flux data for arctic ecosystems are derived from plot-scale studies, a scaling routine that can provide reliable estimates of regional CO2 flux is required. This study combined data collected from chamber measurements of CO2 exchange, meteorology, hydrology, and surface reflectance with simple physiological models to quantify the diurnal and seasonal dynamics of whole-ecosystem respiration (R), gross primary production (GPP), and net CO2 exchange (F) of wet- and moist-sedge tundra ecosystems of arctic Alaska. Diurnal fluctuations in R were expressed as exponential functions of air temperature, whereas diurnal fluctuations in GPP were described as hyperbolic functions of diurnal photosynthetic photon flux density (PPFD). Daily integrated rates of R were expressed as an exponential function of average daily water table depth and temperature, whereas daily fluctuations in GPP were described as a hyperbolic function of average daily PPFD and a sigmoidal function of the normalized difference vegetation index (NDVI) calculated from satellite imagery. These models described, on average, 75–97% of the variance in diurnal R and GPP, and 78–95% of the variance in total daily R and GPP. Model results suggest that diurnal F can be reliably predicted from meteorology (radiation and temperature), but over seasonal time scales, information on hydrology and phenology is required to constrain the response of GPP and R to variations in temperature and radiation. Using these physiological relationships and information about the spatial variance in surface features across the landscape, measurements of CO2 exchange in 0.5-m2 plots were extrapolated to the hectare scale. Compared to direct measurements of hectare-scale F made using eddy covariance, the scaled estimate of seasonally integrated F was within 20% of the observed value. With a minimum of input data, these models allowed plot measurements of arctic ecosystem CO2 exchange to be confidently scaled in space and time.

The effect of soil moisture and thaw depth on CH4 flux from wet coastal tundra ecosystems on the north slope of Alaska

Abstract Results from a 2-year study of wet coastal tundra ecosystems located near Prudhoe Bay, Alaska indicate that CH 4 flux to the atmosphere is strongly controlled by soil moisture content and the depth of the seasonally thawed soil active layer. Daily CH 4 flux from a flooded and well drained site on the Prudhoe Bay oil fields was on average 0.7 and 0.1 gC m −2 day −1 , respectively over both sampling seasons. Daily CH 4 flux in an area located approximately 50 km south of Prudhoe Bay was considerably higher. This area (APL 133-3) contained a flooded, moist, and dry site which emitted on average 1.5, 1.1, and 0.2 gC m −2 day −1 to the atmosphere, respectively over both sampling seasons. These flux rates represent significant inputs of CH 4 to the atmosphere. Evidence of the importance of soil moisture and thaw depth in controlling CH 4 flux, coupled with the predominantly waterlogged, anaerobic conditions of wet tundra soils, suggest the potential for strong interactions of this soil-atmosphere system with global climate change.

Light‐stress avoidance mechanisms in a Sphagnum‐dominated wet coastal Arctic tundra ecosystem in Alaska

The Arctic experiences a high-radiation environment in the summer with 24-hour daylight for more than two months. Damage to plants and ecosystem metabolism can be muted by overcast conditions common in much of the Arctic. However, with climate change, extreme dry years and clearer skies could lead to the risk of increased photoxidation and photoinhibition in Arctic primary producers. Mosses, which often exceed the NPP of vascular plants in Arctic areas, are often understudied. As a result, the effect of specific environmental factors, including light, on these growth forms is poorly understood. Here, we investigated net ecosystem exchange (NEE) at the ecosystem scale, net Sphagnum CO2 exchange (NSE), and photoinhibition to better understand the impact of light on carbon exchange from a moss-dominated coastal tundra ecosystem during the summer season 2006. Sphagnum photosynthesis showed photoinhibition early in the season coupled with low ecosystem NEE. However, later in the season, Sphagnum maintained a significant CO2 uptake, probably for the development of subsurface moss layers protected from strong radiation. We suggest that the compact canopy structure of Sphagnum reduces light penetration to the subsurface layers of the moss mat and thereby protects the active photosynthetic tissues from damage. This stress avoidance mechanism allowed Sphagnum to constitute a significant percentage (up to 60%) of the ecosystem net daytime CO2 uptake at the end of the growing season despite the high levels of radiation experienced.

Direct Effects of Elevated CO2 in Chaparral and Mediterranean-Type Ecosystems

Atmospheric CO2 has increased by about 25% since the beginning of the industrial revolution, and is currently rising by about 1.8ppm per year (Waston et al., 1990). The concentration at the end of the next century will depend both on human use of fossil fuels and the response of natural ecosystems, and could range from 450 ppm CO2 to more than 800 ppm CO2 (Waston et al., 1990). Although the specific nature and regional pattern are uncertain, climate change associated with increases in atmospheric CO2 and other greenhouse-active gases is generally expected (Folland et al., 1992). For much of the contiguous U.S., this may translate to higher temperatures and decreasing soil moisture (Mitchell et al., 1990). In the chaparral of southern California, higher temperatures are likely to result in increased evapotranspiration, and if precipitation does not increase markedly, available soil moisture should decrease (Rind et al., 1990).

Aircraft Regional-Scale Flux Measurements over Complex Landscapes of Mangroves, Desert, and Marine Ecosystems of Magdalena Bay, Mexico

Naturalecosystemsare rarely structurallysimple or functionally homogeneous. This is true for the complex coastal region of Magdalena Bay, Baja California Sur, Mexico, where the spatial variability in ecosystem fluxes from the Pacific coastal ocean, eutrophic lagoon, mangroves, and desert were studied. The Sky Arrow 650TCN environmental research aircraft proved to be an effective tool in characterizing land‐atmosphere fluxes ofenergy, CO2, and watervapor across a heterogeneous landscape at the scale of1km. The aircraftwas capable of discriminating fluxes from all ecosystem types, as well as between nearshore and coastal areas a few kilometers distant. Aircraft-derived average midday CO2 fluxes from the desert showed a slight uptake of 21.32mmolCO2m 22 s 21 , the coastal ocean also showed an uptake of 23.48mmolCO2m 22 s 21 , and the lagoon mangroves showed the highest uptake of 28.11mmolCO2m 22 s 21 . Additional simultaneous measurements of the normalized difference vegetation index (NDVI) allowed simple linear modeling of CO2 flux asafunction ofNDVIforthemangrovesoftheMagdalenaBayregion.Aircraftapproachescan,therefore,be instrumental in determining regional CO2 fluxes and can be pivotal in calculating and verifying ecosystem carbon sequestration regionally when coupled with satellite-derived products and ecosystem models.

Airborne springtime IOP measurements of radiative exchange and albedos in the Barrow, Alaska, region and comparisons to growing season results

On April 8, 1999, at approximately local solar noon, airborne measurements of longwave and shortwave radiation were conducted in the Barrow, Alaska, region. Over the pack ice (100 km north of Point Barrow), albedo averaged 0.83, net radiation flux (average 32.4 W m−2) was into the surface, and surface temperatures were positively correlated to observed cloudiness and measured sky temperatures. Over the coastal “fast” ice and tundra, albedo averaged 0.90, net radiation flux (average 2.7 W m−2) was out of the surface, and surface temperatures were not significantly correlated to observed cloudiness or measured sky temperatures. Albedo was not a significant function of observed variation in cloudiness or sky temperature but may have been affected by measurable snowfall the previous day at Barrow. Compared to growing season albedos, springtime albedos had lower coefficients of variation.

Direct estimates of CO/sub 2/ flux in Arctic environments using a spectral vegetation index

Carbon flux and spectral reflectance data were collected on the North Slope of Alaska during the 1994 growing season. Observations were made at two sites in the coastal plain and two sites in the foothills of the Brooks Mountain Range. The relationship between the normalized difference vegetation index (NDVI) and daily gross primary production (GPP) was investigated. The daily gross primary production of tussock tundra was normalized for variations in photosynthetically active radiation (PAR) and was found to be linearly related to the NDVI. Seasonal changes in the vegetation and solar elevation, and variations in cloud cover conditions during radiometric observations did not have a significant effect on the relationship between GPP/PAR and the NDVI. The contrasting landscapes of the coastal plain and foothills regions (vegetation and moisture characteristics) and experimental manipulations that included changes in soil moisture conditions, illumination and temperature also did not appear to affect the relationship.