Measurements of carbon fluxes in Arctic tundra landscapes are generally obtained through intensive field work and involve the use of chamber and/or micrometeorological tower techniques. However, findings in a variety of nonArctic ecosystems have demonstrated the potential of remote sensing-based techniques (particularly spectral vegetation indices) to provide estimates of CO2 exchange in a more timely and efficient manner. As the firststep towards modelling Arctic regional and circumpolar fluxes of CO2 using remotely sensed data, we investigated the relationships between plot-level fluxes of CO2 and a vegetation spectral reflectance index derived from hand-held radiometric data at two sites. These relationships were evaluated for variations in vegetation cover type and environmental factors using data collected during the short Arctic growing season. Overall, this study demonstrated a relationship between the Normalized Difference Vegetation Index (NDVI) and measurements of mean site gross photosynthesis and ecosystem respiration at two sites in Arctic tundra ecosystems on the North Slope of Alaska.

We propose an upward infiltration method to calibrate a time domain reflectometry (TDR) system. The method is rapid, allows the soil to remain unchanged during the experiment, and provides hundreds of data points. Three experiments were conducted using 18.7-cm-long probes in soils of three textures (nine experiments total). Calibrations also were performed by packing soil cores with soil at progressively higher water contents. The data from the two methods were fitted using regression techniques to two published models of the dielectric constant-water content relationship. The results show that the calibration curves of all three soils, fitted simultaneously to replicate experiments, mere similar in shape and statistically the same. A single calibration curve, fitted simultaneously to the replicate experiments of the three soils, was compared with Topps equation and was statistically the same. The results support the conclusion that the upward infiltration method provides a fast and repeatable calibration method, consistent with conventional calibration.

Models of regional CO2 exchange processes in arctic tundra environments using landscape characteristics may need to incorporate spatial and temporal variations in the depth to the permafrost layer because it influences soil drainage, aeration, decomposition, and nutrient availability. However this depth, or the depth of the active layer (DAL), will have to be estimated indirectly since it is not practical to make a large number of ground measurements on a regular basis. Previous research has demonstrated that spatial variations in DAL are strongly related to aboveground vegetation production in sub-arctic tundra. Since aboveground vegetation production in Alaskan arctic tundra has been related to the Normalized Difference Vegetation Index (NDVI), it was hypothesized that spatial variations in NDVI would follow variations in DAL in these environments. Studies were conducted on the North Slope of Alaska during the summers of 1994 and 1995 to determine the feasibility of estimating DAL at multiple spatial scales using the NDVI. Experiments were performed at sites with distinctly different topographic relief using hand-held and satellite spectral radiometric data and ground measurements of DAL. Overall, our results suggest that there is no relation between NDVI and DAL in areas with little variation in relief. However, in areas where topography strongly controls the flow and redistribution of water, NDVI did account for approximately 40 per cent of the variability in DAL.

AbstractCarbon flux measurements made at an elevated point are associated with an effective upwind area or ‘footprint.’ Since Arctic tundra landscapes can exhibit substantial heterogeneity within the footprint of an eddy correlation tower, it may be necessary to determine the relative point source contributions to the observed flux if landscape properties are to be related to the flux. This study evaluates the potential importance of representing footprint source contributions in relationships that are developed between tower observations of net ecosystem exchange of carbon dioxide (NEE) and a remotely sensed spectral vegetation index. Satellite data collected over the foothills region of the North Slope of Alaska are used to determine spatial patterns of a spectral vegetation index in the calculated footprints of 30 randomly selected tower locations. A previously developed relationship between NEE and the vegetation index is used to calculate NEE at each tower location using two techniques, one that explicitly considers the footprint pattern of relative contributions to tower fluxes and another that ignores these patterns. The results indicate that if carbon fluxes measured at a tower are to be related to remotely sensed spectral vegetation indices, then it is necessary to consider the relative flux contributions from within the tower footprints for sites on the North Slope of Alaska.

Experimental validation of solute transport models is often met with unsatisfying or ambiguous results. Aside from the inherent difficulty in conducting these experiments, the lack of a standard set of methodologies employed in these studies may be an impediment. This study was conducted to determine if the method employed in the sampling of soil solutes could influence the results of model validation attempts. A narrow pulse of KBr was uniformly applied to two 25-m(2) plots, one tilled and the other undisturbed, on a level Ulm clay loam (fine, montmorillonitic, mesic Ustollic Haplargid). Solute movement was then monitored for the next 388 d using replicated ceramic cup solution samplers at depths of 0.15, 0.30, 0.80, 1.20, 1.60, and 2.00 m and by the periodic removal of replicate soil cores in 0.05-m increments to a maximum depth of 3.70 m. Overall, the methods yielded consistent information regarding the mean convective transport of Br-. Both methodologies indicated deeper and more rapid movement of Br- in the nontilled soil, although the shallow solution samplers appear to miss the most rapidly moving solute. By the conclusion of the experiment, the difference between the centers of mass of the Br - plumes in the tilled and nontilled soil was at least 0.6 m. The soil core and solution sampler methods appear to yield inconsistent measures of the solute dispersion but we attribute the differences to the effects of incomplete mass recovery and, we argue, follow naturally from the instrument window of the sampling methodology relative to the observed soil heterogeneity.

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.

The albedo of tussock tundra was measured at two sites on the north slope of Alaska. One site was selected because of its apparent uniformity and undisturbed condition, while the other site had been visibly affected by dust deposition from the Dalton Highway. Albedo measurements were made under varying cloud-cover conditions. It was hypothesized that observed variations in albedo at the undisturbed site was caused by variations in illumination conditions and vegetation cover type. A simple cloud index (CI) was used to characterize the cloud-cover-illumination conditions, and vegetation cover was described using a hierarchical classification scheme and point-quadrat sampling. Albedo at the undisturbed site was found to be related significantly to variations in CI but unaffected by differences in vegetation cover type. The final component of the study compared the albedo-CI relationships at the undisturbed and dust-impacted sites. It was concluded that dust deposition had a significant direct and/or indirect effect on the albedos at this site, particularly under cloudy conditions.