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Summary1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34 -36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

Active-layer thickness was mapped over a 26,278-km2 area of northern Alaska containing complex and highly variable patterns of topography, vegetation, and soil properties. Procedures included frequent probing to ascertain thaw depth in representative land-cover units, extensive thermal monitoring with data loggers, and application of spatial analytic techniques. Geographic information systems technology was used analytically to merge thaw-depth and temperature data with a digital land-cover map, a digital elevation model, and a topoclimatic index, yielding a spatial time series of active-layer thickness for the map area at weekly intervals over the summer of 1995. Although the maps show a strong regional trend in the thickness of the active layer, extreme local variation occurs in complex terrain and in areas with sharp discontinuities in soil moisture content. Because active-layer thickness is influenced strongly by vegetation and soil properties, the relative volume of thawed soil beneath several landscape units is not proportional to the relative surface area occupied by those units. Predicted values of active-layer thickness are within approx. 6 cm of measured mean values in representative 1-km units distributed over the latitudinal extent of the study area. If computed and averaged for a series of years (e.g, one decade), the integrated products yielded by the mapping procedures could be used as baseline documents for comparison with calculations based on climate-change simulations.

Abstract. The morphological, chemical, and physical properties of arctic tundra soils were examined along a 200-km latitudinal gradient in northern Alaska which includes two major physiographic provinces; the Arctic Coastal Plain and the Arctic Foothills. Annual air temperature and precipitation increase along the gradient from north to south. Soils on the Arctic Coastal Plain support wet, nonacidic tundra vegetation and have high carbonate contents. Soil on the Arctic Foothills support moist, nonacidic tundra in the northern part and moist acidic tundra in the southern part. Most arctic tundra soils are characterized by medium texture, poor drainage, and high organic matter content. From north to south along the transect, the base saturation of the active layer decreases and exchangeable aluminum increases from north to south. Most soils have strongly developed cryogenic features, including warped and broken horizons, ice lenses, thin platy structure, and organic matter frost-churned into the ice-rich upper permafrost horizons.