Radiative exchange, near-surface temperature and soil water of forest and cropland in interior Alaska

Abstract

The expansion of the agricultural industry in Alaska during the early 1980's allowed for clearing 50 000 ha of boreal forest. Little is known, however, about the impact of clearing boreal forest on surface energy exchange processes. This study assessed radiative and heat exchange, temperature and soil water at boreal forest and grassland sites near Delta Junction, AK. The sites, located 5 km apart, were on a nearly level alluvial terrace. The grassland site was cleared in 1978 and seeded to grass in about 1985. At both sites, sensors were installed to measure the albedo, net radiation, surface heat flux, near-surface temperatures and soil water content from September 1991 to April 1993. Seasonal variations in albedo were more apparent for grassland (0.19 to 0.85) than for forest (0.08 to 0.18). Net radiation over forest was 20% greater during the summer and 20% smaller during winter compared with grassland. Surface heating consumed 13 and 6% of net radiation during the growing season at the grassland and forest sites, respectively. Soil temperatures at the 0.5-m depth ranged from −14 to 10°C at the grassland site and from −2.5 to 0.5°C at the forest site. The soil at the grassland site was drier and exhibited greater seasonal variation in water content compared with the soil at the forest site. Ice lenses were apparent in the forest soil at a depth of 0.3–0.6 m as the water content exceeded the porosity. This study suggests that clearing forest land will reduce the net absorption of radiation, but enhance soil heating and drying in the subarctic.