The structure and function of Alaskan forest ecosystems was examined across two secondary successional sequences. One, the most common in interior Alaska, follows fire in black spruce stands on permafrost sites. The other, less common sequence, follows fire on warmer, generally south aspect sites and passes through a shrub and hardwood stage to white spruce. On black spruce site, the thickness of the forest floor is a key factor responsible for maintaining moist soil, cold temperature and permafrost. Soil degree days range from 1250 to 1000 degree days 1 to 5 years following fire. In mature black spruce temperature ranges from 500 to 800 degree days. The active layer thickness in later stages of this succession is usually from 40 to 60 cm. Following fire it increases for several years and may reach 1 meter in thickness. In the south aspect white spruce sequence, soil temperature approached 1400 degree days in heavily burned forests in contrast with a 900 to 1000 degree days in mature white spruce. On the black spruce successional sequences, cold soil temperature results in lowest rates of soil biological activity, nutrient cycling, and, in turn, lowest tree productivity. Net annual above-ground tree production ranges from about 7 metric tons per hectare on the warmer, south aspect hardwood-white spruce sequence to less than 1 metric ton per hectare on the black spruce sequence.