Wrangell-St. Elias National Park & Preserve - Permafrost & Forests (U.S. National Park Service)

Visitors often wonder why the forests in and around the park don't all look the same. Some trees are tall and straight. Others look weak and spindly. The absence and presence of permafrost profoundly influences tree growth in park forests.

Permafrost, or permanently frozen ground, develops when soils remain below freezing for two or more years. Permafrost occurs in many areas of the park, but does not occur beneath large lakes, major streams, and south-facing bluffs.

Permafrost may occur one to ten feet below the surface soil and be 100 to 200 feet thick! In summer, some thawing occurs in the active zone-the upper layer of soil that seasonally thaws. The thickness of this active zone affects the size and survival rate of trees and other plants.

Permafrost Image showing active zone and frozen ground

Where the active zone is deeper, forests become more developed. Only black spruce can survive above shallow permafrost.

Shallow Permafrost	No or Very Deep Permafrost
North-facing slopes and valley bottoms Thick moss layer insulates and prevents thawing Cold, poorly drained, saturated soils Woodlands, fens, muskegs, and bogs with primarily black spruce trees. Slow growth and low productivity Slow nutrient cycling	Floodplains and south-facing bluffs. Warm sites Deep and well drained soils Forests of white spruce, poplar, and aspen Fast growing, high productivity Fast nutrient recycling

Where permafrost is near the surface, soils are cold, saturated with surface water that cannot drain through the permafrost, and are low in nutrients. The black spruce forests that survive here are slow growing and stunted.

Where permafrost is deep or not present at all, the soils are well drained and deep. They are rich in nutrients and support a rapidly growing mixed-species forest of white spruce, birch, poplar, and quaking aspen.