Succession and nitrogen Glacier Bay, Alaska

Glacier Bay National Park, Alaska NASA/Landsat ETM+

In 1879, the glacier was 4000ft thick, 20 miles wide and extended 100 miles to the St. Elias Mountain range By 1916, the Grand Pacific Glacier – the main glacier credited with carving the bay – had melted back 60 miles to the head of what is now Tarr Inlet. Glacier Bay (close up) Glacial retreat has opened up new areas available for colonisation By new plant and animal communities… SUCCESSION

Pioneer 0-15 years * Mosses * Slow development of organic soils * Shallow rooted herbs * Avens (Dryas) fixes N, which is added to the soil The seeds of fireweed (above) are designed to be carried to new areas by the wind

Alder thickets * years * alders and willows * development of alder thickets approximately 10 m tall * alders fix N and add to solid * pH lowered from 8 to 5 over 50 years Alders add Nitrogen to soil

Sitka spruce years Sitka spruce Dense mixed forest forms Mixed forest >200 years Gradual invasion by western and mountain hemlock

Young cottonwood trees grow out of round mats of nitrogen-fixing avens (Dryas), which has gone to seed. Lichen spores that land on the appropriate rocky surface will anchor themselves to the rock. As they grow, lichens secret an acid that dissolves the rock around them, creating soil. As soil develops, more seeds and spores arrive, such as those of mosses, avens (Dryas), horsetail and fireweed. These pioneer communities can develop into dense thickets of nitrogen-fixing alder and cottonwood that enrich the soil and provide shelter for other colonising species such as willow

Nitrogen content over time