Vegetation Chronosequence near Exit Glacier, Kenai Fjords National Park, Alaska, U.S.A.

Abstract

A vegetation chronosequence has developed on an outwash plain formed as Exit Glacier (Kenai Fjords National Park, Alaska, U.S.A.) retreated, beginning in the late 1700s. However, much of the area deglaciated since 1950 is partly flooded most years; this has slowed succession compared to other chronosequences on glacial till. Vegetation cover, diversity, density, and plant sizes in several stands of each of seven successional stages (Barren, Isolated Plant, Patchy, Alder, Cottonwood, Spruce-Cottonwood, Spruce-Hemlock) were sampled by systematic points along line transects and belt transects. All major species were found in the Barren stage, but they differed in timing of importance in the remainder of the chronosequence. Populus balsamifera ssp. trichocarpa (black cottonwood) was the most abundant colonizer and dominated many sites up to 150 yr after deglaciation. Picea sitchensis (Sitka spruce) was present throughout the chronosequence, but not in significant numbers until the Cottonwood stage. Tsuga mertensiana (mountain hemlock) was not common until it colonized dead logs and organic soils in the Spruce-Cottonwood stage. Forested sites on hillslopes above the outwash plain were dominated by P. sitchensis and T. mertensiana. Cover of Alnus sinuata (Sitka alder) and Populus balsamifera separated the three youngest stages from the middle two while Picea sitchensis cover distinguished the oldest two stages from the others.