Present vegetation in the Snag-Klutlan area has a general altitudinal zonation; tree line being between 4100 and 4400 ft elevation on almost all slopes. Pollen spectra from surface samples below tree line generally reflect the vegetational composition. Those from above tree line, however, do not always reflect the surrounding vegetation, because they contain a large amount of pollen originating from below tree line.
A pollen diagram from pond sediments suggests the following vegetational sequence for the last 31,000 yrs: 31,000 B.P. through 27,000 B.P., fell-field or sedge-moss tundra followed by shrub tundra; 27,000 B.P. through 10,000 B.P., sedge-moss tundra; 10,000 B.P. through 8700 B.P., shrub tundra; 8700 B.P. through 5700 B.P., spruce woodland; 5700 B.P. through present, spruce forest. The diagram also suggests the following negative departures of July temperatures: 31,000 B.P. through 27,000 B.P., at least 8°F and possibly as much as 16°F; 27,000 B.P. through 13,500 B.P., 13°; 13,500 B.P. through 10,000 B.P., 12°F; 10,000 B.P., through 8700 B.P., 8°F. Precipitation seems to have been lower during cooler intervals than are present levels. Precipitation also seems to have increased over the last 6000 yrs.
Logs above the present tree line imply that summer temperatures have fluctuated above present values between 6000 B.P. and 1220 B.P. Tree-ring studies indicate that temperatures during the 200 yrs preceding 1940 were as much as 2°F cooler than present.