Paleoenvironmental changes at treeline: a 6,500 year long pollen and stable isotope record

Abstract

Combined pollen, macrofossil and stable-isotope records from two lakes in the sub-alpine zone of the north-central Alaska Range indicate significant changes in vegetation and lake productivity during the past 6̃750 ¹⁴C yrs BP. These changes are associated with neoglacial cooling and climate variations during the Little Ice-Age (LIA). Highest spruce densities occurred during a period from 5,000 ¹⁴C yrs BP and 2,500 ¹⁴C yrs BP and coincided with the onset of cooler and moister climate. The shifts in climate, which resulted in increased effective moisture levels in Central Alaska, possibly shifted the competitive balance towards spruce and against tundra taxa. Lake productivity declined as climate cooled. A brief episode of climate amelioration between 1,500 ¹⁴C yrs BP and 800 ¹⁴C yrs was followed by cooking events of the LIA which resulted in decreased spruce densities in the sub-alpine forest-tundra zone and a possible lowering in treeline at higher elevation.