Fossil pollen records from the high Andes of Ecuador and northern Peru show that Alnus increased in abundance at most sites following warming associated with the onset of the Holocene. The peak of Alnus pollen abundance occurred between ∼9000 and 5000 yr BP. However, between 6500 and 4500 yr BP, the abundance of Alnus pollen declined markedly in all of the available records, including sites both within and above its modern dis...