Paleoecology
Hemlock was very abundant in the northeastern USA and eastern Canada during the early Holocene, then it underwent a sudden decline in abundance, almost disappearing from the pollen record around 5,000 years ago. It later returned, but never became as abundant as it had been previously. The causes of this 'hemlock decline' are unknown, but it is likely to have been due to some form of pest or disease.
Read more about this topic: Tsuga Canadensis