ECOLOGICAL IMPACT OF THE MID-HOLOCENE HEMLOCK DECLINE IN SOUTHERN ONTARIO, CANADA

Abstract

Hemlock (Tsuga canadensis) was an abundant tree species in eastern North America before it declined sharply in abundance ∼4700 14C yr BP (∼5400 calendar years BP). The rapid, apparently synchronous, and species-specific nature of the decline suggests that it was the result of a pathogenic outbreak. T. canadensis is a highly shade-tolerant, long-lived conifer, and its sudden removal from the canopy is likely to have had an impact on forest composition, structure, and function. The response of forest vegetation to the mid-Holocene hemlock decline at two sites in southern Ontario, Canada was examined using paleoecological methods. At one site, where T. canadensis declined sharply, several other forest taxa increased in abundance, resulting in a long-term shift in forest composition. At a second site, pollen percentages of T. canadensis did not decrease so dramatically, and less change in forest composition occurred. At both sites, T. canadensis took ∼2000 yr to recover from the decline and reach former abundance levels. This study suggests that the mid-Holocene decline of a forest dominant may have had a long-term impact on forest composition in northeastern North America.