Late Quaternary paleoecology of Thuja and Juniperus (Cupressaceae) at Crawford Lake, Ontario, Canada: pollen, stomata and macrofossils

Abstract

In northeastern North America, paleoecological records of Thuja and Juniperus are problematic due to their indistinguishable and poorly preserved fossil pollen grains. However, macrofossils and stomata of Thuja and Juniperus are distinctive. Difficulties with macrofossil data are that the analysis is time-consuming, and macrofossils are usually scarce in lake sediments. Using stomata as a proxy of macrofossils has two advantages: (1) stomata are more abundant than macrofossils; and (2) they can be counted from pollen preparations. Crawford Lake has 5–15% Thuja-Juniperus (Cupressaceae) pollen through most of the past 13,000 yr except for a dearth during the Pinus pollen zone at ca. 10,000-7500 14C yr BP. Macrofossil, stomatal and pollen results showed the late glacial (ca. 13,000-10,000 14C yr BP) pollen mostly was derived from Juniperus (likely J. communis) indicated by smaller pollen, a few Juniperus stomata, and absence of Thuja stomata and macrofossils, whereas pollen at 7500-0 14C yr BP was from Thuja occidentalis indicated by larger pollen, and abundant Thuja stomata and macrofossils. This bimodal stratigraphic pattern of Cupressaceae pollen appears at other sites in southern Ontario, which suggests the possibility of separating these taxa to an earlier Juniperus and later Thuja at these sites. The late glacial Thuja macrofossils reported in previous studies may indicate early immigration of a small population via favourable habitats along Ontario's Niagara Escarpment. Alternatively, these Thuja macrofossils may be derived from younger sediments, as suggested by the questionable stratigraphies and puzzling 14C dates. The separation of two genera would provide valuable information in paleoecological interpretation of pollen data because the taxa occupy different habitats.