Fossil pollen and conifer stomata reveal 14,000 years of vegetation dynamics on a hypermaritime island on the outer Pacific coast of Canada

Background/Question/Methods

Haida Gwaii is an archipelago of about 150 islands, located 80 km offshore of mainland British Columbia. Proximity to the Pacific Ocean moderates the climate of these islands, producing a hypermaritime environment that is characterized by cool summers, abundant fog, and very wet winters and that supports a mosaic of coniferous rainforest and bog-forest wetlands. Given that the dominant tree species in these coastal rainforests have lifespans that exceed several hundred years, paleoecological methods are necessary to understand long-term plant community dynamics in this region. To this end, fossil pollen analyses were conducted on 3.55 m sediment core retrieved from a small lake on Hippa Island at the western edge of the archipelago. Coastal British Columbia is an ideal setting for pollen analysis because the pollen of many of the dominant taxa can be identified to the species level. Cupressaceae is an important exception as cedar species produce morphologically indistinguishable pollen. Thus, pollen analysis was coupled with conifer stomata analysis to confirm the local presence of conifer species and more specifically to determine which of three possible cedars were present on Hippa Island. This provided a means to directly compare paleovegetation records derived from these two lines of evidence.

Results/Conclusions

Radiocarbon dating indicates that the Hippa Lake sediment record spans the last 14,000 cal yr. Pollen and conifer stomata analyses reveal substantial changes in vegetation communities over this period. Between 14,000 and 13,230 cal yr BP, the island supported diverse herb-shrub communities dominated by Cyperaceae and Artemisia, with up to 15% Salix pollen. Pinus and Picea stomata are present in these basal sediments, indicating that these conifers were present on the island by 14,000 cal yr BP, likely as scattered individuals. Transition to Pinus contorta woodland by 13,000 cal yr BP is followed by increases in Alnus viridis, Alnus rubra, and Picea sitchensis. After 11,000 cal yr BP, the island supported closed Picea sitchensis and Tsuga heterophylla forest with abundant Lysichiton americanus and ferns in the understory. Cupressaceae pollen and Thuja plicata stomata indicate that this species was present by 8600 cal yr BP and then dominated forests along with Tsuga heterophylla and Picea sitchensis after 5500 cal yr BP. Fossil conifer stomata indicate that species may be locally present for thousands of years before expansion of their populations is recorded in pollen analysis of the same sediments.