Phytoliths of Southeastern Vancouver Island, Canada, and their potential use to reconstruct shifting boundaries between Douglas-fir forest and oak savannah

• In Western North America Douglas-fir forests tend to invade undisturbed savannahs.
• We catalogue phytoliths in dominant plants of oak savannahs and Douglas-fir forests.
• Douglas-fir asterosclereid phytoliths are more abundant in forest soils.
• Phytoliths could help establish the timing of local shifts from savannah to forest.

Phytolith assemblages in terrestrial soils have the potential to reveal local shifts between open savannah and conifer forests in the past. On southeastern Vancouver Island, this could help quantify the range of variability of Garry oak savannahs prior to European colonization, and the degree of influence of indigenous people in maintaining open savannahs. We catalogued the phytolith morphotypes found in 72 of the most common plant species in this system, and extracted phytoliths from two soil cores on either side of the boundary between a conifer forest and an oak savannah. Grasses are the most prolific producers of phytoliths in savannahs. In forests, Douglas-fir (Pseudotsuga menziesii) produces large, distinctive asterosclereid phytoliths. Phytolith assemblages from soil cores showed that phytoliths from grasses dominated, even in soils from the forest, where grasses are rare. However, the presence of asterosclereid phytoliths reliably distinguished between Douglas-fir forest and Garry oak savannah habitats within 20 m of the vegetation boundary. This finding may be useful for reconstructing past vegetation not only on Vancouver Island, but also across a large geographic area in North America where grasslands tend to be invaded by Douglas-fir following fire suppression. The use of the phytolith record from terrestrial soils at local sites where the history of human occupation is well-known could help establish the degree of influence of indigenous peoples in maintaining open savannahs on southeastern Vancouver Island.