Combining phytolith analysis with historical ecology to reveal the long-term, local-scale dynamics within a savannah-forest landscape mosaic

Abstract

An understanding of the historical range of variability of an ecosystem can improve management and restoration activities, but this variability depends on the spatial and temporal scale at which it is measured. We examined the extent of local-scale variation in vegetation prior to European settlement across a savannah-forest landscape mosaic on southeastern Vancouver Island, British Columbia, Canada. We used phytoliths extracted from soil surface samples to calibrate an index that differentiates open savannahs from closed canopy Douglas-fir forests and then examined shifts in this index with soil depth at seven local sites. We tested whether changes with depth aligned with known vegetation changes based on land survey records from the mid-1800s, and then inferred vegetation change prior to European settlement. The log ratio of astrosclereids (phytolith specific to Douglas-fir) and rondels (phytolith specific to grasses) in soil surface samples accurately distinguished between current vegetation types, and shifts in this ratio with depth were sensitive to known historical changes in most of the cores. Some sites have supported open savannah vegetation for at least 2,000 years, while others that were formerly open have been filled in by Douglas-fir forest. However, this infilling appears to have begun at different times for different sites. Our findings demonstrate that the degree and timing of historical variation in vegetation can differ between local sites within a broader regional landscape that appears relatively stable.