Vegetation Dynamics

The Quaternary Environments group at Brown has studied how plant taxa have migrated and changed in abundance and association during the late Quaternary (i.e. the past 25,000 years).  Our vegetation reconstructions rely upon fossil pollen records obtained from the sediments of quiet-water environments such as lakes and bogs. The pollen grains can be identified and counted to provide quantitative estimates of past plant abundances, and their age is determined via radiocarbon dating. Pollen records from individual lakes reflect the vegetation history for that region. When many pollen records are aggregated, maps can be created showing the past distribution and abundance of plant taxa.

Click to acess pollen viewer.

Maps of spruce pollen from 21,000 years ago to present in 3000 year intervals.
Green is 20% or higher amounts of spruce pollen.

Click here to access an interactive pollen viewer.

Such reconstructions reveal that, over thousands of years, plant populations in North America and elsewhere have behaved in a highly fluid manner, continuously changing in their distribution and abundance. Migrational histories for no two taxa have been the same.

The primary agent driving these complex vegetation dynamics is climate; over the last 20,000 years the earth’s climate system has switched from glacial to interglacial climates. The key climatic feature of this transition has been an increase in mean annual temperatures, causing the northern hemisphere ice sheets to melt and retreat and inducing the northward migration of many plant taxa.

The pollen record also shows evidence for prehistoric and historic outbreaks of disease among hemlock and chestnut population respectively. About 5400 years ago, hemlock populations decreased in abundance and did not recover until after 3000 years ago. A hemlock looper may have been the cause of this sudden and synchronous decrease in hemlock abundance. Pollen data also record the massive land clearance initiated by European settlers. Earlier records of human disturbance of the vegetation have been difficult to find. (Discover these phenomena using our interactive pollen viewer.)

Some questions currently being addressed by the Quaternary Environments group include

Coastal Studies     Geoarchaeology     Paleoclimates     Vegetation Dynamics