Marshes Tell Story Of Medieval Drought, Little Ice Age, And European Settlers Near NYC

Plants provide an indicator of climate because the well-being of a species is controlled by the temperature and moisture of a region, and whether those conditions suit a type of plant. That's why if you draw latitudinal or horizontal lines around the world you'll find very similar species growing along those lines, like tropical plants around the equator, or tundra and northern or boreal forest species in a circumference south of the North Pole.

From the pollen record found in sediments in Piermont Marsh of the lower Hudson Valley, a Medieval Warm period was evident from 800 to 1300 A.D. Researchers know this from the striking increases in both charcoal, a sign of dry vegetation and fires, and pollen from pine and hickory trees. Prior to this warming spell, there were more oaks, which prefer a wetter climate.

The study which appeared in a recent issue of the journal Quaternary Research is important for showing how climate in this region has changed due to natural causes prior to human interventions in the area. Dee Pederson, a researcher at Columbia University's Lamont-Doherty Earth Observatory (LDEO), Palisades, N.Y., and Dorothy Peteet, a researcher at NASA's Goddard Institute of Space Studies, New York, N. Y., and LDEO, wrote the study.

During this drought period, a core drilled into the marsh bed showed large influxes of inorganic soil particles, a sign of erosion. Plant roots hold soil in place, but with drought and plant deaths, more erosion occurs.

Droughts like this also make the bay saltier, and evidence of this was found by an increase in salty marsh plants, like saltmarsh cordgrass. The changing salinity of the marshes and estuaries could present future water quality issues in the event of a drought. For example, heading north up the Hudson River, the city of Poughkeepsie draws its municipal water directly from the river. Because the salinity of the river changes with drought, causing saltier water to move further north, salinity changes have the potential to affect the water supply of the city.

During the Little Ice Age from the early 1400s to late 1800s, the vegetation changed again to plants that favored cooler and wetter climates. The core records revealed increases in spruce and hemlock that prefer cooler and wetter climates.

Similarly, when Europeans settled the area they cleared the forests for agriculture. The pollen record reflects this with a vast decline in tree pollen and an increase in pollen from weedy plants like ragweed, plantain, sorrel and dock. Inorganic soil particles also went up following European settlement.

Peteet points out that researchers could use these methods to similarly learn about climate in other parts of the world.

The study was funded by the Hudson River Foundation, the LDEO Investment Fund and NASA.