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Paleoclimate
Marshes record climate changes

The Hudson River watershed supplies water to the 8 million inhabitants of New York City, not to mention millions more throughout the watershed. As the climate changes, so too does the water in the river and its tributaries, and the surrounding vegetation. Research coming out of a marsh near the mouth of the Hudson is now providing insight into how the ecosystem has evolved, based on a core that dates back 1,350 years, and could help planners better manage the system in the future.

Estuarine marshes provide “nice records” of climate change because they keep pace with sea-level rise and have higher rates of sedimentation than lakes or bogs, says Dorothy Peteet, a researcher at NASA Goddard Institute of Space Studies in New York City and Columbia University’s Lamont-Doherty Earth Observatory. Marshes, especially the Piermont Marsh of the lower Hudson, also integrate what’s happening throughout the watershed, recording both local and upland changes, Peteet says.

To understand what Piermont Marsh has recorded, Peteet and colleague Dee Pederson drilled a 14-meter core in the mucky peat, a record that reached back about 6,000 years, as reported in the May issue of Quaternary Research. So far, the researchers have examined the top 3 meters of the core, extending back to about A.D. 650, and what they have already seen is surprising, Peteet says.

The pollen, spores, charcoal, macrofossils and marsh sediments in the core reveal major climate change and human effects on the ecosystem, starting with the Medieval Warm Period around A.D. 800. Prior to the warming, beech, oak and hemlock trees characterized the area, Peteet says — species that thrive in cooler, wetter climates.

Then around A.D. 800, the core shows a distinct shift to pine and hickory trees, which prefer drier, warmer climates, and a marked increase in charcoal, indicating fires during a drought. Local plants also shifted from freshwater species to saltwater marsh plants, indicating that as the drought continued, a wedge of saltwater from the Atlantic made its way up the river. The drought persisted for about 500 years, Peteet says, when suddenly the climate appeared to shift again.

The core recorded a forest transition between the years 1292 and 1418, Peteet says. From about 1418 to 1850, roughly when the Little Ice Age occurred, the core recorded distinctly less charcoal, more pollen and a transition to trees preferring cooler, wetter conditions. Starting in the late 1680s, however, another change occurred — tree populations started declining overall, and ambrosia, ragweed and other marsh grasses jumped from 5 percent of the pollen record to 20 to 30 percent, she says. That change directly correlates to the arrival of European settlers, who began clearing the land, as well as introducing invasive species. And the trend only increased over the next centuries.

“Four hundred years ago, this area was filled with incredibly lush and beautiful forests with lots of fish in the rivers,” Peteet says, and today, trees are sparse. Instead, marshy plants like cattails and reedgrass, both invasive species, “grow wild like a jungle.” Some of this change, she says, is due to natural climate variability and some is due to human influences. Regardless, however, it is obvious that the ecosystem can adapt quickly to new climates, says Debra Willard, a researcher at the U.S. Geological Survey in Reston, Va.

High-resolution records such as this “show us how ecosystems respond to climate change, and the more studies we do, the more we understand” the whole region, Willard says. For example, if the climate in New York warmed significantly, as it did during the Medieval Warm Period, scientists might expect to see a salt wedge work its way up the river all the way to Poughkeepsie, threatening the town’s freshwater supplies. Thus, understanding the Hudson’s past may help scientists predict what will happen to the system as the climate changes in the future and help community managers plan for natural resource challenges, she says.

Megan Sever

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