In low places bordering the Chesapeake Bay, it is easy to spot dead, leafless and branchless trees. These ‘ghost forests’, their gray trunks pointing skyward from swamps or open waters, silently bear witness to the creeping flooding of coastal lands as sea levels rise.
The transition from verdant forest to thatch of trunks and snags is so gradual that it is often not noticed until it is obvious. But now, a team of scientists are undertaking what they expect to be a decade-long study of the transition by repeatedly flooding a pair of forest plots at the Smithsonian Environmental Research Center in Edgewater, MD.
Starting in June, each 2,000 square meter plot should be periodically watered with 80,000 gallons of water. Fresh water will be obtained, the equivalent of a 6 inch downpour in 10 hours. This is several times the normal rainfall for the region, but it simulates the type of extreme downpour that is expected to become more common with climate change.
The other will get brackish water pumped from the nearby Rhode River, mimicking episodic flooding from storm surges that gradually reach further inland.
They will monitor changes in the trees on both plots, which are mainly tulip poplar, red maple and American beech. But they will also watch for less visible chemical signs of stress and decline.
“We are interested in how the first storm surge that is pushed into a forest that has never been exposed to seawater begins to impact the biology and ecology of these upland forests. “, said Pat Megonigal, ecosystem ecologist and SERC associate. Director of Research. “We know just one isn’t going to kill the forest, so we’re going to run these 10-hour events… with increasing frequency, basically simulating what we hope will be a 10-year period of sea level rise and of storms moving further. and further into the forest.
More than 150 square miles of forest in the Chesapeake region has turned into swamp since the mid-1800s, according to a study, and the rate of forest loss has accelerated dramatically. Climate change combined with progressive land subsidence around the bay is causing sea levels to rise faster than in other places, killing trees and other vegetation that cannot tolerate salt water.
Other studies have identified changes in coastal plant types as early warning signs of saltwater intrusion. This project aims to search for even older clues.
“We know the forest will die,” Megonigal said. “What interests us is: what is the tipping point? And also, what are the mechanisms that dictate how the forest dies and how it begins to turn into a swamp?
It took three years of planning and preparation to launch this large-scale field experiment, which is aptly called STORM (Terrestrial Ecosystem Manipulation to Probe the Effects of Storm Treatments). This is part of a larger research project called COMPASS (with an even longer full name), funded by the US Department of Energy. With fieldwork in the Great Lakes and Bay watersheds, COMPASS aims to understand how coastal ecosystems respond to short- and long-term changes.
Researchers will deliver fresh water to the forest from large storage reservoirs nearby that must be replenished by truck. They will pump water from the Rhode a short distance upstream to a 20,000 gallon inflatable bladder that will be filled four times during each simulated flood. Water flows evenly into the forest plots through an irrigation system. A network of sensors installed on the trees and in the ground feeds a computer with data, allowing researchers to follow the impacts in real time.
As high-tech as that sounds, it involved some good old-fashioned parts shopping. “I spent a lot of time at Home Depot trying to find the right thing,” said Anya Hopple, a postdoctoral researcher at Pacific Northwest National Laboratory who is leading the project. His lab, which is part of the Department of Energy, is partnering with the Smithsonian Center and several other institutions on the project.
Home Depot produced the caulking needed to seal “flux chambers,” small boxes attached to tree trunks to capture the methane and carbon dioxide that trees absorb or emit through their bark.
Some tree trunks are also equipped with pairs of needles, which measure the flow of sap under the bark. Soil and soil probes measure soil characteristics including temperature, water content and methane before and during flooding.
Like carbon dioxide, methane is a gas that contributes to global warming. Depending on conditions, forests can absorb methane or release it. When the soil is dry, bacteria in the soil consume methane “like little natural gas stoves,” Megonigal said. But when the ground is flooded, different microbes produce methane, releasing it into the atmosphere.
Last year when tested with fresh water, Megonigal said he was surprised at how quickly the ground began to produce methane, as water pooled on the forest floor and some began to run off.
Researchers do not expect to see dramatic changes in the patch of forest that will be flooded with fresh water. These trees can actually benefit from flooding because their growth is usually limited in late summer when there is usually little rain.
The patch to saturate with brackish water should be a different story. The salt stays in the ground when the water drains off, impairing the ability of tree roots to absorb the water they need to grow and survive.
“Each time you do that, you add a little more salt to the system,” he said, “and there will be a tipping point where the trees here are not adapted to stress and they will start to decline. .”
Information from this study will be used in computer modeling to help scientists refine predictions of the speed and extent of upland forest responses to extreme weather and sea level rise.
“It’s not going to stop sea level rise,” Hopple said. “It’s just going to help us understand how it plays out.”