You can see them from space— swaths of bleached trees that were drowned by flooding or poisoned by salt water as sea levels rose. But satellite imagery isn’t detailed enough to tell the whole story. That’s why UVA grad student Henry Yeung started using aerial images and artificial intelligence to identify individual trees.
"We mapped out around 10 million dead trees across the coastal areas."
They are, he says, most common in the southeast.
"Because those are more flat areas with a very gentle slope, but in Maine or like the New England area the places are more steep," Yeung explains.
He and UVA Professor Xi Yang confirmed the accuracy of their AI method with a field trip to the Albemarle-Pamlico Peninsula in North Carolina.
"Actually, it’s great to see that what he found from the satellite matches what we see on the ground," Yang says.
Along with colleagues from Duke and the University of North Carolina, they took soil samples and leaves.
"If there’s no salt in the soil, and those trees are dead, most likely it was caused by just freshwater flooding," Yang says.
Whether death is caused by too much fresh water, sea lever rise, saltwater infiltrating ground water or sea water spray Yeung says trees die, making climate change worse.
"When the trees decompose, they will release that carbon back into the atmosphere, so you start losing that carbon back."
Yang notes that in some cases trees are protected from saltwater by man-made structures.
"You drive alongside the coast of North Carolina, you’ll see one side a lot of dead trees, and the other side not many."
And in other cases, humans have made it easier for ocean water to infiltrate.
"In the past when we tried to empty water from land for crops in agriculture fields, we created a lot of channels, and those channels take water out from the land to the sea, but with sea level rise, you actually get those as conduits for salt water to come in, and – actually – it causes a lot of tree mortality," Yang explains.
Their findings, to be published in the journal Science, could spark mapping of ghost forests around the world and lead scientists to study just how much CO2 is released when living forests become ghosts.
