By Adam Voiland From Earth Observatory

We recently posted several striking pairs of satellite images of Caribbean islands before and after they were hit by Hurricane Irma. The images show lush, green vegetation turning a dark shade of brown. We were curious about what exactly caused the damage and how quickly vegetation might recover, so we checked in with Edmund Tanner, a University of Cambridge ecologist who has authored multiple studies about vegetation damage and recovery after Caribbean hurricanes.

EO: What were the primary mechanisms that turned the islands brown? Leaves blown away by the wind? Mud and debris covering them up? Salt spray killing them off?
Away from the sea (100 meters or so), most of the green to brown is the loss of green leaves because they were blown off. Native vegetation on these islands has been through hundreds of hurricanes since the last major change of climate (10,000 years ago, the end of the most recent ice age) and have been naturally selected to lose leaves and small branches and re-sprout. I doubt if it is mud since the fairly heavy rain will have washed that off. Most of the water falling on the vegetation would have be fresh—rain—not sea spray.

EO: How long will it take for the vegetation to recover?
My guess is the greening up in the lowlands will take 6 months, with a lot happening in the first 3-4 months. You can probably measure that from satellite images. On the ground, there will be lots of sprouts on tree trunks. For larger trees, the surviving branches will produce leaves and small branches and slowly these will shade out and kill the epicormic shoots produced on the lower parts of trunks
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EO: What about storm surge? Was that a major factor in killing trees and other vegetation?
Salt water from storm surge may have killed trees whose roots were inundated with it. Those trees will take much longer to recover because the soil will need to be desalinated naturally by rain, and seeds will have to germinate and grow. The areas involved are not likely to be large—a fringing zone of a few hundred hectares in some places.

EO: How useful are satellites in assessing vegetation damage caused by hurricanes?
Very useful. Especially for measuring the change in green from before the hurricane to brown immediately after the hurricane, to greening up over weeks and months after the hurricane. Modern analyses of satellite images could also show the decrease in individual tree canopy size if trees lost big branches and only regrew them slowly.

IMAGES:

NASA Earth Observatory images of the virgin islands by Joshua Stevens, using Landsat data from the U.S. Geological Survey.

NASA Earth Observatory images of Barbuda by Joshua Stevens, using Landsat data from the U.S. Geological Survey.

For more on this story go to: https://earthobservatory.nasa.gov/blogs/earthmatters/2017/09/18/why-caribbean-islands-went-brown-and-how-long-will-they-stay-that-way/