Mangrove forests are some of the most biologically diverse and productive ecosystems on the planet. Made up of trees and large shrubs at the boundary between land and sea, these coastal wetlands are important carbon sinks and are home to a wide array of plant and animal life. Millions of people depend on mangroves for fishing, tourism, and protection against storm surges from tropical cyclones.
But agricultural development, urbanization, sea level rise, and damage from wind and tropical cyclones threaten the health of mangroves in some places. Tracking the health of these ecosystems by measuring forest extent and tree height is critical to informing sustainable management of mangroves and the services they provide.
Researchers use a number of remote sensing instruments to detect the presence of mangroves and to measure their size and shape, and new technologies allow researchers to track the height and carbon content of mangrove canopies.
Mangroves skirt tropical and subtropical coastlines in narrow bands, and optical imagery is often used to map the extent of these important forests and distinguish them from surrounding ecosystems. But these coastal areas are often shrouded by cloud cover which makes it difficult for researchers to get images on a regular basis.
One of the most cited maps of the world’s mangroves was developed by the U.S. Geological Survey (USGS) in 2011. Chandra Giri and team gathered over 1,000 cloud-free images of tropical and sub-tropical coastlines from the joint NASA/USGS Landsat 5 satellite and used them to map the extent of mangroves across the globe. Giri’s research mapped mangroves over 118 countries, occupying 137,760 square kilometers of coastal areas. This map is often used as a baseline in many studies to further investigate mangrove characteristics with other remote sensing tools.
Synthetic Aperture Radar
Synthetic aperture radar (SAR) instruments complement optical imagers because of their different properties. As active sensors, SAR instruments emit electromagnetic waves in the microwave portion of the electromagnetic spectrum, and they measure how much of that radiation makes it back to the sensor. The differences in energy return tells researchers about the characteristics of the surface.
These instruments are able to penetrate cloud cover and work in both day and night conditions, making them a useful complement to optical sensors that are limited by cloud cover and sunlight. SAR instruments can also penetrate forest structure in a way that optical imagery can’t. With optical sensors, the returned signal is an indication of the chlorophyll concentration of a leaf. Using microwave sensors, the returned signal is proportional to the size, shape, and water content of the leaf. (Read more about this in The SAR Handbook: Comprehensive Methodologies for Forest Monitoring and Biomass Estimation.)
The SRTM instrument used C-band microwaves, which are about 5.6 centimeters in length. Small microwave bands, such as C (5.6 cm) and X-band (3.1 cm) microwaves, interact with small objects such as branches and scatter within tree canopies. Because of this scattering, SRTM DEM measurements in forested areas actually correspond to the height between the ground elevation and the top of the canopy.
Longer L-band (24 cm) microwaves can penetrate deeper into forest canopies. SAR measurements using L-bands can be used to distinguish between different types of mangroves, based on their root structure. Red mangroves have large, interwoven root systems, called stilt roots, that result in different scattering mechanisms than black and white mangroves with smaller roots.
Simard used SRTM-derived mangrove height within the mangrove areas mapped by Giri to estimate forest aboveground biomass and carbon density. His team estimated that mangroves store about 1.75 billion metric tons of carbon above ground.
But SAR doesn’t provide an accurate estimate of tree canopy height. Simard and colleagues used lidar data from the Geoscience Laser Altimeter System (GLAS), the primary instrument aboard NASA’s Ice, Cloud and land Elevation Satellite (ICESat-1) satellite (in orbit from 2003 to 2009), to get measurements of maximum canopy heights for mangroves.
Lidar works much the same way as radar instruments, except that lidar instruments use electromagnetic waves in the optical and infrared wavelengths instead of microwaves.
“The reason lidar is better than radar at getting canopy heights is because it’s operating in the near-infrared, in this case, and interacts with everything it encounters, starting with the leaves at the very top of a canopy. Radar, on the other hand, saw through the leaves, and interacted with larger branches,” said Simard. “Lidar gives you a really good measurement of the actual height of the trees.”
Yet there are downsides to lidar. So far, spaceborne lidar datasets don't provide global coverage.
“You cannot make images out of spaceborne lidar data yet because the measurements are sparse. Lidar measurements are also sensitive to clouds, so the resulting dataset is even more sparse,” said Simard. “We collected all of the lidar data that intersected Giri’s mangrove map and used that to calibrate the SRTM data to get a global map of mangrove canopy height.”
Simard’s study found that mangroves are significantly taller than previously reported. Equatorial West Africa and South America are hot spots for giant mangroves. The tallest mangroves were found in the Gabon Estuary in Africa and towered 65 meters (213 feet) tall.
Simard’s group is now working on developing new maps of mangrove size and extent by using lidar data from NASA’s Global Ecosystem Dynamics Investigation (GEDI) and ICESat-2, and radar data from the TerraSAR-X add-on for Digital Elevation Measurements (TanDEM-X), a German Space Agency mission. His research group is also looking into drivers of mangrove loss around the world.
Simard, M., Fatoyinbo, L., Smetanka, C. et al. 2019. Mangrove canopy height globally related to precipitation, temperature and cyclone frequency. Nature Geoscience 12, 40–45: doi:10.1038/s41561-018-0279-1.