Have you ever felt your feet get heavy and your legs sting and sweat? If so, you've probably visited a mangrove forest. In the mangroves it is difficult to walk because the mud is very sticky. It wraps around your boots, making them heavy. Sweat attracts hundreds of insatiable mosquitoes and no amount of repellent will keep them at bay. It is like working in hell and it is for this very reason that we look for more efficient ways to monitor these enchanted sites.
Estefania and I grew up in Ecuador, a country where mangroves have been cleared to make way for shrimp farms, new real estate development, roads and more. At first we thought that the mangroves were forests no different from others. But with the passage of the years we have realized that they are much more.
Mangroves are tropical forests that grow in the mouths of rivers and are recognized, mainly, for their strange roots and their ability to live between salt water and fresh water. Mangroves grow in more than 105 countries and yet only cover an area of 137,000 km2, that is they wouldn’t even cover Uruguay's surface (which is approximately 176,000 km2). Despite their small extension, the goods and services provided by these forests have a global reach.
Mangrove forests help fight climate change, as they sequester more carbon than any other type of rainforest. Mangroves use carbon dioxide – emitted by factories, vehicles and thermoelectric plants – as an energy source and store carbon in their leaves, branches, trunks and roots. When the leaves and branches fall from the trees, they are usually buried in the mud where they decompose slowly, leaving the carbon trapped instead of contributing to the warming of the earth. But mangroves are much more than carbon sinks; These forests act as natural barriers and protect coastal communities from tropical storms and storm surges. The intricate roots of trees act as breakwaters and thus prevent large human and material losses in coastal communities.
Obtaining field data about mangrove forests has several advantages. For one, we can see how trees grow, flower and reproduce. But obtaining this information requires a lot of time, money and effort, and we can only visit a limited number of sites. For these and other reasons, we resort to satellite images to monitor mangroves.
Satellite images are revolutionizing the way we look at the earth and do science. From the temperature of the water to the height of the trees, satellites help us monitor these rainforests. Although our visits to the field are important, satellite images play a critical role in monitoring mangroves for several reasons: 1) satellite images acquire information over large areas (thousands of kilometers) of mangroves each day; 2) satellites know no borders and provide us with information about ecosystems around the world.
But perhaps the greatest advantage of using satellite images is that they provide us with an unprecedented look at the past. That’s because there are satellites that have been monitoring the surface of the earth for more than 30 years that allow us to see, month by month, the changes that have taken place in the mangroves. To our eyes, mangroves are evergreen forests; that is, they do not defoliate in winter. But satellites can detect minute changes in the greenness of the mangroves that indicate the beginning and end of their growth period.
Our research involves using thousands of satellite images taken between 1988 and 2018 to see how and when the greenness of the mangroves changes. When the mangroves are at their peak, we see a peak in satellite information that is related to the increase in the number of leaves that are growing in the trees and, vice versa, the mangroves look 'less green' when they lose many of their leaves. Knowing when mangroves are 'more' or 'less' green allows us to understand the impact of climate change on these forests.
To identify the periods of growth of the mangroves, we use hundreds (or thousands) of satellite images that we obtain for free from the United States Geological Survey (USGS). We sort them by date and place them one on top of the other as if they were the ingredients of a hamburger. When we have all the images stacked, we extract the information from each pixel and analyze it using statistical models and different visualizations.
In Latin America, there are several examples of the use of satellite images for the monitoring of mangroves. For example, Juan C. Mejía-Rentería at ECOMANGLARES in Cali and his colleagues have found that, despite the effects of climate change and other socio-economic conflicts, the area of the Colombian mangroves has remained relatively stable since 1966. In 2018, López-Angarita and his team of researchers used satellite images to compare deforestation rates in Colombia, Panama and Costa Rica. The team found that mangroves are cleared to make way for watermelon, melon, sugar cane and oil palm cultivation as well as for pasture for livestock.
Day after day, mangroves protect and feed millions of people in Latin America. Daily we obtain satellite images that help us describe the changes that occur in these ecosystems and provide us with unique information that we can use to protect them.
Nicolas Younes and Estefania Erazo-Mera work in the Centre for Tropical Environmental and Sustainability Science and School of Science and Engineering, James Cook University, Townsville, QLD 4814, Australia.