Blog
08.16.2018

Mangroves from Three Perspectives

Blog, General

This work is possible through the support of Pew Charitable Trusts.

Drone Work

By Savannah Longoria

A team from Engineers for Exploration went to Baja Sur California with the Aburto Lab from Scripps Institution of Oceanography (SIO) from July 11-18 to collect aerial and on-the-ground data to document several mangrove sites. The primary purpose of this expedition was to improve and implement new data acquisition techniques from previous expeditions in May 2018 and July 2017, in order to calculate mangrove biomass and classify species effectively and train new people to gather data. This will allow us to support and improve the other stages of our project such as machine learning techniques and data analysis.

An orthomosaic (left) of a mangrove forest created in AgiSoft from imagery taken from a drone.

On this trip, we made specific improvements to our data collection process.  During past expeditions, we flew the drone at 50m and 120m. This time, we added additional perimeter surveys at 10m. This data allows us to look at the mangrove species in high resolution, which we believe is necessary to perform mangrove species identification. The higher resolution data improves training data for our algorithms and helps us understand the resolution required to perform classification.

Additionally, we created orthomosaics (composite image made up several individual photos) of our data in the field. This allowed us to ensure the data the drone is collecting is adequate enough for our research purposes, and advise the drone pilot to collect more data if needed. Likewise, in previous trips, the exposure and color varied in the images due to changes in sunlight throughout the day. By including a color calibration photo at the end of each flight, we standardized these different lightings and created a consistent exposure.

On this trip I learned that drones provide a new opportunity for researchers to gain insight on sites that would otherwise be inaccessible. When we went in the field we learned that mangroves are dense and really hard to navigate.  However, using drone imagery to classify mangrove species and calculate biomass is also its own unique challenge. There are always improvements to be made, and something new to learn about taking our technology out in the field.  We hope for our next expedition to be able to collect ground control points. These ground control points will allow us to have a reference when aligning different layers and establish points with known elevations.

 

Mangroves

By Sara Ye

For the on-the-ground side of the project, our goal was to collect data to conduct biomass estimations of the mangrove forests. The experimental setup was straightforward: we would take transects of three quadrats each, measured from the shoreline, for mangroves at high intertidal, mid-intertidal, and low intertidal zones. We would measure the diameter at breast height (DBH) for all the trees in the quadrat for biomass estimation calculations and measure downed wood by counting the dead branches and roots that intersected a five-meter line from the quadrat. On paper, these boxes and straight lines made plain sense, but once we headed out into the field we quickly realized that our desired data would be nearly as complicated as the tangled mangrove roots we measured.

Experimental setup for on-the-ground surveys.

Our large group pushed our way through some red mangroves (Rhizophora mangle) before reaching the site of the first quadrat, where our team leader, Astrid, showed us how to set up the two-meter-by-two-meter quadrat with transect tape and meter-high PVC pipes at each corner. This involved snaking of our equipment through the mangrove branches. Then, we measured DBH. Since red mangroves pretty much grow from top down, sending prop roots down to the soil from the branches above, identifying an individual red mangrove tree was like a 3-D puzzle, solved by tracing the junctions of the branches and roots down to the oldest part of the tree. Eventually, we recorded the circumferences of all the trees in our quadrat. Finally, the downed wood measurement, which also proved to be less simple than a five-meter straight line from the quadrat. Instead, we spooled five meters of transect tape over and through mangrove branches and maneuvered carefully around the line, so we wouldn’t trample branches into the measurement line.

Measuring downed wood across a 5 meter transect through a dense patch of red mangrove.

Indeed, our first quadrat demonstrated to us that collecting mangrove data in the field was more complicated than our carefully constructed plans in the lab suggested. But over the three days in the mangroves, we soon proficiently set up quadrats at different sites around La Paz, measured DBH, downed wood, and canopy height, all while facing new challenges at each site. At times, high tide rose very quickly and chased us out of the mangrove roots. Other times, we had to find ways to go over, under, and through mangroves of different sizes. We learned tricks to keep our water cool despite the heat, the importance of dive booties, how to climb through dense mangroves thickets without falling or getting stuck, among many other experiences. Overall, our experience in the field, though challenging, was extraordinarily fulfilling. I know I came back from the field feeling that I had gained some more insight about mangroves, which augmented what I gleaned from reading papers.

 

Bahía Magdalena

By Kathy Qi

For the next part of our trip, we traveled to Puerto San Carlos in Magdalena Bay with a much smaller team than we started out with. The main purpose for this segment was to meet and train with the local office of Mexico’s National Commission of Protected Natural Areas (known in Spanish as CONANP.) In addition, our team would also be surveying past sites through on the ground measurements and drone work with improved methodologies developed on the last trip. I learned that mangrove sites could vary significantly by location even in La Paz, so I was interested in seeing how the forests grew in a different region.

Location of our two study sites for this trip: La Paz and Puerto San Carlos.

After a long drive and a night’s rest at the hotel, our team went to work at one of the larger sites that we had previously flown at. Almost immediately, we noticed that the geography of this region was distinct from La Paz as the terrain was considerably flatter. The difference showed its effect in the many of the black mangrove (Avicennia germinans) leaves, as they exhibited fewer salt crystals compared to the trees from our previous sites. Since there was almost no change in elevation throughout the forest expanse, the smoother transition of salinity seemed to cause more mixing of the three species. This interesting observation can be seen with our aerial data and is an example of the essential role fieldwork has in distinguishing the fascinating patterns and anomalies of wetland ecosystems. We continued to conduct aerial and ground surveys for a couple more regions, collecting more data to analyze and measure in the lab.

A clear line separates red and black mangroves in La Paz.
Red, black, and white mangroves growing along the edge in Puerto San Carlos.

A main goal for this project is to have local actors fly drones over mangrove forests and provide us the aerial imagery to be constructed into models that will provide us important information for research. During the trip, we met with CONANP share our work doing aerial surveys with the drone and discuss establishing a working collaboration between our groups. This collaboration could provide future connections to increase our potential for collecting data efficiently and promote better management efforts of mangroves together. From this experience, I learned an important cultural lesson that sometimes research not only requires the knowledge and tools for acquiring the data we need, but also create lasting partnerships with regional actors that can help improve our research.

 


About the authors:

Savannah Longoria is an undergraduate at UC Berekley majoring in Geography, and is part of E4E’s summer REU program.

Sara Ye is a Biology undergraduate at UC San Diego studying mangrove soil.

Kathy Qi is an undergraduate at UC San Diego majoring in Marine Biology and minoring in computer science, and is part of E4E’s summer REU program.

From left to right, Sara, Savannah, and Kathy demonstrating an essential part of field research: teamwork.

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