Today a little more land may belong to the sea, tomorrow a little less. Always the edge of the sea remains an elusive and indefinable boundary.
Rachel Carson
Mangroves
These online mapping products are a part of the larger Marine Habitat Mapping Project CRRF is undertaking to delineate all the habitats between the shoreline and 100 meters deep, to soon have a reliable baseline of habitats across Palau. The goal is to understand the spatial distribution and gather an inventory of shallow and deep-water habitats across Palau at a significantly higher level of accuracy than what currently exists. One of the first habitats to be completed was mangroves. Historical mangrove mapping in Palau has largely been reliant on images (optical data). Instead, CRRF used a ground elevation model to derive mangrove habitat from LiDAR (Light Detection And Ranging) data to gain a more coherent understanding of the mangrove-land interface which is so often hidden under a continuous canopy of green.
WHY WE MAP
Knowing where the island’s mangroves are, and even more so how they are changing, is largely important because mangroves offer protection from storm surge. The strength of waves is reduced as they travel through the forest, which will become more critical for communities with the rising sea level and increasingly frequent and intense storms predicted. Mangrove forests also provide habitat and foraging opportunities for various birds, bats, and vertebrates. There is constant pressure of degradation on mangroves, due to harvest or destruction from humans extracting the lumber or the space for development projects. In this regard, knowing where, and how much mangrove exists, as well as understanding the forest structure, can be incredibly valuable.
Though the need to monitor mangrove loss is often in the spotlight, with accurate mapping and a more optimistic lens, we can also gain an understanding of where mangroves may be expanding. In Palau we see this in some sites, generally in response to increased sedimentation from destabilized soils in the watershed transported via a river. An example of a growing stand of mangrove can be found at the mouth of the Ngerikiil River, in Airai Bay. Here the mangrove edge has expanded from 1946 to 2000 (shown in white) by approximately 2.8 ha, and another 1.4 ha from 2000 until 2021 (shown in yellow). This is visible in a different view, the 2021 mangrove edge with the 1946 (orange) and 2000 (white) edges outlined for comparison.
HOW WE MAP
Fine-scale habitat mapping can be even more important in a small island nation, where resources are sparse. Unfortunately, mapping efforts are usually undertaken at a scale much too large for the target area, designed to capture extensive forest area but often exclude the smaller stands. With minimal suitable area for a habitat such as mangroves which relies on a fairly strict regime of flooding and ebbing tidal inundation, the management of Palau’s existent mangrove stands is even more high-stake. CRRF’s mapping methodology was built to be specific to Palau, relying on an incredibly high-resolution dataset and intensive ground truthing to confirm the results from the model.
The key component of this model was remotely identifying respective elevations of the water-mangrove and the land-mangrove interfaces. To do this, a recent sample of the average tidal range recorded via a local tide gauge was identified and values were converted to match the datum of the elevation data available from the LiDAR survey performed in 2021, by the US Department of Defense. A suitable range of tidal inundation experienced by the local mangroves informed the foundation of the model. To refine the results, a mask was added to exclude areas of high slope (e.g., rock island shorelines) and tree heights below a minimum (to eliminate mud flats and areas of cleared mangrove). The refinements continued with extensive ground truthing. Read more in our Technical Report.
GROUNDTRUTHING
Groundtruthing techniques varied based on the accessibility of the mangroves. Techniques included drone surveys, shoreline observation via boat, observation of landlocked stands by foot, and remote validation based on high resolution aerial imagery collected during the LiDAR surveys. Remote observation from aerial imagery was extrapolated when canopy characteristics matched other areas of confirmed mangrove.
Accessing the inland edge of mangroves usually includes a treacherous walk or partial kayak and a particularly muddy hike. The GPS signal within the forest often drops because of extensive canopy cover blocking satellite connections.
MANGROVE TREE HEIGHTS
Once the mangrove coverage was confidently mapped, the final part of our work focused on characterizing the forest height structure. Along with ground elevation, LiDAR collects information on the “surface” elevation, or more simply, the height of all the features on the ground. Fortunately, this includes tree canopy, and having the data available at a 1-m grid allows the extraction of estimated tree height within the known areas of mangrove. This allowed us to create a canopy height model across all the identified mangrove and provide metrics as to how much of the local mangrove is comprised of “short” trees (1-3 m), “medium” trees (3-6 m), or “tall” trees (over 6 m). This is important because there are records of “dwarf” trees (generally considered below 3 m tall) across Palau and new research shows they may offer a higher rate of carbon sequestration due to the extensive below-ground roots and soil complex.
MEASURING MANGROVE IMPACT
Utilizing measured values of annual productivity for dwarf mangroves (8.9 Mg C/ha yr-1) and tall mangroves (2.0 Mg C/ha yr-1), and with the results of our tree height classification, we estimate the annual carbon sequestration rates may fall between 12,500-15,000 megagrams of carbon each year in Palau. This would be offsetting the carbon emitted by tens of thousands of tourist flights each year!
Looking forward, we hope more research will be done applying the information we gathered on tree heights and mangrove distribution. These data are available to view online at Mangrove Extent & Mangrove Height Classes and data requests can be made through the Palau Automated Land And Resource Information Office (PALARIS).
This mangrove mapping summary for Palau was supported by the Office of Insular Affairs, Department of Interior with further support from the US Forest Service.
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