Abstract
Airborne lidar can observe saltmarshes on a regional scale, targeting phenological and tidal states to provide the information to more effectively utilize frequent multispectral satellite observations to monitor change. Airborne lidar observations from NASA Goddard Lidar Hyperspectral and Thermal (G-LiHT) of a well-studied region of saltmarsh (Plum Island, Massachusetts, United States) were acquired in multiple years (2014, 2015 and 2016). These airborne lidar data provide characterizations of important saltmarsh components, as well as specifications for effective surveys. The invasive Phragmites australis was observed to increase in extent from 8374 m2 in 2014, to 8882 m2 in 2015 (+6.1%), and again to 13,819 m2 in 2016 (+55.6%). Validation with terrestrial lidar supported this increase, but suggested the total extent was still underestimated. Estimates of Spartina alterniflora extent from airborne lidar were within 7% of those from terrestrial lidar, but overestimation of height of Spartina alterniflora was found to occur at the edges of creeks (+83.9%). Capturing algae was found to require observations within ±15° of nadir, and capturing creek structure required observations within ±10° of nadir. In addition, 90.33% of creeks and ditches were successfully captured in the airborne lidar data (8206.3 m out of 9084.3 m found in aerial imagery).
Highlights
This multi-year study investigated how airborne light detection and ranging observations can characterize saltmarsh ecosystems by providing detailed classifications, representations of vegetation and geomorphology, and snapshots of phenological state
To evaluate airborne lidar retrievals of the tidally active hydrological features in the saltmarsh the union of components in the 2015 airborne lidar classification that were found below the height of the marsh platform were compared to aerial imagery (Figure 6)
Vegetation heights from 2015 airborne lidar (0.75 m resolution) produced a Spartina alterniflora volume estimate of 693.3 m3 (±21.6 m3 based on ranging error), which was 83.9% higher than that derived from terrestrial lidar vegetation heights (0.25 m resolution) acquired simultaneously (373.2 m3, ±23.98 m3 based on ranging error)
Summary
This multi-year study investigated how airborne light detection and ranging (lidar) observations can characterize saltmarsh ecosystems by providing detailed classifications, representations of vegetation and geomorphology, and snapshots of phenological state. Each satellite observation can include a highly variable amount of water of varying turbidity, depending on the specific footprint of the observation, the tidal state at the time of overpass, and sediment loading and run-off Extreme inundation events such as storm surges that submerge upland areas of saltmarshes can restructure hydrology [19,56], and cause substantial changes in biology [57] over a short time-scale, further confounding satellite observations. This regional comparability of saltmarshes that belong to distinct watersheds and bays has long made the saltmarshes of New England an appealing subject of study, since they have been subjected to a variety of anthropogenic impacts dating back to the 17th Century [103,104] These anthropogenic impacts include extensive modification of hydrological features, such as the addition of ditches branching off from natural tidal creeks [103]. Terrestrial lidar acquisitions provided validation for some G-LiHT structural products, were used to produce pathfinders for hybrid models to refine satellite interpretations, and were used to form simulations to guide specifications for future airborne lidar data acquisitions in temperate saltmarshes
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