Remote sensing salt marsh biomass: a dialogue between technical approach and spatial patterns of ecogeomorphological complexity

Salt marshes provide a multitude of ecosystem services while simultaneously being susceptible to habitat loss and degradation in response to climatic and anthropogenic disturbances. Thus, the spatially explicit characterisation, monitoring and sustainable management of these environments is crucial. Particularly as salt marshes are considered a blue carbon ecosystem due to their enhanced ability to produce and sequester organic carbon, acting as long-term reservoirs with a role in climate change mitigation. Since tidal wetlands are bio-geomorphologically intricate, biotic and abiotic coevolution is a key factor in the landscape development. Given the complexity of the processes and the interactions which underlie the system, research in this field requires a multidisciplinary approach. Remote sensing is a facet of this approach, which can enable the cost-effective analysis of salt marsh systems across a range of temporal and spatial scales.

Herein we analyse the application of unmanned aerial vehicle (UAV) based light detection and ranging (LiDAR) and optical sensors as tools to derive salt marsh biomass spatial distribution and structure in the Venice Lagoon (Italy). All validation and empirical relations are based on in situ data. Our results allow us to (1) derive digital terrain and vegetation models (DTM and DVM) and canopy structure using an efficient and open-access procedure; (2) examine the effect which scan angle, post-processing and variation in ecogeomorphological characteristics have on the accuracy of remote sensing results and; (3) further elucidate good practice guidelines for UAV based remote sensing of salt marsh topography and biomass. The results indicate that a linear feedback exists between the LiDAR scan angle and the DTM elevation error, notable for angles above 10 degrees. Furthermore, there is a dialogue between the accuracy of the remote sensing derived data and the spatial patterns driven by salt marsh ecogeomorphological complexity. Thus, characteristics such as vegetation density, elevation transitions between geomorphological structures and differences along the marsh gradient result in spatially variable levels of uncertainty. Overall, our analyses support salt marsh sustainable management as well as enhance the understanding of salt marsh ecogeomorphological complexity.