Analysis of airborne LiDAR surveys to quantify the characteristic morphologies of northern forested wetlands

A new technique for quantifying the geomorphic form of northern forested wetlands from airborne LiDAR surveys is introduced, demonstrating the unprecedented ability to characterize the geomorphic form of northern forested wetlands using high-resolution digital topography. Two quantitative indices are presented, including the lagg width index (LWI) which objectively quantifies the lagg width, and the lateral slope index (LSI) which is a proxy measurement for the dome shape or convexity of the wetland ground surface. For 14 forested wetlands in central Ontario, Canada, northwestern Ontario, Canada, and northern Minnesota, United States, these indices were systematically correlated to metrics of topographic setting computed from LiDAR digital elevation models. In particular, these indices were strongly correlated with a Peatland Topographic Index (PTI, r(2) = 0.58 and r(2) = 0.64, respectively, p <= 0.001) describing the relative influence of upslope contributing area on the hydrology and biogeochemistry of individual wetlands. The relationship between PTI and the LWI and LSI indices was interpreted as geomorphic evolution in response to the spatially varying influence of upslope runoff on subsurface hydrochemistry. Spatial patterns of near-surface pore water chemistry were consistent with this interpretation. Specifically, at four wetland sites sampled extensively for pore water chemistry, the mean and variance of near-surface pore water methylmercury (MeHg) concentrations were higher within the zone of enhanced upland-wetland interactions, as inferred from the LiDAR-derived LWI estimates. Use of LiDAR surveys to measure subtle topographic gradients within wetlands may therefore help quantify the influence of upland-wetland interactions on biogeochemical cycling and export in northern forested landscapes.