The uncertainty of modelled ICESat-2 returns across a sparse boreal forest structure gradient

We use a radiative transfer model to examine the uncertainty of the planned ICESat-2 single photon LiDAR instrument to measuring vegetation structure across a northern Siberian Larix forest gradient in the taiga-tundra ecotone. Model simulations provide insight into future spaceborne LiDAR measurements of vegetation in two ways. First, they show how the next generation of spaceborne LiDAR measurements of sparse vegetation structure will compare to those from the previous generation. Second, the simulations demonstrate how measurements from single photon LiDAR, which provides a fundamentally different measurement of vegetation structure compared to previous LiDAR systems, may perform across a boreal forest to non-forest structure gradient. We use a modified version of the FLIGHT radiative transfer model to simulate the acquisition parameters of ICESat-2 by using a 10m diameter footprint spaced every 70cm. We analyze modelled returns along 20m-70m transects through simulated forest stands initialized with parameters drawn from field surveys in Larix forests. Canopy and ground returns are compiled for 100 simulated transects for each 10 Mg ha-1 interval in the 0-100 Mg ha-1 above-ground biomass density forest gradient (AGB), and canopy height metrics are computed. We examined the uncertainty associated with distribution of these metrics within each AGB interval across the gradient. Preliminary results suggest that the theoretical sensitivity of single photon LiDAR gathered along shorter transects across this gradient do not discern significant differences in vegetation structure in sparse forests characteristic of the taiga-tundra ecotone.