The uncertainty of biomass estimates from modeled ICESat-2 returns across a boreal forest gradient P.M. Montesano a,b,c, ⁎, J. Rosette d,e , G. Sun a,c , P. North d , R.F. Nelson c , R.O. Dubayah a , K.J. Ranson c , V. Kharuk f a University of Maryland, Department of Geographical Sciences, College Park, MD 20742, USA b Sigma Space Corp., Lanham, MD, 20706 USA c Code 618, Biospheric Sciences Branch, NASA/Goddard Space Flight Center, Greenbelt MD 20771, USA d Swansea University, Department of Geography, Singleton Park, Swansea SA2 8PP, UK e Forest Research, Northern Research Station, Roslin, Midlothian EH26 9SY, UK f Sukachev Institute of Forest, Siberian Branch, Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036 Russia abstract article info Article history: Received 24 February 2014 Received in revised form 3 September 2014 Accepted 24 October 2014 Available online 1 December 2014 Keywords: Ecotone LiDAR Radiative transfer model Forest biomass Uncertainty Spacebourne The Forest Light (FLIGHT) radiative transfer model was used to examine the uncertainty of vegetation structure measurements from NASA's planned ICESat-2 photon counting light detection and ranging (LiDAR) instrument across a synthetic Larix forest gradient in the taiga–tundra ecotone. The simulations demonstrate how measurements from the planned spaceborne mission, which differ from those of previous LiDAR systems, may perform across a boreal forest to non-forest structure gradient in globally important ecological region of northern Siberia. We used a modified version of FLIGHT to simulate the acquisition parameters of ICESat-2. Modeled returns were analyzed from collections of sequential footprints along LiDAR tracks (link-scales) of lengths ranging from 20 m–90 m. These link-scales traversed synthetic forest stands that were initialized with parameters drawn from field surveys in Siberian Larix forests. LiDAR returns from vegetation were compiled for 100 simulated LiDAR collections for each 10 Mg · ha -1 interval in the 0–100 Mg · ha -1 above-ground biomass density (AGB) forest gradient. Canopy height metrics were computed and AGB was inferred from empirical models. The root mean square error (RMSE) and RMSE uncertainty associated with the distribution of inferred AGB within each AGB interval across the gradient was examined. Simulation results of the bright daylight and low vegetation reflectivity conditions for collecting photon counting LiDAR with no topographic relief show that 1–2 photons are returned for 79%–88% of LiDAR shots. Signal photons account for ~67% of all LiDAR returns, while ~50% of shots result in 1 signal photon returned. The proportion of these signal photon returns do not differ significantly (p N 0.05) for AGB intervals N 20 Mg · ha -1 . The 50 m link-scale approximates the finest horizontal resolution (length) at which photon counting LiDAR collection provides strong model fits and minimizes forest structure uncertainty in the synthetic Larix stands. At this link-scale AGB N 20 Mg · ha -1 has AGB error from 20–50% at the 95% confidence level. These results suggest that the theoretical sensitivity of ICESat-2 photon counting LiDAR measurements alone lack the ability to consistently discern differences in inferred AGB at 10 Mg · ha -1 intervals in sparse forests characteristic of the taiga–tundra ecotone. © 2014 Elsevier Inc. All rights reserved. 1. Introduction 1.1. Global relevance of the taiga–tundra ecotone At the northern edge of the boreal forest in the taiga–tundra ecotone (TTE), vegetation cover and structure is changing (Elmendorf et al., 2012; Epstein et al., 2013; Myers-Smith et al., 2011; Kharuk et al., 2013; Ropars & Boudreau, 2012). These changes can be subtle yet occur across broad scales, and can alter the magnitude and direction of biome-level and continental scale feedbacks to climate (Bonan, 2008; Bonfils et al., 2012; Chapin, Sturm, & Serreze, 2005; Chapin et al., 2000; Lawrence & Swenson, 2011; Loranty & Goetz, 2012; Loranty et al., 2011, 2013; Myers-Smith et al., 2011; Pearson et al., 2013; Swann, Fung, Levis, Bonan, & Doney, 2010). Broad-scale, but spatially discontinuous and heterogeneous, changes in forest structure are expected in northern Siberia, where the TTE reaches its northern-most limit extending above 72°N (Bondarev, 1997). At spe- cific sites in the TTE canopy closure and expansion of Larix in tundra have been observed (Kharuk, Ranson, Im, & Naurzbaev, 2006). Evidence shows that dark-needle conifers have begun moving into Larix forests Remote Sensing of Environment 158 (2015) 95–109 ⁎ Corresponding author at: Code 618, Biospheric Sciences Branch, NASA/Goddard Space Flight Center, Greenbelt MD 20771, USA. Tel.: +1 301 614 6642. E-mail address: paul.m.montesano@nasa.gov (P.M. Montesano). http://dx.doi.org/10.1016/j.rse.2014.10.029 0034-4257/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse