Estimating boreal shrub cover and biomass: Scaling from laborious field measurements to 3D structural quantification using UAV and airborne lidar

Woody shrubs are an increasingly dominant feature of boreal forest landscapes but are not currently included in Forest Inventory and Analysis (FIA) estimates of aboveground biomass. In Alaska, alder and willow species can grow to the size of small trees (≥20 cm DBH) and are thus an important terrestrial carbon sink. Moreover, shrubs are the first colonizers of disturbed or marginal lands, adding both carbon and nitrogen to build soils for future forests. Indeed, shrub cover expansion is one early indicator of climate-driven shifts in community composition in boreal forests and tundra.

Explicitly accounting for the shrub component of aboveground biomass using remote sensing has proven difficult due to a lack of in-situ measurements and allometric equations relating diameter root collar (DRC) measurements to stem volume. In the present study, we make use of contemporaneously collected DRC measurements, FIA plot-level structure from motion (SfM) from UAV, and high-resolution Goddard Lidar Hyperspectral Thermal (G-LiHT) imagery to develop relationships between shrub structure as viewed from above, and carbon storage. Data collection was carried out in summer 2018 at approximately 40 FIA subplots along gradients of elevation and shrub cover in Chugach State Park east of Anchorage, AK. We processed the highly-overlapped UAV photos into dense (>1000 pts/m²) RGB-colored point clouds using SfM techniques allowing for estimation of shrub patch volumes and exclusion of the tree component of plot biomass. Volume estimates from UAV and G-LiHT were compared with field-based estimates to understand the increased uncertainty from scaling up to remote sensing measurements.

Preliminary results indicate the potential for SfM and lidar-based estimates of shrub biomass particularly in sub-alpine environments where overstory forest cover is minimal. Given a warming-driven increase in high-elevation habitat suitability, remote sensing-based inquiry will be useful for examining the speed, extent, and variability of shrub colonization in a manner not possible from the ground.