title: ICESat-2 measurements of forest structure &amp; their utility for driving large-scale albedo simulation
Creators: name: Purslow, Matthew Alastair Bell
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: ICESat-2
subject: Plant Structural Traits (PSTs)
subject: Canopy:ground reflectance
subject: Radiative transfer modelling
subject: Land surface albedo

description: Forests are a vital part of the Earth system, with many processes depending on their distribution and structure. The ICESat-2 spaceborne lidar mission collects global measurements of 3D structure, with potential applications in radiative transfer modelling, biomass mapping, biodiversity assessment and more. However, the ICESat-2 mission currently processes only canopy height and Relative Height metrics in its vegetation product, which limits the utility of ICESat-2 data in many fields. In this thesis, I assess how measurements of additional stand-level Plant Structural Traits (PSTs) like canopy cover and Plant Area Volume Density can be extracted from data collected by ICESat-2. I go on to test the applicability of such measurements to the challenge of land surface albedo modelling over boreal forest. In Chapter Two, I explore how to extract the input parameters required for the calculation and interpretation of canopy height, canopy cover, Relative Height (RH) metrics and Plant Area Volume Density (PAVD) from data collected by ICESat-2. In particular, I investigate how the ratio of canopy and ground reflectances - key variables for the accurate estimation of these stand-level PSTs - can be estimated. I demonstrate that it is possible to calculate the canopy:ground reflectance ratio at site scale from ICESat-2 data with accurate photon classification. In Chapter Three, I apply these canopy:ground reflectance ratios to simulate ICESat-2 data from discrete-return Airborne Laser Scanning (ALS) data at five sites. I calculate canopy height, canopy cover, RH metrics and PAVD profiles from this simulated ICESat-2 data and compare these with the same PSTs calculated from colocated data collected by ICESat-2 itself. As ALS is known to be sensitive to these stand-level PSTs, similarity between simulated and observed ICESat-2 metrics shows that ICESat-2 is sensitive to PSTs where the canopy:ground reflectance ratio is well constrained. In Chapter Four, I assess whether ICESat-2 canopy height and cover can be used to drive the implicit 3D SPARTACUS radiative transfer model to simulate land surface albedo over boreal forest in Finland. I find that, while SPARTACUS is able to simulate the full range of land surface albedo values observed by MODIS, driving it with ICESat-2 canopy height and cover does not offer an improvement over the more simple Sellers two-stream radiative transfer model. However, I identify mapping of snow cover and Clumping Index as key areas where ICESat-2 measurements could aid in improving the modelling of land surface albedo. This thesis concludes that ICESat-2 is sensitive to canopy structure and can be used to retrieve measurements of canopy cover and Plant Area Volume Density alongside the canopy height and Relative Height metrics currently offered by ICESat-2. Correction for canopy:ground reflectance ratio is necessary to minimise bias in these PST measurements. Using ICESat-2-derived canopy height and cover to drive an implicit 3D radiative transfer model does not offer improved land surface albedo simulation compared with a more simple two stream model. However, ICESat-2 PST measurement could contribute to land surface albedo modelling through use for snow mapping, alongside potential contributions to many other fields such as biodiversity assessment, fire risk mapping or aboveground biomass measurement.