Spaceborne height models reveal above ground biomass changes in tropical landscapes

Michael Schlund,Martyna M Kotowska,Fabian Brambach,Jonas Hein,Birgit Wessel,Nicolò Camarretta,Mangarah Silalahi,I Nengah Surati Jaya,Stefan Erasmi,Christoph Leuschner,Holger Kreft

doi:10.1016/j.foreco.2021.119497

Abstract

The area-wide estimation of aboveground biomass (AGB) and its changes as a proxy for the sequestration and emission of carbon are currently associated with high uncertainties. Here we combined interferometric synthetic aperture radar (InSAR) height models derived from TanDEM-X with repeated ground-based inventories from the years 2012 and 2019 to estimate InSAR height and AGB changes in a structurally diverse and dynamic landscape in Sumatra, Indonesia. The results suggested that the InSAR height models were highly accurate and the relationship between InSAR height and AGB change resulted in a coefficient of determination R2 of 0.65 and a cross-validated root mean square error (RMSE) of 2.38 Mg ha−1 year−1, equivalent to 13.32% of the actual AGB difference range. The estimated AGB changes with TanDEM-X were further related to the initial canopy height and fire activities in the study area. Initial canopy heights and the occurrences of fires had a significant effect on the AGB change. In general, low canopy heights tend to be associated with increasing AGB over time, whereas high canopy heights tend to be associated with stable or decreasing AGB. As expected, fires had a negative impact on the AGB changes being more pronounced in forest areas compared to oil palm concessions. The results of this study are relevant for the utilization of spaceborne InSAR height models and its potential to estimate canopy height and AGB change on large spatial scales. It was demonstrated that these changes can be related to their sources and ecosystem processes. This AGB change estimation technique can be used to model the impacts of fires on AGB change and carbon emissions, which are important for sustainable forest management.

Highlights

Tropical landscapes are increasingly dominated by humans, even in formerly inaccessible tropical wilderness areas (Lewis et al, 2015; Venter et al, 2016; Watson et al, 2018)
The TanDEM-X interferometric synthetic aperture radar (InSAR) height achieved an root mean square error (RMSE) of 3.46 m in com parison with the light detection and ranging (LiDAR) height in non-vegetated areas
Our study illustrates the high potential of TanDEM-X height models and their change to estimate aboveground biomass (AGB) change, which was confirmed by a significant relationship between the TanDEM-X InSAR height and AGB changes with an R2 of 0.65

Summary

Introduction

Tropical landscapes are increasingly dominated by humans, even in formerly inaccessible tropical wilderness areas (Lewis et al, 2015; Venter et al, 2016; Watson et al, 2018). The extent of this offset and the overall carbon balance of most human-modified tropical landscapes is unknown, as the dy namics of land use and carbon depend on a complex interplay of man agement decisions such as plantation conversion, selective logging, clear-cutting or slash-and-burn clearance with ecosystem processes such drought-induced mortality or pathogen outbreaks (Hudak et al, 2012). A better understanding of the influence of these drivers on AGB change is relevant for land use management and can be included in carbon modelling to predict future landscape dynamics and de velopments. This is important in structurally diverse and dynamic landscapes (Brun et al, 2015)

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