Tropical-Forest Structure and Biomass Dynamics from TanDEM-X Radar Interferometry

Robert Treuhaft,Maxim Neumann,Yang Lei,João Santos,André Almeida,Michael Keller,Fabio Gonçalves

doi:10.3390/f8080277

Abstract

Changes in tropical-forest structure and aboveground biomass (AGB) contribute directly to atmospheric changes in CO 2 , which, in turn, bear on global climate. This paper demonstrates the capability of radar-interferometric phase-height time series at X-band (wavelength = 3 cm) to monitor changes in vertical structure and AGB, with sub-hectare and monthly spatial and temporal resolution, respectively. The phase-height observation is described, with a focus on how it is related to vegetation-density, radar-power vertical profiles, and mean canopy heights, which are, in turn, related to AGB. The study site covers 18 × 60 km in the Tapajós National Forest in the Brazilian Amazon. Phase-heights over Tapajós were measured by DLR’s TanDEM-X radar interferometer 32 times in a 3.2 year period from 2011–2014. Fieldwork was done on 78 secondary and primary forest plots. In the absence of disturbance, rates of change of phase-height for the 78 plots were estimated by fitting the phase-heights to time with a linear model. Phase-height time series for the disturbed plots were fit to the logistic function to track jumps in phase-height. The epochs of clearing for the disturbed plots were identified with ≈1-month accuracy. The size of the phase-height change due to disturbance was estimated with ≈2-m accuracy. The monthly time resolution will facilitate REDD+ monitoring. Phase-height rates of change were shown to correlate with LiDAR RH90 height rates taken over a subset of the TanDEM-X data’s time span (2012–2013). The average rate of change of phase-height across all 78 plots was 0.5 m-yr - 1 with a standard deviation of 0.6 m-yr - 1 . For 42 secondary forest plots, the average rate of change of phase-height was 0.8 m-yr - 1 with a standard deviation of 0.6 m-yr - 1 . For 36 primary forest plots, the average phase-height rate was 0.1 m-yr - 1 with a standard deviation of 0.5 m-yr - 1 . A method for converting phase-height rates to AGB-rates of change was developed using previously measured phase-heights and field-estimated AGB. For all 78 plots, the average AGB-rate was 1.7 Mg-ha - 1 -yr - 1 with a standard deviation of 4.0 Mg-ha - 1 -yr - 1 . The secondary-plot average AGB-rate was 2.1 Mg-ha - 1 -yr - 1 , with a standard deviation of 2.4 Mg-ha - 1 -yr - 1 . For primary plots, the AGB average rate was 1.1 Mg-ha - 1 -yr - 1 with a standard deviation of 5.2 Mg-ha - 1 -yr - 1 . Given the standard deviations and the number of plots in each category, rates in secondary forests and all forests were significantly different from zero; rates in primary forests were consistent with zero. AGB-rates were compared to change models for Tapajós and to LiDAR-based change measurements in other tropical forests. Strategies for improving AGB dynamical monitoring with X-band interferometry are discussed.

Highlights

Tropical forests, which account for about 50% of the world’s forested biomass, play a critical role in the control of atmospheric carbon dioxide through emissions from forest removals and uptake through forest accruals [1]
Interferometric SAR (InSAR), with its broad, all-weather coverage, and sensitivity to vegetation vertical structure [7,8], has the potential to play a key role in aboveground biomass (AGB)-change monitoring strategies at local to regional to global scales
A model is introduced to convert phase-height rate to AGB-rate, and AGB-rates are produced for the 78 plots

Summary

Introduction

Tropical forests, which account for about 50% of the world’s forested biomass, play a critical role in the control of atmospheric carbon dioxide through emissions from forest removals and uptake through forest accruals [1]. Monitoring of changes in aboveground biomass (AGB) in tropical forests at the fine spatial scales at which disturbance takes place, 25–100 m [4], is required for global carbon cycle characterization. Climate change mitigation efforts designed to reduce emissions from deforestation and forest degradation (REDD+) will require monitoring much more frequently than the ≈1 time per year realized by optical satellites in the tropics [5]. Monitoring on the temporal scales at which disturbance takes place, ≈1 month, will be required to enable REDD+ [6]. Interferometric SAR (InSAR), with its broad, all-weather coverage, and sensitivity to vegetation vertical structure [7,8], has the potential to play a key role in AGB-change monitoring strategies at local to regional to global scales. The high temporal density of measurements was possible because, unlike optical signals, radar signals at X-band are not attenuated by clouds and rain

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