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    Author(s): Robert Treuhaft; Yang Lei; Fabio Gonçalves; Michael Keller; João Santos; Maxim Neumann; André Almeida
    Date: 2017
    Source: Forests. 8(8): 277
    Publication Series: Scientific Journal (JRNL)
    Station: International Institute of Tropical Forestry
    PDF: Download Publication  (9.0 MB)

    Description

    Changes in tropical-forest structure and aboveground biomass (AGB) contribute directly to atmospheric changes in CO2, 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.

    Publication Notes

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    • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.

    Citation

    Treuhaft, Robert; Lei, Yang; Gonçalves, Fabio; Keller, Michael; Santos, João; Neumann, Maxim; Almeida, André. 2017.Tropical-Forest Structure and Biomass Dynamics from TanDEM-X Radar Interferometry. Forests. 8(8): 277-. https://doi.org/10.3390/f8080277.

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    Keywords

    tropical forest dynamics, aboveground biomass, interferometric SAR, TanDEM-X.

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