Assessment of Aboveground Woody Biomass Dynamics Using Terrestrial Laser Scanner and L-Band ALOS PALSAR Data in South African Savanna

Forests. 2016;7(12):294 DOI 10.3390/f7120294

 

Journal Homepage

Journal Title: Forests

ISSN: 1999-4907 (Print)

Publisher: MDPI AG

LCC Subject Category: Science: Botany: Plant ecology

Country of publisher: Switzerland

Language of fulltext: English

Full-text formats available: PDF, HTML

 

AUTHORS

Victor Onyango Odipo (Department of Earth Observation, Friedrich-Schiller-University Jena, Grietgasse 6, 07743 Jena, Germany)
Alecia Nickless (Primary Care Clinical Trials Unit, Nuffield Department of Primary Care Health Sciences, University of Oxford, Radcliffe Primary Care Building, Woodstock Rd., Oxford OX2 6GG, UK)
Christian Berger (Department of Earth Observation, Friedrich-Schiller-University Jena, Grietgasse 6, 07743 Jena, Germany)
Jussi Baade (Department of Geography, Friedrich-Schiller-University Jena, Löbdergraben 32, 07743 Jena, Germany)
Mikhail Urbazaev (Department of Earth Observation, Friedrich-Schiller-University Jena, Grietgasse 6, 07743 Jena, Germany)
Christian Walther (Federal Institute for Geosciences & Natural Resources (BGR), Remote Sensing, Wilhelmstr. 25-30, 13593 Berlin, Germany)
Christiane Schmullius (Department of Earth Observation, Friedrich-Schiller-University Jena, Grietgasse 6, 07743 Jena, Germany)

EDITORIAL INFORMATION

Blind peer review

Editorial Board

Instructions for authors

Time From Submission to Publication: 11 weeks

 

Abstract | Full Text

The use of optical remote sensing data for savanna vegetation structure mapping is hindered by sparse and heterogeneous distribution of vegetation canopy, leading to near-similar spectral signatures among lifeforms. An additional challenge to optical sensors is the high cloud cover and unpredictable weather conditions. Longwave microwave data, with its low sensitivity to clouds addresses some of these problems, but many space borne studies are still limited by low quality structural reference data. Terrestrial laser scanning (TLS) derived canopy cover and height metrics can improve aboveground biomass (AGB) prediction at both plot and landscape level. To date, few studies have explored the strength of TLS for vegetation structural mapping, and particularly few focusing on savannas. In this study, we evaluate the potential of high resolution TLS-derived canopy cover and height metrics to estimate plot-level aboveground biomass, and to extrapolate to a landscape-wide biomass estimation using multi-temporal L-band Synthetic Aperture Radar (SAR) within a 9 km2 area savanna in Kruger National Park (KNP). We inventoried 42 field plots in the wet season and computed AGB for each plot using site-specific allometry. Canopy cover, canopy height, and their product were regressed with plot-level AGB over the TLS-footprint, while SAR backscatter was used to model dry season biomass for the years 2007, 2008, 2009, and 2010 for the study area. The results from model validation showed a significant linear relationship between TLS-derived predictors with field biomass, p < 0.05 and adjusted R2 ranging between 0.56 for SAR to 0.93 for the TLS-derived canopy cover and height. Log-transformed AGB yielded lower errors with TLS metrics compared with non-transformed AGB. An assessment of the backscatter based on root mean square error (RMSE) showed better AGB prediction with cross-polarized (RMSE = 6.6 t/ha) as opposed to co-polarized data (RMSE = 6.7 t/ha), attributed to volume scattering of woody vegetation along river valleys and streams. The AGB change analysis showed 32 ha (3.5%) of the 900 ha experienced AGB loses above an average of 5 t/ha per annum, which can mainly be attributed to the falling of trees by mega herbivores such as elephants. The study concludes that SAR data, especially L-band SAR, can be used in the detection of small changes in savanna vegetation over time.