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Interactions of savanna vegetation structure with C-band

Synthetic Aperture Radar (SAR) at pixel level

V. Odipo1*, C. Berger, C. Schmullius

Intr

od

ucti

on

1 *,2,3Friedrich-Schiller-University JenaGrietgasse 6, 07743 Jena*victor.onyango@uni-jena.de; christian.berger@uni-jena.de,

c.schmullius@uni-jena.de+49 3641 94 89 26/ +49 3641 94 88 80 / 94 88 82C

on

tact

Savanna vegetation structure especially aboveground biomass (AGB) and canopy cover (CC) are important for assessment of the

biome’s ability to provide ecosystem services under various disturbance scenarios. Studies aimed at tree-level structure mapping

provides the best way to monitor intrinsic changes when performed at resolutions permitting detection of forest degradation at localized

scales. ESA’s Sentinel-1 synthetic aperture radar (SAR) data is freely available at high temporal and spatial resolutions which can solve

problems of atmospheric interference experienced with optical data in the tropics. We assess feature detectability (cover classes) and

error propagation as a result of changing SAR resolution from high to low within a Lowveld Savanna using terrestrial laser scanner (TLS)

derived CC and AGB as reference data over a 9 sq.Km area in KNP. The study found that whereas changes are visible at higher

resolutions, the RMSE in savanna structure detectability reduced with downscaling SAR resolution, irrespective of polarization and

season.

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• High correlation on SAR backscatter and AGB in dry season

due to open canopy caused by „leaf-off“

• High correlation on SAR backscatter and CC in the wet

season due to closed canopy

• RMSE reduced while R-squared increased with

downgrading SAR resolution, and vise versa

• High AGB values – Inability to reconstruct trees with H<5 m

and DBH <10 cm (shrubs left our)

Refe

ren

ces • Odipo, V.O.; Nickless, A.; Berger, C.; Baade, J.; Urbazaev, M.; Walther, C.; Schmullius, C. Assessment of aboveground woody biomass dynamics using terrestrial laser scanner and L-band ALOS

PALSAR data in South African savanna. Forests 2016, 7, 294; doi:10.3390/f7120294

• Colgan, M.S.; Asner, G.P.; Levick, S.R.; Martin, R.E.; Chadwick, O.A. Topo-edaphic controls over woody plant biomass in South African savannas. Biogeosciences 2012, 9, 1809–1821. Doi: 10.5194/bg-

9-1809-2012

• Mathieu, R.; Naidoo, L.; Cho, M.A.; Leblon, B.; Main, R.; Wessels, K.; Asner, G.P.; Buckley, J.; van Aardt, J.; Erasmus, B.F.N.; Smit, I.P.J. Toward structural assessment of semi-arid African savannahs

and woodlands: The potential of multitemporal polarimetric RADARSAT-2 fine beam images. Rem. Sens. Of Environ. 2013,138: 215-231. Doi: 10.1016/j.rse.2013.07.011

• TLS tree H & DBH used in AGB (t/ha) calculation

AGB=0.109D(1.39+0.14In(D))H0.73ρ0.80 …..Colgan et al 2013 )

• LAStools: TLS derived CHM for CC (%)

• GAMMA: SAR processing: Rad. Calib.> Geocod.> Topo.

Normalization and Linear to σ0 (dB) conversion

DBH (cm) Height (m) Tree biomass CC (%)

tree plot tree plot kg/tree t/ha plot

Min 10 10 5.3 5.3 274.4 1.8 0

Median 31 41 10.7 10.7 6669.9 46.5 11.3

Mean 39.6 40.2 10.7 10.7 7793.4 54.3 15.6

Max 75 75 16.1 16.1 35285.2 226 90.2

• Heterogeneous

savanna in S.

Africa

• Mix of grass,

shrubs and trees.

• Disturbance &

heterogeneity

drivers: fire,

herbivory,

nutrient, climate

• Dry winter: May-

Sept, Wet

summer: Oct-

April

• MAP = 650mm,

MAT= 22°C

Fig.1: LC map of Skukuza study area, KNP,S. Africa

• Field data (Mar., 2015): 42 plots (30x30

m); H (>1m H); DBH at 1m H. Use:

validation of TLS measured H & DBH

• TLS data (Sept., 2015); Use: Tree H &

DBH, DTM, DSM & CHM; LC

classification in OBIA

Fig.3: Graphical summary of methodology used in the study

• Ground range detected (GRD) C-band

Sentinel-1A; IWS; Ascending pass;

• Period: Feb. 2015 - Mar. 2017 (VH &

VV)

Tab.1: Summary of

dependent variables:

AGB and CC for both tree-

and plot-level

Fig. 2: Datasets used in the study- tree DBH, H; TLS data; C-band data & soil

moisture

• AGB and CC vs. SAR

variables (VH/VV;

Wet/Dry) using RF („mlr“)

• Trimble RealWorks and

QuickTerrainReader

used for tree H & DBH

measurement from TLS

• High resolution- ease of change monitoring and life form

separation (visual)-pixel mixing at low resolutions

• C-band SAR is critical in savanna monitoring: high temporal

& spatial resolution; freely available

• TLS data – identification and measurement of tree H & DBH

• TLS data-lower vegetation not captured due to insufficient

point to reconstruct the tree trunk

• Error propagation-limited validation data (TLS area, few

plots with corse resolution)

• Possibility to assess effects of multi-temporal filters in a

heterogenous savanna

• Predicted AGB &

CC at different

resolutions: 10,

30, 70 and 75 m

• Average σ0 (dB)

per LCC, and

life-form

separability

• RMSE in AGB &

CC mapping

using C-band

SAR

Acknow

ledg

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ent