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| Titel |
Does EO NDVI seasonal metrics capture variations in species composition and biomass due to grazing in semi-arid grassland savannas? |
| VerfasserIn |
J. L. Olsen, S. Miehe, P. Ceccato, R. Fensholt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 14 ; Nr. 12, no. 14 (2015-07-29), S.4407-4419 |
| Datensatznummer |
250118037
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| Publikation (Nr.) |
 copernicus.org/bg-12-4407-2015.pdf |
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| Zusammenfassung |
| Most regional scale studies of vegetation in the Sahel have been based on
Earth observation (EO) imagery due to the limited number of sites providing
continuous and long term in situ meteorological and vegetation measurements.
From a long time series of coarse resolution normalized difference vegetation
index (NDVI) data a greening of the Sahel since the 1980s has been
identified. However, it is poorly understood how commonly applied remote
sensing techniques reflect the influence of extensive grazing (and changes in
grazing pressure) on natural rangeland vegetation. This paper analyses the time
series of Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI metrics
by comparing it with data from the Widou Thiengoly test site in northern
Senegal. Field data include grazing intensity, end of season standing biomass
(ESSB) and species composition from sizeable areas suitable for comparison
with moderate – coarse resolution satellite imagery. It is shown that
sampling plots excluded from grazing have a different species composition
characterized by a longer growth cycle as compared to plots under controlled
grazing or communal grazing. Also substantially higher ESSB is observed for
grazing exclosures as compared to grazed areas, substantially exceeding the
amount of biomass expected to be ingested by livestock for this area. The
seasonal integrated NDVI (NDVI small integral; capturing only the signal
inherent to the growing season recurrent vegetation), derived using absolute
thresholds to estimate start and end of growing seasons, is identified as the
metric most strongly related to ESSB for all grazing regimes. However
plot-pixel comparisons demonstrate how the NDVI/ESSB relationship changes due
to grazing-induced variation in annual plant species composition and the NDVI
values for grazed plots are only slightly lower than the values observed for
the ungrazed plots. Hence, average ESSB in ungrazed plots since 2000 was 0.93 t ha−1, compared to 0.51 t ha−1 for plots subjected to controlled
grazing and 0.49 t ha−1 for communally grazed plots, but the average
integrated NDVI values for the same period were 1.56, 1.49, and 1.45 for
ungrazed, controlled and communal, respectively, i.e. a much smaller
difference. This indicates that a grazing-induced development towards less
ESSB and shorter-cycled annual plants with reduced ability to turn additional
water in wet years into biomass is not adequately captured by seasonal NDVI
metrics. |
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