Does EO NDVI seasonal metrics capture variations in species composition and biomass due to grazing in semi-arid grassland savannas?

[摘要] Most regional scale studies of vegetation in the Sahel have been based onEarth observation (EO) imagery due to the limited number of sites providingcontinuous and long term in situ meteorological and vegetation measurements.From a long time series of coarse resolution normalized difference vegetationindex (NDVI) data a greening of the Sahel since the 1980s has beenidentified. However, it is poorly understood how commonly applied remotesensing techniques reflect the influence of extensive grazing (and changes ingrazing pressure) on natural rangeland vegetation. This paper analyses the timeseries of Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI metricsby comparing it with data from the Widou Thiengoly test site in northernSenegal. Field data include grazing intensity, end of season standing biomass(ESSB) and species composition from sizeable areas suitable for comparisonwith moderate – coarse resolution satellite imagery. It is shown thatsampling plots excluded from grazing have a different species compositioncharacterized by a longer growth cycle as compared to plots under controlledgrazing or communal grazing. Also substantially higher ESSB is observed forgrazing exclosures as compared to grazed areas, substantially exceeding theamount of biomass expected to be ingested by livestock for this area. Theseasonal integrated NDVI (NDVI small integral; capturing only the signalinherent to the growing season recurrent vegetation), derived using absolutethresholds to estimate start and end of growing seasons, is identified as themetric most strongly related to ESSB for all grazing regimes. Howeverplot-pixel comparisons demonstrate how the NDVI/ESSB relationship changes dueto grazing-induced variation in annual plant species composition and the NDVIvalues for grazed plots are only slightly lower than the values observed forthe 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 controlledgrazing and 0.49 t ha⁻¹ for communally grazed plots, but the averageintegrated NDVI values for the same period were 1.56, 1.49, and 1.45 forungrazed, controlled and communal, respectively, i.e. a much smallerdifference. This indicates that a grazing-induced development towards lessESSB and shorter-cycled annual plants with reduced ability to turn additionalwater in wet years into biomass is not adequately captured by seasonal NDVImetrics.

[学科分类] 地球化学与岩石