Abstract:
Reliable estimates of savanna vegetation constituents (i.e., woody and herbaceous vegetation) are essential as they are both responders and drivers of global change. The savanna is a highly
heterogenous biome with high variability in land cover types while also being very dynamic at both
temporal and spatial scales. To understand the spatial-temporal dynamics of savannas, using Earth
Observation (EO) data for mixed-pixel analysis is crucial. Mixed pixel analysis provides detailed
land cover data at a sub-pixel level which are essential for conservation purposes, understanding
food supply for herbivores, quantifying environmental change, such as bush encroachment, and fuel
availability essential for understanding fire dynamics, and for accurate estimation of savanna biomass.
This review paper consulted 197 studies employing mixed-pixel analysis in savanna ecosystems. The
review indicates that studies have so far attempted to resolve the savanna mixed-pixel issues by
using mainly coarse resolution data, such as Terra-Aqua MODIS and AVHRR and medium resolution
Landsat, to provide fractional cover data. Hence, there is a lack of spatio-temporal mixed-pixel
analysis for savannas at high spatial resolutions. Methods used for mixed-pixel analysis include
parametric and non-parametric methods which range from pixel-unmixing models, such as linear
spectral mixture analysis (SMA), time series decomposition, empirical methods to link the green
vegetation parameters with Vegetation Indices (VIs), and machine learning methods, such as regression trees (RT) and random forests (RF). Most studies were undertaken at local and regional scale,
highlighting a research gap for savanna mixed pixel studies at national, continental, and global level.
Parametric methods for modeling spatio-temporal mixed pixel analysis were preferred for coarse
to medium resolution remote sensing data, while non-parametric methods were preferred for very
high to high spatial resolution data. The review indicates a gap for long time series spatio-temporal
mixed-pixel analysis of savannas using high resolution data at various scales. There is potential to
harmonize the available low resolution EO data with new high-resolution sensors to provide long
time series of the savanna mixed pixel, which, according to this review, is missing.
