Abstract:
Assessments of tree/grass fractional cover in savannahs using
remote sensing are challenging due to the heterogeneous mixture
of the two plant functional types. Time-series decomposition models
can be used to characterize vegetation phenology from satellite
data, but have rarely been used for attributing phenological
signal components to different plant functional types. Here, tree/
grass dynamics are assessed in savannah ecosystems using timeseries
decomposition of 14 years of Moderate Resolution Imaging
Spectroradiometer (MODIS) normalized difference vegetation
index data acquired from 2002 to 2015. The decomposition
method uses harmonic analysis and tests the individual harmonic
terms for statistical significance. Field data of fractional cover of
trees and grasses were collected for 28 plots in Kruger National
Park, South Africa. Matching MODIS pixels were analysed for their
tree/grass phenological signals. Tree/grass annual and interannual
variability were then assessed based on the harmonic models. In
most harmonic cycles, grass-dominated sites had higher amplitudes
than tree-dominated sites, while the tree green-up started
earlier than grasses, before the start of the wet season. While
changes in tree phenology are gradual, grasses present higher
variability over time. Tree cover showed a significant correlation
with the amplitude (r (correlation coefficient) = −0.59, p = 0.001)
and phase of the first harmonic term (r = −0.73, p = 0.0001) and
the number of cycles of the second harmonic term (r = 0. 56,
p = 0.002). Grass cover was also significantly correlated with the
amplitude (r = 0. 51, p = 0.005) and phase of the first harmonic
term (r = 0.55, p = 0.002) and the number of cycles of the second
harmonic term (r = −0.52, p = 0.005). The positive correlation of
grass cover with phase and negative correlation with number of
cycles is indicating a late greening period and higher variability,
respectively. Tree cover estimated from the phase of the strongest
harmonic term showed a positive correlation with field-measured tree cover (R2 (coefficient of determination) = 0.55, p < 0.01,
slope = 0.93, root mean square error = 13.26%). The estimated
tree cover also had a strong correlation with the woody cover map
(r = 0.78, p < 0.01) produced by Bucini. The results show that
MODIS time-series data can be used to estimate the fractional tree
cover in heterogeneous savannahs from the phase of the plant
functional type’s phenological behaviour. This study shows that
harmonic analysis is able to discriminate between fractional cover
by trees and grasses in savannahs. The quantitative analysis of
tree/grass phenology from satellite time-series data enables a
better understanding of the dynamics of the tree/grass competition
and coexistence.
