Assessing the Phenology of Southern Tropical Africa: A Comparison of Hemispherical Photography, Scatterometry, and Optical/NIR Remote Sensing

The seasonal cycle of tree leaf display in the savannas and woodlands of the seasonally dry tropics is complex, and robust observations are required to illuminate the processes at play. Here, we evaluate three types of data for this purpose, comparing scatterometry (QuikSCAT σ⁰) and optical/near-infrared MODIS EVI remotely sensed data against field observations. At a site in Mozambique, the seasonal cycles from both space-borne sensors are in close agreement with each other and with estimates of tree plant area index derived from hemispherical photography . This agreement results in similar estimates of the start of the growing season across different data types (range 13 days). Ku-band scatterometry may therefore be a useful complement to vegetation indices such as EVI for estimating the start of the growing season for trees in tropical woodlands. More broadly, across southern tropical Africa there is close agreement between scatterometry and EVI time series in woody ecosystems with 25 % tree cover, but in areas of 25 % tree cover, the two time series diverge and produce markedly different start of season (SoS) dates (difference 50 days). This is due to increases in σ⁰ during the dry season, not matched by increase in EVI. The reasons for these increases are not obvious, but might relate to soil moisture, flowering, fruiting, or grass dynamics. Further observations and modeling of this phenomenon is warranted to understand the causes of these dry season changes in σ⁰. Finally, three different definitions of the SoS were examined and found to produce only small differences in estimated dates, across all types of data.