Drought Influence on Forest Plantations in Zululand, South Africa, Using MODIS Time Series and Climate Data
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Date
2018-08-30
Journal Title
Journal ISSN
Volume Title
Publisher
MDPI
Abstract
South Africa has a long history of recurrent droughts that have adversely affected its
economic performance. The recent 2015 drought has been declared the most serious in 26 years and
impaired key agricultural sectors including the forestry sector. Research on the forests’ responses
to drought is therefore essential for management planning and monitoring. The effects of the latest
drought on the forests in South Africa have not been studied and are uncertain. The study reported
here addresses this gap by using Moderate Resolution Imaging Spectroradiometer (MODIS)-derived
normalized difference vegetation index (NDVI) and precipitation data retrieved and processed using
the JavaScript code editor in the Google Earth Engine (GEE) and the corresponding normalized
difference infrared index (NDII), Palmer drought severity index (PDSI), and El Niño time series data
for KwaMbonambi, northern Zululand, between 2002 and 2016. The NDVI and NDII time series
were decomposed using the Breaks for Additive Seasonal and Trend (BFAST) method to establish
the trend and seasonal variation. Multiple linear regression and Mann–Kendall tests were applied
to determine the association of the NDVI and NDII with the climate variables. Plantation trees
displayed high NDVI values (0.74–0.78) from 2002 to 2013; then, they decreased sharply to 0.64 in
2015. The Mann–Kendall trend test confirmed a negative significant (p = 0.000353) trend between
2014 and 2015. This pattern was associated with a precipitation deficit and low NDII values during a
strong El Niño phase. The PDSI (−2.6) values indicated severe drought conditions. The greening
decreased in 2015, with some forest remnants showing resistance, implying that the tree species had
varying sensitivity to drought. We found that the plantation trees suffered drought stress during
2015, although it seems that the trees began to recover, as the NDVI signals rose in 2016. Overall,
these results demonstrated the effective use of the NDVI- and NDII-derived MODIS data coupled
with climatic variables to provide insights into the influence of drought on plantation trees in the
study area.
Description
Peer reviewed article published under Forests Journal, Volume 9 Issue 9
Keywords
drought, NDVI, NDII, precipitation, El Niño, PDSI, forest disturbance, Climatology
Citation
Xulu, S., Peerbhay, K., Gebreslasie, M. and Ismail, R., 2018. Drought influence on forest plantations in Zululand, South Africa, using MODIS time series and climate data. Forests, 9(9), p.528.