Drought Influence on Forest Plantations in Zululand, South Africa, Using MODIS Time Series and Climate Data

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.

Time Series Analysis of MODIS-Derived NDVI for the Hluhluwe-Imfolozi Park, South Africa: Impact of Recent Intense Drought

“…These observations are consistent with a study by Xulu et al [10], who investigated the response of commercial forestry to the recent strong and broad El Niño event in a region which is 70 km south-east of the HiP. In their study, Xulu et al [10] reported a significant decline of NDVI values which corresponded to the 2014-2016 El Niño years [10]. Although the influence of the 2014-2016 El Niño in the HiP seems to be the strongest, it follows the same pattern as that reported by Anyamba et al [40] in their study of the influence of both El Niño and La Niña in the vegetation status over eastern and southern Africa.…”

“…There is an expected resemblance between the NDVI and EVI observations in both the monthly mean time series and the smooth trend, with a clear indication of the effect of the 2014-2016 drought period. These observations are consistent with a study by Xulu et al [10], who investigated the response of commercial forestry to the recent strong and broad El Niño event in a region which is 70 km south-east of the HiP. In their study, Xulu et al [10] reported a significant decline of NDVI values which corresponded to the 2014-2016 El Niño years [10].…”

“…In general, there is a steady trend of NDVI measured at the HiP beside some anomalies observed in specific parts of the time series. This seems to be the case for southern Africa because other studies also indicated a steady trend for this region [10]. Remarkably, during the 2014-2016 period, a period that coincided with the recent intense El Niño, there was a sudden decrease in the NDVI values which reduced to the lowest minimum value of about 0.3 in November 2015.…”

“…These studies have enhanced our understanding of how vegetation reacts to drought events over time [22][23][24][25].Hitherto, numerous studies have explored vegetation changes using NDVI in response to climatic variability. Most have shown that vegetation is largely swayed by the El Niño/Southern Oscillation (ENSO) phenomenon and have been established to respond well to climatic variables [10,24,26,27]. These studies in different climatic regions have revealed climate-induced effects in key economic sectors such as agriculture [28] and forestry [24,29].…”

“…First, Bond et al [31] established that droughts largely influence the extent of grazing vegetation in the reserve. More recently, Xulu et al [10] showed how the recent intense drought moderated the vegetation health of commercial plantations located~70 km from the park. Second, the HiP is an important conservation area and ecotourism destination in South Africa [32], so the resultant socio-economic impacts of ecosystem changes are of great concern.…”