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

Sifiso Xulu,Michael Gebreslasie,Kabir Peerbhay,Riyad Ismail

doi:10.3390/f9090528

Sifiso Xulu, Michael Gebreslasie + Show 2 more

Open Access

https://doi.org/10.3390/f9090528

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Journal: Forests	Publication Date: Aug 30, 2018
Citations: 36	License type: CC BY 4.0

Affiliation: University of KwaZulu-Natal, University of Zululand

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.

Highlights

Defined as an extended period of rainfall shortage coupled with higher than normal temperatures, droughts result in water shortages and plant water stress [1]
These results demonstrated the effective use of the normalized difference vegetation index (NDVI)- and normalized difference infrared index (NDII)-derived Moderate Resolution Imaging Spectroradiometer (MODIS) data coupled with climatic variables to provide insights into the influence of drought on plantation trees in the study area
Severe droughts that affected the country were apparently established during a mature El Niño phase [61]

Summary

Introduction

Defined as an extended period of rainfall shortage coupled with higher than normal temperatures, droughts result in water shortages and plant water stress [1]. In part, induced forest fires [5,6], provoked the outbreak and spread of tree-attacking insect pests [7], amplified tree moisture deficits [8], stimulated the establishment of opportunistic plant invaders [9,10], and engendered land degradation and fragmentation in and around forested landscapes [11]. These effects can result in severe tree mortality and incur heavy economic costs for the forestry industry [12,13]. The effects of droughts cannot be ignored, because they present a compounding challenge to forest health and productivity [8,15,16]

Methods

Results

Discussion

Conclusion