# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 3 Results and Discussion
### The Temporal Response of Vegetation Health to Oil Spill Exposure  
Figure 11 shows the change in NDVI (\(\Delta\)NDVI) at spill sites (SSs) with a spill volume above 225 bbl and their corresponding non-spill control sites (CSs), relative to a pre-spill baseline NDVI value, for a number of years after an oil spill occurrence for dense vegetation (Figure 11a), Sparse vegetation (Figure 11b) and Mangrove vegetation (Figure 11c). Additionally, Figure 11d presents a boxplot that summarises \(\Delta\)NDVI for both SSs and their corresponding CSs for each vegetation type. A negative \(\Delta\)NDVI suggests signs of degradation in vegetation's health condition (a sign of stress) relative to the condition prior to the occurrence of a spill. Both graphical and statistical analyses using paired \(t\)-tests were used to determine the significance of oil spill-induced differences in \(\Delta\)NDVI values of SSs and CSs relative to a pre-spill baseline NDVI value for each site (Figure 11d). As previously mentioned, \(\Delta\)NDVI was used instead of absolute NDVI values at different time points because the magnitude of NDVI at some sites may be inherently higher than at others due to the additional influence of other factors that affect vegetation health. These include soil type and both water and nutrient availability, among others [67]. For instance, vegetation at oil spill sites could be healthier (and, hence, have a higher NDVI) before a spill than vegetation not exposed to a spill if located in an area with better water and nutrient availability. Calculating the NDVI changes at a site relative to a pre-spill baseline NDVI, therefore, acts to isolate the effect of exposure to oil from the influence of these external factors. The influence of these factors was further mitigated by carefully selecting CS and SS pairs with corresponding conditions (e.g., sunlight, soil composition, climate). The correlation between the \(\Delta\)NDVI relative to pre-spill baselines for each site and the number of days after a spill shows that the vegetation health varied considerably during the study period at both the SSs and CSs (Figure 11). Although both positive and negative \(\Delta\)NDVI values are observed at SSs and CSs for all vegetation types, the negative \(\Delta\)NDVI values have a greater magnitude at the SSs, especially for sparse and mangrove vegetation. Dense vegetation at SSs exhibited 38% positive \(\Delta\)NDVI values within the first 730 days (2 years) after a spill, sparse vegetation had 24% positive \(\Delta\)NDVI values, whilst the mangrove vegetation displayed only negative \(\Delta\)NDVI values within the first two years (Figure 10a-c). Dense vegetation at CSs exhibits a negligible decreasing trend in \(\Delta\)NDVI  
Figure 10: Relationship between post-spill NDVI and time after the spill for (**a**) dense vegetation, (**b**) sparse vegetation, and (**c**) mangrove vegetation. during the time period, whilst the vegetation health at SSs remains stable following an initial decrease in NDVI (\(\Delta\)NDVI = \(-\)0.05) immediately after oil spills. For the sparse vegetation (Figure 11b), the CSs exhibit an increasing trend in positive \(\Delta\)NDVI during the entire time period, indicating that the vegetation health is improving over time. Although vegetation at the SSs shows signs of recovery through a similar increasing trend in \(\Delta\)NDVI following exposure to oil, the vegetation health does not return to its pre-spill baseline condition until the ninth year (3285 days). The \(\Delta\)NDVI trends for mangroves are similar to those observed for dense vegetation in that there is a negligible decrease in the vegetation health over time at CSs, whilst the vegetation health at SSs decreases (\(\Delta\)NDVI = \(-\)0.1) immediately following exposure to oil and then remains consistent for the remainder of the time period.