| In the study of long-term climatic change, increased mean temperatures have been shown to correlate with changes to vegetation phenology, specifically with regard to phenological metrics such as advance in onset of the growing season, overall extension of growing season length, and delay to the end of the growing season. However, these long term trends tend to mask the effects of relatively short yet extreme climatic events that can alter phenology. In this study, remote sensing data was utilized to assess the effects of temperature fluctuations from volcanic activity on vegetation phenology on various sites around the world over a period of 11 years (2001-2011). The phenological metrics were derived from a normalization ratio performed on the Enhanced Vegetation Index (EVI) values with linear interpolation used to determine the exact day of greenness onset and greenness offset on a pixel-by-pixel basis. A trend analysis methodology known as Breaks for Additive and Seasonal Trend (BFAST) was used in conjunction with the Wilcoxon rank sum test and boxplot visualizations to assess how changes in temperature trends from volcanic activity during the periods of greenness onset and greenness offset would affect the start of greenness onset and offset. The results demonstrate that accurate abrupt temperature changes from volcanic activity can be picked up if the magnitude of the volcanic activity is large enough. Additionally, abrupt temperature change from volcanic activity has a statistically significant effect on the period of greenness onset in that temperature decreases result in a later start to growing season onset. Temperature changes resulting from volcanic activity can alter vegetation phenology and, generally speaking, such trends can be picked up through the aforementioned statistical methodologies. Overall, this study provides evidentiary support that the timing of an extreme temperature event is more detrimental to plant phenology if the extreme temperature event occurs early in the growing season when compared to other portions of the growing season. Future work on this topic would be improved by taking into consideration additional environmental factors. |
