Analysis of Phenological and Growing season dynamics in Savannah ecosystem of Karamoja sub-region, Uganda.

Abstract

Vegetation phenology is an important control on the global fluxes of energy, water, and carbon from terrestrial ecosystems and is a useful indicator of ecosystem response to climate variability and change. However, few studies have focused on the phenology dynamics of different savannah vegetation types and associated fluxes as indicators for length of growing season. This study i) identified savannah phenological characteristics, ii) determined the effect of rainfall on savanna phenology and, iii) determined the length of growing season in Karamoja sub-region. The Moderate Resolution Imaging Spectro-radiometer’s Normalized Difference Vegetation Index (MODIS NDVI) imagery (MOD13Q1) was used. Rainfall data obtained from the National Centers for Environmental Prediction (NCEP) with a daily temporal resolution and gridded resolution of 0.2 x 0.2 degrees was used. Phenological attributes from four major savannah vegetation types (bushland, grassland, thickets and woodland) with eight savannah phenological characteristics (Time of onset of greenness ,Time of end of greenness, Duration of greenness ,Time of maximum NDVI, Value of onset of greenness, Value of end of greenness, Value of maximum NDVI, Range of NDVI which were categorized as Temporal NDVI metrics and NDVIvalue metrics) and rainfall effect on phenological dynamics and length of growing season were analyzed in R and TIMESAT analytical software. Phenological characteristics results revealed that thickets had the earliest onset of greenness (85th day) and earliest end of greenness time (244th day) with maximum greenness, onset NDVI value, and end NDVI value of 0.68, 0.42 and 0.57 respectively. In addition, woodland revealed the highest Max NDVI value, Onset NDVI value, End of greenness NDVI value and least Range of NDVI value of 0.73, 0.56, 0.67 and 0.18 respectively. Rainfall as a driver of phenological change had a positive and significant effect over the 18 year period of analysis on bushland, grassland, thickets and woodland with correlation coefficient of 0.704, 0.722, 0.701, and 0.65 respectively. On average, the start, end and length of growing season were found to occur in pentad 23 (April 19th - 20th), pentad 47 (August 22rd - 23rd) and 123 days respectively. These results reveal that green up of savannah vegetation has positive relationship with rainfall but the start and end of green up is less influenced by the start and end of the rains. Therefore, it is recommended that there is need to couple other drivers such as fires with rainfall for a comprehensive monitoring of savannah vegetation phenological dynamics in Karamoja sub-region.