Phenological parameters trend of the southern Zagros forests based on MODIS-NDVI time series during 2000-2017

Climate change and human interventions as a whole have had negative and remarkable effects on the quantity and quality of forests. Beside the change in the forests extent, which has always been considered and monitored, its phenological changes have also been investigated in the last decade. The NDVI Vegetation Index derived from satellite data is an appropriate proxy for quantifying and expressing forest status including phenological changes. This study aimed to characterize the trend of start, end, and length of growing season using NDVI satellite time series dataset over 18-year time periods and then assess their relationship with precipitation and temperature parameters. This study was carried out over the southern Zagros forests using MODIS-NDVI time series with temporal and spatial resolution of 16-day and 250 meters, respectively. The precipitation and temperature datasets were also collected from regional synoptic meteorological stations. After preprocessing steps, 414 NDVI images during 2000-2017 were analyzed pixel by pixel to extract the start, end, and length of the growing season using Midpoint method, considering 50% and 35% thresholds of NDVI annual amplitude for start and end of the growing season, respectively. Then, the statistical significance of phenological metrics was assessed. Based on the results, the mean dates for the start and the end of growing season were 16th March and 15th August, respectively, with the mean length of growing season of 151 days in the study period from 2000 to 2017. Considering 35% of NDVI annual amplitude for the end of growing season, the mean date for the end of growing season was 8th September and the mean length of growing season was 190 days. The seasonal trend showed that the start and the end of growing season has occurred respectively 0.02 and 1.04 days earlier per year in the southern Zagros forests during 2000-2017. The length of growing season has been shortened 1.02 day per year. However, variation in the start and the end dates and the length of growing season have not been significant in 93%, 81% and 81% of the region, respectively, at 90 % confident level. Generally, the change in the occurrence of the end of growing season was greater than the start of growing season. A weak correlation was observed between phenological changes and climate parameters like temperature and precipitation in the study area.