A study of the changes in the forest phenology in Turkey through MODIS satellite data

The Start of Growing Season (SGS) marks the time of year where a type of plant starts its growing season; therefore, forming green leaves. The End of Growing Season (EGS) is the opposite, where plants end their growing seasons and start defoliation. The difference in green leaf changes are much more dramatic in deciduous forests than coniferous forests, all plants show annual differences after SGS and EGS. As all plants undergo SGS and EGS, they look for environmental cues in order to keep up with their annual cycles. To detect variations in SGS and EGS, MODIS imagery was used. MODIS satellites have a polar orbit and capture images in 7 spectral bands daily. To detect the SGS and EGS from MODIS satellite data by comparing the surface reflectance in red versus green through the GRVI index was used. The threshold for the detection of SGS and EGS is set to GRVI = 0. If GRVI > 0, then it is currently the growing season; if GRVI < 0 then it is the senescence season. Yearly GRVI estimates are analyzed to detect the SGS and EGS by catching GRVI zero crossing. We tried to detect the yearly shifts of SGS and EGS using this GRVI estimates. For these measurements, daily MODIS Terra MOD09GA V006 products were downloaded for the period 2007-2017, then analyzed through an R script using RASTER package. Noise reduction was applied on the results using Exponential Moving Average and the GRVI=0 crossings were detected. Later, the refined data were compared annually and among regions with different vegetation to look for SGS and EGS shifts over time. Although in all plots there were obvious peaks during Growing Season, in some cases of mountain ranges and coniferous forests, GRVI method was unable to detect SGS or EGS; as there was no 0 crossing. In the regions where GRVI was successful, SGS shifted to an earlier DOY and EGS shifted to a later DOY on a scale of 11 years. Overall, the GRVI has been successful in showing seasonal patterns in a given region.