Proof of evidence of changes in global terrestrial biomes using historic and recent NDVI time series.

Climate change affects plant dynamics and functioning of terrestrial ecosystems. This study aims to investigate temporal changes in global vegetation coverage and biomes during the past three decades. We compared historic annual NDVI time series (1982, 1983, 1984 and 1985) with recent ones (2015, 2016, 2017 and 2018), captured from NOAA-AVHRR satellite observations. To correct the NDVI time series for missing data and outliers, we applied the Harmonic Analysis of Time Series (HANTS) algorithm. The NDVI time series were decomposed in their significant amplitude and phase given their periodic fluctuation, except for ever green vegetation. Our findings show that the average NDVI values in most biomes have increased significantly (F-value<0.01) by 0.05 ndvi units over during the past three decades, except in tundra, and deserts and xeric shrublands. The highest rates of change in the harmonic components were observed in the northern hemisphere, mainly above 30° latitude. Worldwide, the mean annual phase reduced by 9° corresponding to a 9 days shift in the beginning of the growing season. Annual phases in the recent time series reduced significantly as compared to the historic time series in the five major global biomes: by 14.1, 14.8, 10.6, 9.5, and 22.8 days in boreal forests/taiga; Mediterranean forests, woodlands, and scrubs; temperate conifer forests; temperate grasslands, savannas, and shrublands; and deserts, and xeric shrublands, respectively. In tropical and subtropical biomes, however, changes in the annual phase of vegetation coverage were not statistically significant. The decrease in the level of phases and acceleration of growth and changes in plant phenology indicate the increase in temperature and climate changes of the planet.