Investigating the trend of vegetation changes (greening and browning) using MODIS-NDVI time series in Mazandaran province

Vegetation is one of the main components of the biosphere and a vital element in the climate system, and knowing its changes and activity trends can affect the optimal productivity of agricultural land, natural ecosystems, climate change, and biodiversity. Vegetation indices derived from satellite data have provided a powerful tool for monitoring and investigating vegetation dynamics in large temporal and spatial scales. The aim of this study is to investigate the long-term trend of vegetation in pixels to pixels in Mazandaran province using the NDVI time series of MODIS sensor.

The current research was conducted in the province of Mazandaran, which includes 53% of Hyrcanian forests and 12050 square kilometers of rangeland. In this study, the vegetation index products of the MODIS sensor with a spatial resolution of 250 meters and an interval of 16 days, named MOD13Q1 and MYD13Q1, have been used. The NDVI index images available in these two products were combined and a time series of 828 images with an 8-day interval and a spatial resolution of 250 meters was created from the NDVI index for the period from 2003 to 2021.In order to increase the quality of the time series, the pixels affected by clouds, snow and ice in each image of the time series based on MODIS quality assurance information and the seasonal component within the time series that may make long-term trend investigation difficult, using the Anomaly method were removed. Then the time series was analyzed using parametric ordinary least squares (OLS) regression and non-parametric Theil-Sen and Mann-Kendall in Google Earth Engine (GEE).

The results of this study showed that the output of OLS, Theil-Sen and Mann-Kendall methods were close to each other and Greening and Browning occurred in about 87% and 13% of the province, respectively. Also, the significance of Man-Kendall trends shows that 77.21% of trends in the province, include 70.65% of positive trends (Greening) and 6.56% of negative trends (Browning) at ρ < 0.01, 6.48% and 2.95% of trends at ρ < 0.05 and ρ < 0.10 are significant, respectively. The spatial distribution of the trends also showed that a large part of the Browning trends occurred in the plains of the north of the province and most of the Greening trends occurred in forest lands and rangeland.

In general, this study showed that greening occurred on a large scale, especially in natural vegetation including forests and rangeland. Changes in natural vegetation are affected by climate change, because human activities are limited in them. Therefore, the process of global warming can be the most important factor of Greening in Mazandaran province.