China's deserts greening and response to climate variability and human activities.

Xiaoyu Liu,Liangjie Xin,Gerald Forkuor

doi:10.1371/journal.pone.0256462

Abstract

Vegetation, which is a good indicator of the impacts of climate variability and human activities, can reflect desert ecosystem dynamics. To reveal the vegetation variations in China’s deserts, trends in the monthly, seasonal, and annual normalized difference vegetation index (NDVI) from 2000 to 2017 were measured both temporally and spatially by the Theil-Sen estimator and Mann-Kendall test. Additionally, correlation coefficients and residual analysis were employed to evaluate the correlations between the NDVI and climatic factors and to distinguish the impacts of climate variability and human activities. The results showed that China’s deserts underwent greening. The annual NDVI showed a significant increasing trend at a rate of 0.0018/yr, with values of 0.094 in 2000 and 0.126 in 2017. Significant increasing trends in NDVI were observed in all four seasons. The NDVI were higher in summer and autumn than in spring and winter. Both the monthly NDVI and its trends showed an inverted U-shaped curve during the year. Spatially, the greening trends were mainly distributed on the southern edge of the Gurbantunggut Desert, in the northwestern part of the Taklimakan Desert, and in the Kubuqi Desert. The correlations between the NDVI and climatic factors at the monthly and seasonal scales were stronger than those at the annual scale. Temperature and precipitation had positive effects on NDVI at the monthly and seasonal scales, but only precipitation had a positive effect at the annual scale. Human activities, especially oasis expansion and sand stabilization measures, were two major causes of large increasing areas of desert greening in China indicated by the NDVI.

Highlights

Deserts are one of the major land-cover types, covering approximately one-third of the terrestrial area of the earth [1]
In contrast to ordinary least squares used for linear regression, the Theil-Sen estimator is an unbiased estimate of a linear trend and is more robust; it has been widely used for estimating the slope of the normalized difference vegetation index (NDVI) [55,56,57,58]
The mean values of the monthly, seasonal, and annual NDVI, temperature, and precipitation of the whole study area were calculated for use in the analysis of the temporal variations, and the trends were identified by using the Theil-Sen estimator and Mann-Kendall test

Summary

Introduction

Deserts are one of the major land-cover types, covering approximately one-third of the terrestrial area of the earth [1]. Atmospheric circulation, latitude, ocean currents, etc., deserts, as a product of arid climatic conditions, are mostly located on the western coasts of continents near 30 ̊ north and south latitude [2,3]. There is still no universally accepted common or technical definition for deserts, only distinguished by characteristics (e.g., climate, weather, and hydrology) [4]. Deserts are dynamic, and their boundaries are gradual and not clearly defined.

Objectives

Methods

Results

Discussion

Conclusion