Temporal Changes in Coupled Vegetation Phenology and Productivity are Biome-Specific in the Northern Hemisphere

Lanhui Wang,Rasmus Fensholt

doi:10.3390/rs9121277

Abstract

Global warming has greatly stimulated vegetation growth through both extending the growing season and promoting photosynthesis in the Northern Hemisphere (NH). Analyzing the combined dynamics of such trends can potentially improve our current understanding on changes in vegetation functioning and the complex relationship between anthropogenic and climatic drivers. This study aims to analyze the relationships (long-term trends and correlations) of length of vegetation growing season (LOS) and vegetation productivity assessed by the growing season NDVI integral (GSI) in the NH (>30°N) to study any dependency of major biomes that are characterized by different imprint from anthropogenic influence. Spatial patterns of converging/diverging trends in LOS and GSI and temporal changes in the coupling between LOS and GSI are analyzed for major biomes at hemispheric and continental scales from the third generation Global Inventory Monitoring and Modeling Studies (GIMMS) Normalized Difference Vegetation Index (NDVI) dataset for a 32-year period (1982–2013). A quarter area of the NH is covered by converging trends (consistent significant trends in LOS and GSI), whereas diverging trends (opposing significant trends in LOS and GSI) cover about 6% of the region. Diverging trends are observed mainly in high latitudes and arid/semi-arid areas of non-forest biomes (shrublands, savannas, and grasslands), whereas forest biomes and croplands are primarily characterized by converging trends. The study shows spatially-distinct and biome-specific patterns between the continental land masses of Eurasia (EA) and North America (NA). Finally, areas of high positive correlation between LOS and GSI showed to increase during the period of analysis, with areas of significant positive trends in correlation being more widespread in NA as compared to EA. The temporal changes in the coupled vegetation phenology and productivity suggest complex relationships and interactions that are induced by both ongoing climate change and increasingly intensive human disturbances.

Highlights

Vegetation is of great importance in the interaction of the biosphere and the atmosphere in the Earth system through biophysical and biogeochemical feedbacks to modulate regional and global climate [1,2,3,4]
The quantitative assessment and spatial patterns indicate that extending length of vegetation growing season (LOS) in the Northern Hemisphere (NH) is caused by both advancing start of growing season (SOS) and delaying end of growing season (EOS) in most regions, whereas shortening LOS is due to both delaying SOS and advancing EOS
Areas of pixels of the NH with significant positive trends in growing season Normalized Difference Vegetation Index (NDVI) integral (GSI) (Figure 2D) (51.22%) are considerably larger than those in LOS (23.24%) (Table 2), whereas the areas of pixels with negative trends in GSI (9.95%) are much smaller than those in LOS (19.21%) (Table 2). This suggests that areas of significant greening trends are dominating the NH, with LOS appearing comparably mixed in lengthening and shortening trends over the past three decades, indicating that some areas are experiencing increasing vegetation productivity without significant changes in LOS

Summary

Introduction

Vegetation is of great importance in the interaction of the biosphere and the atmosphere in the Earth system through biophysical (e.g., albedo and water cycle) and biogeochemical (e.g., carbon cycle) feedbacks to modulate regional and global climate [1,2,3,4]. Long-term time-series satellite data have been widely used to study vegetation activity at various spatial and temporal scales [6,16,17,18,19], such as the NDVI (Normalized Difference Vegetation Index) dataset that is produced by the Global Inventory Monitoring and Modeling Studies (GIMMS) group from the Advanced Very High Resolution Radiometer (AVHRR) of National Oceanic and Atmospheric Administration (NOAA). Long-term time-series NDVI dataset from AVHRR have revealed significant greening trends of vegetation (based on seasonal NDVI integrals) in the NH from the early 1980s to the late 1990s [7,17,18,20], which have been interpreted as an increase in photosynthesis and vegetation productivity through terrestrial ecosystem models [20,27,28,29]. The observed ‘northern greening’ has been central for an increasing carbon sink in the northern ecosystems over the past 30 years [16,30,31,32,33,34]

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