Water availability may not constrain vegetation growth in Northern Hemisphere

With global climate warming, the variability of climate and weather tends to increase driving the water resources available for vegetation become more uncertain. Therefore, there is still a debate as to how vegetation response to, and to which extent it is constrained to water availability. In this study, we analyzed the correlation between vegetation growth and water availability in the Northern Hemisphere from 1982 to 2018. The condition of vegetation growth was characterized by Normalized Difference Vegetation Index (NDVI) and Leaf Area Index (LAI), while the Standardized Precipitation Evapotranspiration Index (SPEI) represented water condition. Our results suggest that the water deficit stress on vegetation growth in the Northern Hemisphere caused by global warming may not be as severe as described in previous studies. Although there was an increasing trend in areas where vegetation growth was constrained by water deficit in the Northern Hemisphere, such trend was not significant; in contrast, there was a significant decreasing trend in areas where vegetation growth was constrained by surplus water. In addition, there was a spatial variability in water availability for vegetation growth at mid and high latitudes. We further found that the response time of vegetation to both water deficit and water surplus was increasing, indicating that the cumulative effect of water availability on vegetation growth was delaying, and that vegetation tends to be affected by climatic extreme events such as drought with longer time scales. Our results highlight the need to reconsider the response of vegetation activities to water availability, and shed light on a more divers reaction pattern of vegetation to water in the context of climate change.