Legacy effects of spring phenology on vegetation growth under preseason meteorological drought in the Northern Hemisphere

Abstract

Revealing the physiological mechanisms and the implications of changes in spring phenology is crucial for understanding ecosystem productivity and functioning. Previous studies reported an increased vegetation growth associated with warming-induced earlier spring phenology, which can in turn make feedback to the climate systems and increase summer drought. The effects of preseason drought on spring phenology, especially their combined implications for vegetation growth, however, remain largely unknown. We first used four standard methods to extract dates for the start of spring phenology (SOS) from Normalized Difference Vegetation Index (NDVI3g) data across extratropical ecosystems in the Northern Hemisphere (>30°N) during 1982–2015, then calculated the sensitivity of SOS to preseason meteorological drought (preseason drought) using the Standardized Precipitation–Evapotranspiration Index (SPEI), and finally analyzed the coupling legacy effects of preseason drought and advanced SOS on seasonal vegetation growth across biomes. Our findings indicated that between 1982 and 2015 (1) SOS was positively correlated with preseason SPEI in 68.5% of pixels, 33.3% of which were significantly correlated (p < 0.05); the dominant timescales of preseason SPEI (preseason length) were 1 and 6 accumulated months. Mean SOS under extreme preseason drought also advanced in each biome. (2) Interestingly, SOS, rather than preseason drought or other climatic factors, had a dominant but opposite effect on spring (positively correlated) and summer (negatively correlated) vegetation growth. Most (86.4%) areas with increased spring growth were in regions with an earlier SOS, even under extreme preseason drought, but the combined effects of preseason drought and an earlier SOS greatly decreased summer vegetation growth in all biomes. Our results demonstrated that the preseason droughts during 1982–2015 had not yet reached the point beyond which they could overwhelm the role of warming in advancing spring phenology in most regions, but the earlier SOS induced by warming could exacerbate the effects of preseason drought and eventually have profound negative effects on vegetation growth in the middle and later stages of the growing season. This synthesis will advance our current understanding of mechanisms underlying atmosphere-biosphere feedbacks, and benefit biogeochemical models in simulating vegetation growth.