Response of vegetation growth and productivity to spring climate indicators in the conterminous United States derived from satellite remote sensing data fusion

Spring temperatures strongly influence plant phenology, including budburst, canopy development, and crop planting period. Recent spring warming coincides with earlier and longer non-frozen season trends, and generally earlier spring canopy onset and vegetation productivity increases over the conterminous US (CONUS). However, earlier spring onset increases frost damage risk, with potential negative impacts to productivity. Frost sensitivity and vulnerability is heterogeneous over the CONUS domain, while the occurrence, intensity and regional impact of frost events are difficult to monitor from sparse weather stations. To enhance regional frost risk monitoring capabilities, we developed spring frost day (SFD) and spring frost damage day (SFDD) metrics spanning a long-term (>30 year) record by integrating a satellite microwave remote sensing record of daily landscape freeze-thaw (FT) status and optical-IR sensor based phenology record of start of season (SOS) and day of peak (DOP) canopy cover. We find a decreasing regional SFD trend (-1.6 days decade(-1); p< 0.1) coincident with spring warming, while the SFDD is generally increasing (1.5 days decade(-1); p< 0.1). Spring warming is reducing frost occurrence, but an earlier SOS trend is paradoxically increasing vegetation frost damage risk. The ecological significance of the SFD and SFDD changes were evaluated using satellite derived vegetation gross primary production (GPP) and vegetation greenness (EVI2) anomalies. Higher SFD and SFDD levels coincide with reduced vegetation growth in spring, but only the SFDD shows significant (p <0.1) correlation with EVI2 summer growth anomalies. Apparent vegetation sensitivity to the SFDD varies across regional biomes and elevation zones, while an increasing SFDD trend indicates potentially larger negative impacts on regional vegetation growth with continued warming. (C) 2014 Elsevier B.V. All rights reserved.