Earlier green-up and senescence of temperate United States rangelands under future climate

Climate and vegetation phenology are closely linked, and climate change is already impacting phenology in many systems. These impacts are expected to progress in the future. We sought to forecast future shifts in rangeland growing season timing due to climate change, and interpret their importance for land management and ecosystem function. We trained a model on remotely sensed land surface phenology and climate data collected from 2001 to 2014 in temperate United States rangelands. We used this model to forecast annual growing season start dates, end dates, and season length through 2099 among six general circulation models and under RCP 4.5 and 8.5 scenarios. Growing season start was projected to shift earlier throughout our study area. In 2090–2099, start of season advanced by an average of 10 (RCP 4.5) to 17 (RCP 8.5) days. End of season also advanced by 12 (RCP 4.5) to 24 (RCP 8.5) days, but with greater heterogeneity. Start and end of season change mainly offset one another, so growing season length changes were lesser (2 days in RCP 4.5, and 7 in RCP 8.5). Some mountainous areas experienced both earlier start of season and later end of season, lengthening their growing season. Earlier phenology in rangelands would force adaptation in grazing and impact ecosystem function. Mountainous areas with earlier start and later end of season may become more viable for grazing, but most areas may experience slightly shortened growing seasons. Autumn phenology warrants greater research, and our finding of earlier autumn senescence contradicts some prior research.