Worldwide observations that the timing (or phenology) of the onset of plant leaf-out in the spring (hereafter green-up) is occurring earlier and moving more northward in the northern hemisphere offer compelling evidence for global climate change. Plant phenology is driven by environmental conditions such as increases in mean temperatures and the subsequent availability of water precipitation. The physical and biochemical processes that govern green-up also feedback and modify local climate and microclimate, i.e., air and soils temperatures, net surface radiation, albedo, hydrology, and carbon uptake. Currently, regional variation in trends in vegetation phenology is not well understood and previous analyses have offered contradictory results. However, newer satellite data allow more reliable mapping of green-up trends throughout North America. Here, we present the first major analysis of green-up time-series throughout North America using MODIS satellite vegetation phenology data. We investigate trends in green-up on a continental and ecoregional basis and explore climatic drivers of those trends.
We report two major gradients in green-up trend over the 2001-2012 period: i) A longitudinal gradient from advancing green-up in Northeastern United States to green-up getting later in Northwestern United States. This gradient had not been previously observed and may represent a switch in green-up trajectory in Northwestern United States from earlier to later. ii) A latitudinal gradient from later green-up in Mexico to earlier green-up in the Canadian arctic. The trends in green-up adhere broadly to major ecoregion bounds. The trends are also consistent with regional trends in environmental drivers such as the timing and accumulation of precipitation and thermal degree-days, and the timing of snowmelt. These recent trends in spring vegetation phenology demonstrate the dynamic nature of phenologies and their climatic drivers and have important implications for ecological communities responding to changing climatic conditions.