Code 614.4 Branch Brown Bag Seminar - Dr. Xiaoyang Zhang

Earth Resources Technology, Inc. (ERT) at NOAA
Abstract: Warming climates have been widely recognized to advance spring vegetation phenology. However, the delayed responses of vegetation phenology to rising temperature and their mechanisms are poorly understood. Using satellite and climate data from 1982 to 2005, we reveal a latitude transition zone (in subtropical climate) of greenup onset in vegetation community that has diversely responded to warming temperature in North America. From 40 degrees N northwards, winter chilling requirement for plant dormancy release is far exceeded and the decrease in chilling days by winter warming temperature has little impacts on thermal-time requirement for greenup onset. Thus, spring warming temperature has constantly advanced greenup onset by 0.32 days/year. However, from 40 degrees N southward, the shortened winter chilling days are insufficient for fulfilling plant chilling requirement, so that the thermal-time requirement for greenup onset during spring increases gradually. Consequently, vegetation greenup onset changes progressively from an early trend to a later trend along the latitude transition zone from 40-31 degrees N, where the switch occurs around 35 degrees N. The greenup onset is delayed by 0.15 days/year below 31 degrees N. Finally, by combining phenology modelling, we find that the phenological transition zone shifts poleward with a rate of 0.1 latitude degree per year. This rate coincides well with the widening in tropical belt derived from meteorological data.