Little change in heat requirement for vegetation green-up on the Tibetan Plateau over the warming period of 1998–2012

We investigated spatial and temporal variations in the accumulated growing degree-days (AGDD) requirement for spring vegetation green-up onset on the Tibetan Plateau (TP) during the rapid warming period of 1998–2012, which is essential for understanding and modeling the spring vegetation phenological responses to climate change. Annual green-up date was determined by using a satellite-derived normalized difference vegetation index. The annual AGDD requirement was then determined as the sum of daily mean air temperatures above 0°C over the dormancy period. Linear regression between the AGDD requirement and year was used to determine its temporal trend. Partial correlation analysis was used to assess factors controlling inter-annual variations in the AGDD requirement. We found the AGDD requirement was significantly different for different vegetation types and showed large spatial variations ranging from a few °C-days in cold and wet areas to more than 1000 °C-days in warm and dry areas of the TP. Despite the rapid warming and subsequent earlier green-up date, the AGDD requirement did not increase significantly over the period 1998–2012. Interestingly, we found that the inter-annual variations in AGDD requirement were extensively driven by the number of chilling days, while precipitation sum and insolation affected the AGDD requirement in limited areas. Although the winter warming reduced chilling accumulation on the TP, the AGDD requirement for vegetation green-up onset did not necessarily increase over 1998–2012, being insensitive to decline in the chilling accumulation. This suggested that the chilling requirement to break vegetation dormancy might be still fulfilled. In other words, the winter warming might not substantially affect the vegetation green-up process, since the reduction in chilling accumulation may not affect the dormancy break. However, the negative temporal correlation between the AGDD requirement and number of chilling days indicates that continued future warming may lead to a deficiency in chilling and thus an increase in AGDD requirement.