Predicting Future Temperate and Boreal of Growing Season Start With a Land Surface Model

Controlled ecological experiments show that temperate and boreal trees require chilling in winter for rapid leaf out in spring. If the amount of chilling falls below a species specific threshold then an exponentially increasing amount of warming is required to initiate leaf out - potentially actually delaying it in a future warmer climate. The boreal areas could be particularly affected as climate predictions indicate strong warming in these regions. Moreover, currently a large part of the land carbon sink is located in temperate and boreal regions and a changing growing season start might have a large impact on this important sink. Warming-chilling models for green-up, which have been calibrated with remotely sensed normalized difference vegetation index from the years 1983-1995, indicate that in future the chilling requirements reduce the rate of advance of the start of the growing season to earlier times compared to advance rates in the last two decades. Climate scenarios with large warming (IPCC A2 scenarios) show lower advance rates of green- up to earlier times than predictions with a smaller warming (B1 scenarios) due to the reduced chilling in high warming scenarios. When incorporated into a coupled land-surface carbon cycle model based on JULES (the Joint-UK-Land Environment Simulator) the chilling requirements lead to a early growing season photosynthetic carbon up that is correspondingly lower than in simulations where the start of the growing season as simply modelled as responding to warming only. Thus the phenological response in effect provides a positive feedback to global warming.