Direct observations as well as Normalized Difference Vegetation Index\ud (NDVI) data from satellites have shown earlier leaf appearance in the northern\ud hemisphere, which is believed to result from climate warming. The advance of leaf\ud out to earlier times in the year could be limited or even reversed, however, as\ud temperate and boreal trees require a certain amount of chilling in winter for rapid\ud leaf out in spring. If this chilling requirement is not fulfilled, an increasing amount\ud of warming is required. Implications of these chilling requirements at the biome\ud level are not clear. One approach to estimate their importance is to generalize\ud the exponential relationships between chilling and warming established for single\ud species. Previous work using NDVI data suggests that this is indeed feasible but much\ud has been limited to specific biomes or a very few years of data for the modelling.\ud We find chilling requirements for northern temperate and boreal biomes by fitting\ud various phenology models to green-up dates determined from NDVI using various\ud methods and 12 years of data. The models predict that in northern middle and high\ud latitudes the advance of green-up will be limited to a total of 4 to 5 days on average\ud (but up to 15 days regionally) over the time period 2000–2060 as estimated using\ud two contrasting climate simulations. This results from the exponentially increasing\ud warming requirements for leaf out when winter chilling falls below a threshold as\ud shown by a comparison with models that consider only spring warming. The model\ud evaluation suggests an element of regional adaptation of the warming required for\ud leaf out in large biomes.Peer-reviewedPost-prin
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