Influence of snowfall and melt timing on tree growth in subarctic Eurasia

The causes of a reduced sensitivity of high-latitude tree growth to variati ons in summer temperature for recent decades(1,2), compared to earlier this century, are unknown. This sensitivity change is problematic, in that rela tionships between tree-ring properties and temperature are widely used for reconstructing past climate. Here we report an analysis of tree-ring and cl imate data from the forest-tundra zone, in combination with a mechanistic m odel of tree-ring growth, to argue that an increasing trend of winter preci pitation over the past century in many subarctic regions(3-5) led to delaye d snow melt in these permafrost environments. As a result, the initiation o f cambial activity (necessary for the formation of wood cells) has been del ayed relative to the pre-1960 period in the Siberian subarctic. Since the e arly 1960s, less of the growth season has been during what had previously b een the period of maximal growth sensitivity to temperature. This shift res ults not only in slower growth, but also in a reduced correlation between g rowth and temperature. Our results suggest that changes in winter precipita tion should be considered in seeking explanations for observed changes in t he timing of the 'spring greening' of high-latitude forests(6), and should be taken into account in the study of the role of the Siberian subarctic fo rest in the global carbon cycle.