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At this time of year we fixate on the effect of weather patterns, especially warmer autumns like the record-breaking October that we’re having, on leaf-peeping. Now a fascinating study has looked at the effect of warmer autumns on when leaves appear in the spring.

Their experiments on several European tree species, which they subjected to various conditions in greenhouses over the course of years, found that a warmer September and October makes leaves appear later in the spring, which is odd. A warmer winter, surprisingly, had no effect on that pattern while a warmer spring, as you’d expect, made trees leaf out earlier:

As expected, warmer spring temperatures led to earlier leaf-out. … Most interestingly, however, warming in October had the opposite effect and delayed spring phenology by 2.4 days/°C on average; despite six months between the warming and the flushing. The switch between the delaying and advancing effect occurred already in December. We conclude that not warmer winters but rather the shortening of winter, i.e., warming in autumn, is a major reason for the decline in spring phenology.

The reason, the paper says, involves what is called establishment of the dormant phase, in which buds enter a sort of winter hibernation after shedding leaves in the fall. Warmer autumns makes them put off going to bed: “later dormancy establishment delays the whole dormant phase, with a later peak and later release of dormancy. The buds become sensitive to warm temperatures later and, consequently, leaf-out later.”

This is important stuff for agriculture, modeling how climate change will affect crop yields.

The whole paper, written by four German researchers, can be read here. It is surprisingly understandable even by us layfolk.

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