Autumn feels the heat

I’VE just been checking an oak tree at the bottom of my little wood. It grew from an acorn from the Killarney National Park and I planted it out, as a young sapling, about 25 years ago.

Since then it has thrived and, with the help of a little pruning of the lower branches, turned into a beautifully formed tree.

But the most satisfying thing is that this year, for the first time, it produced acorns of its own. There weren’t many of them but then this is very young for an oak tree to fruit —- they’re often 50 to 80 years old before they produce a decent crop of acorns. It was early December and the acorns had all gone. I couldn’t work out whether something had eaten or whether they’d fallen off into the undergrowth.

But the other amazing thing was the amount of leaves still left on the tree. The lower ones were yellow blotched with brown —- oaks aren’t the most satisfying species to grow for autumn colour —- but most of the leaves on the upper branches were still green. This is very late in the year for a deciduous tree to still be in leaf.

Several people have commented to me on how fine the autumn colour has been this year and on how long it has lasted, and the subject has been mentioned on the radio and in newspapers. There are several factors contributing to this but one of them seems to be climate change.

Phenology, the study and timing of seasonal changes, is a very ancient science but one that has acquired a new relevance since we started to suspect that human activity was changing global climate. Because of this autumn leaf colour in deciduous woods in Europe and North America has been monitored by satellite since 1982. The results show that autumn has been happening four days later every decade. A simple sum shows that autumn colour now occurs nearly two weeks later than it did in the early 1980s.

The only credible explanation for this is climate change, though the general trend was reinforced by the weather this year. A slow, late spring inhibited the breaking of buds and the growth of young leaves, this was compensated for by an unusually sunny summer when photosynthesis could do a bit of over-time and produce extra starches and sugars in the leaves. Then autumn and early winter have been relatively mild, with few heavy frosts and, even more importantly, a lack of powerful gales to strip the leaves off the trees.

All this has resulted in one of the most glorious autumns I can remember, and one of the longest.


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