Plantwatch: Acorn crop dips as climate changes

Oak leaves in Britain are still mostly green. The oak is one of the last trees to change colour in autumn, although many have already dropped their acorns. But the days of finding lots of big acorns may be drawing to a close since they are falling victim to climate change.

A study of acorns has revealed that the warmer the weather the smaller the crop of acorns. Tim Sparks, a professor at Coventry University, has examined more than 160,000 observations of oaks and found that the more the first dates of flowering vary in springtime the poorer the acorn crop. In warmer springs the oak trees flower in a less synchronised fashion over a longer period and this gives a smaller crop of acorns in autumn, a drop of about 20%.

In cooler springs the oak flowers within a shorter period and the blooming is more synchronised, allowing more successful cross-pollination.

These days acorns are ripening about 13 days earlier than they did 10 years ago, a phenomenon that matches a widespread earlier shift in fruit and nuts ripening on British trees.

Another sign of the changes is autumn tree-leaf colours tending to appear later in the year and leaves staying on the trees longer; oak leaves now seem to lose their leaves about a week later than was the case 30 years ago.

The observations of the oaks and other trees came from a yearly survey of spring and autumn called Nature’s Calendar, run by the Woodland Trust. Each year the public is asked to record signs of those seasons in the natural world. It has built up into an invaluable, and timed, record.