Bumper acorn crop – is 2020 a ‘mast year’?

You may have noticed a bumper crop of acorns and beech nuts in Jubilee Gardens and other parks this year, keeping the squirrels, mice and other critters happy and well-fed. It looks likely that 2020 will be declared a ‘mast year’ – one in which there is a much heavier crop of fruits and seeds such as acorns, conkers, berries or pinecones, from certain tree species than in a normal year.

Close-up of acorns and English Oak leaf - abundance in a mast year

The reasons for this aren’t fully understood, but Andrew Smith, Director of Westonbirt, The National Arboretum told Forestry England1:

“We experienced a warm and dry spring, which are the perfect conditions for flowers to ‘set’ seeds. This, along with no late frost meant that flowers and young fruit survived into summer. The warm and moist summer has meant the nuts, fruits and berries have filled out well and are continuing to ripen nicely.”

Weather and climate do have an impact, but mast years tend to occur in cycles – for Oak trees this is usually every four years according to Smith. There is a major evolutionary advantage to mast years for the tree; producing nuts and seeds requires a lot of energy and as a result slightly stunts the tree’s growth, however, with such an abundance of seeds there is an increased likelihood that at least some of the crop will germinate into new saplings.

Another theory about the reason for mast years, according to the Woodland Trust2 is ‘predator satiation’:

“Animals like squirrels, jays, mice and badgers feed on the acorns and beech nuts. When the trees produce smaller crops for a few consecutive years, they are in effect keeping the populations of these animals in check. But during a mast year, the trees produce more food than the animals can possibly eat. This abundance causes a boom in populations of small mammals like mice. More importantly, it guarantees some will be left over to survive and grow into new trees.”

Mast years are not just one-off events for individual trees. The vast majority of trees in a particular species will have a fantastic crop all across the UK in the same year. How the trees co-ordinate this when they’re so far apart is one of nature’s many mysteries.


Find out more about the different tree species in Jubilee Gardens here.



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