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What is a 'mast year' and why is it making autumn bigger than ever?

With autumn in full swing it’s that time of year again when trees produce beautiful colours and then sadly start to lose their leaves. We have received a number of reports of early tinting of leaves this year, but have you noticed anything unusual about woodland trees' fruit?

Leaves are a distinct indicator of autumn as warm colours light up woodland (Photo: WTML)
Leaves are a distinct indicator of autumn as warm colours light up woodland (Photo: WTML)

What is a mast year?
Mast is the term used to describe the fruit of forest trees such as acorns and beech seeds. A mast year is when the trees that produce these fruits have a bumper crop and produce much more fruit than they normally would. Trees such as oak and beech fluctuate massively year on year in regards to the fruit they produce, some years no fruit will be seen and other years an exceptional crop is observed.

Why do trees do this?
One of the major theories amongst scientists for this behaviour is predator satiation. Many animals including mammals and birds feed on this woodland fruit, the trees in effect holding back for a few years keep these populations low. Then during a mast year more food is produced than these frugivores can possibly eat, ensuring that some seeds will start to grow. This has a major evolutionary advantage for the tree; producing seeds is costly work and therefore it needs to be ensured that some of their fruit will grow. Producing fruit in a mast year does stunt growth of the tree but as this only occurs every five to ten years it’s a worthy pay off to ensure the production of more saplings. 

How do trees do this?
Mast years are not just one off events occurring with one specific tree - the vast majority of woodland trees across the UK will have a fantastic crop. So how do trees seemingly miles apart communicate with one another? This is one of nature’s mysteries; however, it is thought that it is probably to do with the weather. The right combination of temperature and rainfall in the spring is thought to trigger this response.

Implications of mast years
It is thought that in mast years incidences of Lyme disease across the UK could increase. The increase in foraging materials for small mammals such as mice causes booms in their populations. Mice carry this tick-spread disease so the more that survive the winter the more ticks they will infect in spring.

Our Nature’s Calendar website records a number of seasonal events including amount of fruit for species such as oak and beech. Have you seen an exceptional amount of fruit on these species this year? Get recording and help us to determine whether or not we are in a mast year!

Noticed the seasons changing? Record your nature observations here:

Nature's Calendar