Have you noticed that the Oak trees have been a bit bare this autumn? We’ve seen hardly any acorns but there were plenty last year. What’s going on?
The acorn crop does naturally vary from year to year, most noticeably due to the phenomenon of ‘masting’. In a ‘mast year’ Oak trees produce a bumper crop, with up to 10,000 acorns per tree. This creates such a super-abundant food supply for Jays, Wood Mice, Grey Squirrels and other small mammals that they cannot possibly eat them all – a strategy known as ‘predator satiation.’ Producing such a glut of acorns pretty much guarantees that some will survive to put down roots as the next generation of Oak saplings. It’s a successful strategy but it does come at a cost. Tree growth is stunted in a mast year, since nearly all of the Oak’s resources are diverted to acorn production. As a result, far fewer acorns will be produced in subsequent years as the tree recovers from its mammoth reproductive effort. Interestingly, producing fewer acorns for several consecutive years may also benefit the tree by keeping mice and other acorn predators under control, limiting their numbers via short rations.
Last year was a mast year, so this year was always going to be lighter on acorns – but they are still scarcer than expected. This is because the number of acorns produced also depends on weather conditions experienced in spring, when Oak trees are in flower. Only pollinated flowers will turn into acorns and Oaks are wind-pollinated, so they really need a warm, dry spring to create the ideal conditions for a good acorn crop. A cold, wet spring with late frosts will have the opposite effect, so this year’s challenging spring conditions on top of last year’s mast have resulted in a shortage of acorns.
Mast years tend to occur every 5-10 years but they don’t necessarily happen on a regular basis and are difficult to predict, due to the huge number of underlying factors that can influence when they might occur. The really fascinating thing about mast years is the coordination: if it’s going to happen, then it will happen right across the UK and nearly every single Oak tree will produce a bumper crop in the same year. Other trees also experience mast years, most notably Beech, and major mast years can even be synchronised right across northern and western Europe.