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Country diary: a mast year for the mighty oak

Matt Shardlow
·2-min read

The eastern entrance to Old Sulehay forest is roofed by great oaks, still canopied with autumnal foliage, in contrast to the bare ash, hazel and field maple. The ground underneath crunches with a layer of swollen acorns. There are more than I can ever recall seeing – a magnificent acorn bounty repeated in reports from across England. And the size of them! More like artillery shells than nuts. This is what we call a mast year.

Mast is a traditional term for nuts produced by trees in the beech family – including oaks and sweet chestnuts, but also hazel (which has now been reclassified as a birch). Oak and beech go for several years producing small crops and then, boom, all the trees of the species, at sub-continental scales, are laden. The frequency of these mast years varies, sometimes every seven to 10 years, and then quickening like a heartbeat to every two or three years; consecutive mast years are fabled, but rare.

Related: From little acorns: into the woods in search of Britain’s ancient oak trees

The causes, mechanisms and coordination of this phenomenon continue to be debated, but it is thought that the stochastic abundance benefits the trees. The sudden glut of acorns may enable the oaks to first put them past the gall wasps and then to overwhelm acorn-feeding birds and mammals. Indeed, with so many nuts, the jays must be less likely to re-excavate their dispersed hoards.

Dusting away the fallen leaves reveals acorns bursting at the tip, with a wormlike root delving into the soil. This is not the ideal place for an oak to germinate: they are much more vulnerable to defoliation caused by winter moths than their parents, and are more likely to thrive if they do fall, or are transported away from the tree and its caterpillars.

The word mast comes from the Scandinavian “mat”, meaning food. Acorns were once collected and fed to pigs, a tradition that persists with pannage in the New Forest. Although toxic to horses, acorns are edible, if bitter, and would once have been a staple in human diets as well.

The last big mast year for oaks was in 2013 and the Woodland Trust, which monitors these things, is waiting to see if 2020 has eclipsed that year.