Mast year for Oak trees: Why are there so many acorns in 2025?
- Amanda Zambon
- Oct 19
- 5 min read
If you’re in the UK, you may have noticed that there are tons of acorns on the ground at the moment!
As much as I love the crunching sound under my feet as I walk, I have found myself wondering if this is normal. Perhaps you have felt the same.
Well, it turns out that this year is a bumper year for acorns, also known as a mast year! I hadn’t heard of this before so got curious and started exploring what that means. Read on if you’d like some answers too …
What exactly is a mast year for acorns?
Certain trees, like Oaks and Beeches, do not produce the same amount of nuts each year. Instead, every 3-5 years they decide to have a “mast year” where they produce an absolute glut of nuts.
The reason behind this is that it completely overwhelms their predators who could not possibly eat ALL the nuts produced - this is called “predator satiation”. It means that some then nuts are able to avoid being eaten and thus to germinate and grow into new trees.
Quite clever, really!
What determines a mast year for oaks?
It’s often brought on by climate - so this year we had a dry spring which allowed pollen to stay viable for longer and travel farther.
However what’s interesting is that the trees within a region communicate with one another and they ALL decide to produce a glut of acorns. One tree will not go it alone.
As Robin Wall Kimmerer notes in her book “Braiding Sweetgrass”, it’s a phenomenal act of collaboration between the trees - very different from the sort of individualistic approach taken in our market economies.
Scientists are still exploring exactly how the trees communicate about this or the exact mechanism behind behind their decision. Theories include:
Mycorrhizal communication via the trees’ underground fungal networks (often referred to as the wood wide web!).
A “mother tree” determining decisions and how resources are shared across tree communities.
Pollen coupling - When one oak releases huge amounts of pollen, it increases pollination success for nearby trees. Over time, this mutual boost synchronises their reproduction, a kind of atmospheric “conversation” through pollen.
How does this play out internationally?
Fascinatingly, my recent post on threads about it being a mast year in the UK also garnered attention from elsewhere in the world, with fellow threaders in the Netherlands and the USA also commenting that they had experienced mast years.
Given that the UK is an island, what’s going on here? Are the trees communicating across oceans? This would be phenomenal.
But it’s not the case!! More likely, similar climate conditions are triggering processes within tree populations internationally. However even trees hundreds of kilometers apart can be within the same population from this perspective, though they don’t cross oceans!
How does a mast year impact predators?
As you might expect, a mast year means a feast for predators of acorns such as squirrels. This then sees populations boom though when there is a more meagre availability of food the following year they find themselves hungry and the population is then regulated and many animals then die of starvation.
This seems like a pretty brutal reality but it is how nature regulates different populations and creates and maintains ecosystems.
How about gall wasps?
Some people have noticed that as well as seeing more acorns, there have been more wasp galls, as pictured below.
Gall wasps are incredible creatures which lay their eggs in oak buds, leaves, or developing acorns. The larvae trigger the tree to form a gall, a little “house” of plant tissue they can live and feed inside.
The knopper gall in particular lives inside the acorn as pictured above. What is fascinating about knopper galls in particular is that they are dependent on two types of oak - they cannot complete their life cycle without both the English Oak and also the Turkey Oak, which incidentally was only introduced to the UK in the 18th Century. Gall wasps were first recorded in the UK in the 1950s.
During a mast year, people may report seeing more knopper galls, but studies have shown that they proportionally make up less of the acorn population during a mast year than in other years.
Concluding thoughts
I started this blog wondering why there were so many acorns crunching beneath my feet - and I’m ending it with a sense of wonder at how interconnected everything is.
How these incredible cycles of abundance and scarcity are being coordinated on such a vast scale all around us, quietly, without us even knowing. Even the tiny knopper gall wasp’s fate is impacted by this rhythm.
While we may get caught up in politics and celebrity gossip, all around us nature is moving to a wonderfully delicate dance if only we pause to notice and appreciate it.
If you enjoyed this blog consider signing up to my newsletter, Wild Soul Whispers, where I provide an in depth plant profile each month and explore ways we can be connected to nature’s seasons.
Bibliography
Braiding Sweetgrass – Robin Wall Kimmerer (2013). Milkweed Editions.
Kimmerer’s reflections on mast years and the gift economy of nature inspired the section on oak collaboration.
Woodland Trust. “What is a mast year?” (2020).
Scientific American. “This Fall Is Full of Acorns, Thanks to a Mast Year.” (2021).
Kelly, D. et al. (2023). “Mast seeding: Study of oak mechanisms carries wider implications.” Current Biology.
Koenig, W. D. & Knops, J. M. H. (2015). “What drives masting? The phenological synchrony hypothesis.” Ecology Letters.
Nussbaumer, A. et al. (2021). “Contrasting Resource Dynamics in Mast Years for Oaks.” Frontiers in Forests and Global Change.
Suzanne Simard. Finding the Mother Tree. (2021). Knopf Canada.
Referenced for ideas about communication, resource sharing, and “mother trees.”
Met Office. “A review of the UK’s climate in 2024.”
Centre for Ecology & Hydrology. “Record dry conditions follow months of wet weather.” (2025).
Selva, N., et al. (2012). Mast pulses shape trophic interactions between fluctuating consumers. PLOS ONE, 7(12): e51267.
Shows how rodent populations surge after mast events and then crash when food resources decline, reshaping woodland food webs.
Greenberg, C. H. & Zarnoch, S. J. (2018). A test of the predator satiation hypothesis, acorn predator size, and acorn preference. USDA Forest Service Research Paper SRS-57.
Tests how predator abundance and acorn removal rates change with mast intensity, providing evidence for predator satiation.
Zwolak, R., Bogdziewicz, M., & Steele, M. A. (2022). Global patterns in the predator satiation effect of masting: A meta-analysis. Ecology Letters, 25(5), 1055–1067.
A large-scale meta-analysis showing that mast events effectively reduce seed predation and cause population declines in seed-eating animals during low-mast years.