What Is a Mast Year? When Every Tree Agrees to Fruit at Once
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Some years oak trees drop acorns in overwhelming quantity and other years almost none, and the trees across a whole region do it in unison. That synchrony is not a response to a good summer, and the leading explanation is that it is a strategy aimed at the animals that eat the seeds.
Starving and swamping
The predator satiation hypothesis holds that a tree producing a steady moderate crop every year supports a stable population of seed eaters that consumes essentially all of it. Producing nothing for several years starves that population down, and then producing an enormous crop simultaneously across the region overwhelms what the reduced population can eat, so a proportion of seeds survives to germinate. The strategy only works if trees synchronise, since a single tree fruiting heavily in a lean year would simply attract every animal in the area, which is why the behaviour is regional rather than individual. Supporting evidence includes measured seed survival rates that are much higher in heavy years, documented crashes and booms in rodent and bird populations tracking the cycle, and the fact that the pattern is strongest in species whose seeds are heavily predated. A complementary explanation concerns pollination efficiency, since wind-pollinated trees flowering together achieve much better fertilisation than isolated individuals do.
How trees coordinate
Synchrony across hundreds of kilometres requires a mechanism and several contribute:
- •Weather cues shared across a region, particularly temperature in the spring of flowering and in the preceding year, which act as a common signal rather than as a direct cause
- •Resource depletion, since a heavy crop exhausts stored reserves and forces a recovery period, which imposes a rhythm
- •Flowering success, with frost during flowering capable of eliminating a crop across a whole area at once and resetting the cycle
- •Internal hormonal signalling within a tree that suppresses flowering after a heavy year
- •Pollen coupling, where trees that flower in step fertilise each other better and reinforce the synchrony
- •A growing body of work suggesting the cue is a temperature difference between successive years rather than an absolute value, which would explain why simple weather correlations have been inconsistent
What it does to everything else
A mast year propagates through the food web with a delay, and the sequence is well documented in oak and beech woodland. Rodent populations rise sharply the following spring on the abundant food, which increases predator numbers a season later, and also increases tick numbers since ticks feed on rodents, which has been connected in several studies to subsequent peaks in tick-borne disease including Lyme disease, giving the cycle a direct public health dimension. Wild boar and deer populations respond similarly where they are present. Seed-eating birds including jays cache large numbers of acorns and forget enough of them to plant substantial numbers of oaks, which is the principal dispersal mechanism for the tree. Conversely a failed year causes hardship, and historic records of pannage, meaning the right to graze pigs on fallen acorns, show communities tracking the variation because it determined whether livestock could be fattened for winter.
What is changing
Long-term records of seed production, some running for many decades at forest research sites, indicate that masting patterns are changing in several species, with evidence of weakening synchrony and altered frequency associated with warming. That matters because the strategy depends on synchrony, so trees that fruit out of step with their neighbours lose the satiation benefit and may suffer higher seed predation, which has implications for regeneration. Changes also propagate to the animals that depend on the cycle. Predicting mast years remains difficult and the forecasts made for forestry and for seed collection are approximate, which is a problem for restoration projects that need large quantities of tree seed and can only collect it when it exists. For the same reason, nurseries and conservation programmes maintain seed stores. The phenomenon is also a good illustration of a general point in ecology, which is that variability itself, rather than the average, can be the adaptive feature.
The takeaway
Producing nothing for years starves the seed eaters down, and then an enormous synchronised crop overwhelms what remains, so seeds survive, which only works if trees across a region act together. Shared weather cues, resource depletion and pollen coupling maintain the synchrony. Rodent numbers rise the following spring and tick numbers with them, which links the cycle to peaks in tick-borne disease.