Feast and famine in the forest
If you walk under oaks or beeches over several autumns, you may notice something odd. One year the ground is covered with acorns or beechnuts, crunching under your boots. The next year, hardly any can be found. This pattern, where a tree species produces very large seed crops at irregular intervals and little in between, is called masting. The big years are called mast years.
The term “mast” originally referred to nuts and acorns used as feed for pigs and wild animals. Today it is used for the pattern in general, and it occurs in many kinds of plants, including pines, spruces, dipterocarp trees in parts of Southeast Asia, and some grasses and shrubs.
What makes it mast seeding
Three features define the pattern:
- Variability. Seed output from year to year swings much more than you would expect from weather alone.
- Synchrony. Many trees of a species in an area, sometimes over a wide region, produce large crops in the same year.
- Intervals. Big years come at uneven gaps. For some species they are every two to five years; for others longer. Because timing varies by species and place, it is best not to expect a fixed cycle.
Why would a tree do this?
Making seeds is expensive. A heavy crop uses sugars and nutrients that could have gone into growth. Why not produce a moderate crop every year? Several explanations are widely discussed, and they are not mutually exclusive. Researchers continue to refine them.
Predator satiation
This is the leading explanation. Seeds are food for rodents, deer, boars, birds, insects and others. If a tree produced the same modest crop every year, seed eaters would keep their numbers up and eat most of it. But if there are several lean years followed by a very big year, seed eaters starve or decline during the lean years, and then in the mast year there are more seeds than they can eat. Some survive and may grow into trees.
Pollination efficiency
For wind-pollinated trees, such as oaks, beeches and many conifers, synchronized flowering fills the air with more pollen, which raises the odds that each flower is fertilized. A single tree flowering alone in a sparse stand would have little chance, but a whole forest flowering together does. This idea is often called the pollen coupling hypothesis.
Resource accumulation
Another idea holds that trees build up reserves over lean years and spend them in a big year. This makes sense for a tree: after a heavy crop, it may need time to rebuild its reserves. Observations that growth of trees such as oaks and beeches is often reduced in mast years are consistent with this idea, although the full mechanism is still debated.
What triggers a mast year
Synchrony across many trees needs some kind of shared cue. Weather is the usual candidate. In many species, the conditions during flower formation or flowering, such as a warm, dry summer the year before, or a favorable spring, can influence how heavy the crop will be. Because the same weather affects trees across a region, it can line them up. The cue differs among species and regions, so no single rule applies.
Resource limits probably also play a role: a tree that has just had a heavy crop is less likely to have another in the following year, which helps create gaps between mast years.
Who benefits, and who suffers
A mast year ripples through the whole food web.
- Rodents. Mice and voles often increase after a heavy crop, sometimes sharply in the following season. Their numbers can then fall when food becomes scarce.
- Predators. Animals that eat rodents, such as owls, foxes, and martens, may do better when rodent numbers are high.
- Larger seed eaters. Deer, wild boar, squirrels, bears and birds like jays and pigeons may feed heavily on seeds. Their body condition and reproduction can be influenced in some regions.
- Other links. Several studies suggest that rodent booms after mast years can affect nesting birds, because predators switch to nests when rodent numbers decline. Such effects have been reported but depend heavily on the local system, so they are best treated as possibilities rather than rules.
Because many animals have evolved around this pattern, some store seeds for the lean years. Jays and squirrels bury acorns and nuts, and those they do not recover may germinate. In this way the seed eaters double as dispersers, an aspect of seed dispersal.
Mast years and forest regeneration
For a forest, the practical result is that new seedlings tend to appear in pulses. A large cohort of oak or beech seedlings may establish in the year after a heavy crop, if conditions are right, while the intervening years give few. Whether they survive depends on light, browsing and weather. A mast year that coincides with a canopy gap can be especially important for establishing the next generation.
Foresters who rely on natural regeneration have to take this into account. If a harvest or thinning is planned to encourage natural seedlings, timing it with a good seed year, or being prepared to wait for one, is often part of the plan. Seed collectors for nurseries and tree planting programs also have to plan around unpredictable supplies, since good seed crops of some species may come only every few years.
Climate and changing patterns
Because weather cues seem to matter, there is interest in how a changing climate may alter masting. Some studies in Europe and elsewhere suggest that warming may shift the frequency or the synchrony of mast years in certain species, but findings vary and the topic is still being investigated. The effects may reach seed eaters and the animals that depend on them.
How to observe it yourself
You can notice masting without special equipment. Note each fall how many acorns, beechnuts or cones you see under familiar trees, and compare years. Local records from landowners, hunters and naturalists are often informative, and some projects collect these observations as part of citizen science. A few seasons of notes are usually enough to see that the forest does not produce its seed evenly.