In 2015 Southeastern Wisconsin experienced an exceptional amount of fruit being produced, causing an unpleasant problem for many homeowners. A walnut falling from the canopy can hit you on the head, acorns covering a sidewalk can cause you to slip and fall, mulberry and ginkgo fruit may ferment and produce a horrendous odor. It doesn’t happen every year, but when trees produce bumper crops of fruit it is easy to take notice. Why do trees make so much fruit, and when does it happen? The term used for the exceptional fruit production in a tree is mast year. Masting occurs in cycles, not every year will we see huge amounts of fruit falling from trees. As it turns out, masting is an evolutionary adaptation for successful reproduction. There are two functions trees need to pull this off; variability and synchronization.
Variability refers to the idea that not every year’s fruit production is the same. It was thought that mast years could be predicted by resource tracking. This term refers to years with an abundance of resources available such as water, light, and nutrients. The theory is that when this occurs trees will be happy and produce a high volume of fruit. However it isn’t always true, and is difficult to correlate. Instead we view masting as a cycle, due to the fact that fruit production takes a lot of energy. After a tree has a mast year it has worn out its energy reserves, and will need to recuperate and accumulate new reserves before having enough energy to mast again. If a tree produced a lot of fruit one year it is unlikely there will be a high production level the following year. The idea of cycling is a better predictor for when trees might mast next.
Fruit contains the genes necessary to produce new trees if allowed to germinate. Seeds are also food for predators such as deer, squirrels, and mice. As an important food source, fruit production cycles directly correlate with predator populations. In years with low fruit production there is less food to go around, and fewer predators will survive into the next year. In low production years all of the fruit may be eaten by predators, but it also lowers predator population for the next year. In mast years fruit production is so high that the reduced predator population can’t possibly consume all the fruit, allowing some to germinate and become new trees. This is the concept of predator satiation.
Outsmarting predators with variability doesn’t explain the whole picture, as trees also need to synchronize to be successful. Trees of the same species should all mast at the same time to increase the odds of pollination and produce viable offspring. This is another great evolutionary adaptation when cross pollination is needed. It makes sense that if pollinating with other trees is required; they all do it at the same time and in the same year. This is the idea of pollen coupling.
Trees need to know when to synchronize. So how do trees talk to one another? Our best guess is the Moran effect. This states that because of climate, not weather, similar trees can respond to similar environmental cues together. Trees that are exposed to the same climate cues and respond in sync will be able to successfully reproduce, but trees that are out of sync will be removed from the gene pool. The end result is trees that are synchronized in quantity and timing being able to maximize their reproductive potential over large land areas.
Masting is great for trees but it has other implications for the ecosystem at large. Predator population manipulation can be both good and bad. Certain bird species that rely on a particular tree fruit may disappear entirely from an area when several low yield years occur in a row. Conversely, when squirrel and mice pollutions peak with a mast year, so do tick populations that may carry human borne illnesses. Following a mast year we see an increase in Lyme’s Disease and Alpha-gal allergy (which makes us allergic to red meat). Masting increases reproduction success by working in unison, but that can also mean if you have allergies, those years can be far more difficult for outdoor enjoyment. We know if a mast year occurred this year it is unlikely to occur next year, but as of now we still don’t know how to effectively predict a mast year. Ongoing research hopes to eventually find an answer, until then stay out from under trees when they mast so you don’t think the sky is falling.