Masting is the synchronized and highly variable production of seeds by a population of plants, from oak trees producing acorns to certain types of grasses. In some years, known as mast years, these plants will produce a massive, coordinated abundance of seeds, while in other years, their output is minimal or nonexistent. This “feast or famine” cycle is a widespread and coordinated natural event, driving many ecological processes.
The term “mast” originates from an Old English word describing the accumulation of nuts on the forest floor, which were used to fatten livestock. Today, it refers to the fruits and seeds of forest trees and shrubs that serve as a food source for wildlife. This irregular pulse of food production is a fundamental aspect of forest dynamics. The strategy is particularly common in wind-pollinated trees but has been documented across various plant types.
Evolutionary Advantages of Masting
The evolution of masting is driven by significant reproductive benefits. One primary advantage is a strategy known as predator satiation. During a mast year, the sheer volume of seeds produced overwhelms seed-eating animals like squirrels, mice, and birds. These animals can only consume a fraction of the available food, ensuring that a substantial number of seeds survive to germinate and grow into new plants.
In the years between mast events, the scarcity of seeds keeps the populations of these predators in check. With a limited food supply, the number of seed-eating animals cannot grow to a level that would threaten the entire seed crop of the next mast year, giving the plants a better long-term chance of successful reproduction.
Another significant advantage, particularly for wind-pollinated species, is increased pollination efficiency. When all the plants in a population flower at the same time, it dramatically increases the density of pollen in the air. This synchronization maximizes the likelihood of successful fertilization for each individual plant. The coordinated release of pollen ensures that a higher percentage of flowers are pollinated and develop into viable seeds.
Triggers of a Mast Year
The coordination of a mast year across a wide geographic area is a response to specific environmental and internal factors. Large-scale weather patterns often serve as the primary environmental cue. For many species, a specific combination of conditions, such as a warmer-than-average summer in one year followed by another the next, can signal that it is time for a synchronized reproductive effort.
This external signal is coupled with an internal process of resource management. Plants spend several non-mast years accumulating and storing energy, primarily in the form of carbohydrates. Reproduction on such a massive scale is energetically expensive, so plants must build up sufficient reserves before they can invest in a mast event.
Once a plant has accumulated enough energy and receives the correct environmental signal, it allocates its stored resources to one massive reproductive event. This combination of resource availability and environmental triggers allows for the highly synchronized and variable seed production that defines masting.
Ecological Ripple Effects
Masting events create dramatic “boom-and-bust” cycles. A mast year, with its super-abundance of seeds, provides a massive food source for seed-eating animals, known as granivores. Populations of animals like mice, voles, squirrels, and even larger animals like deer and bears can increase significantly in response to this sudden glut of nutrition.
This boom in herbivore populations has a cascading effect up the food chain. The predators that feed on these seed-eaters, such as owls, foxes, and weasels, find themselves with an abundance of prey. This can lead to higher reproductive success and survival rates for these predators, causing their populations to grow in the year following the initial mast event.
Conversely, the lean years that follow a mast event trigger a “bust” cycle. The scarcity of seeds leads to widespread food shortages for the animals that had thrived just a year before. This results in sharp declines in the populations of granivores, which in turn leads to a decline in their predator populations. This cyclical fluctuation of animal numbers is driven by the plants’ reproductive strategy.
Human Health and Masting Cycles
The ecological consequences of masting extend beyond the forest and can have direct implications for human health. There is a well-documented link between mast years, particularly of oak trees, and the incidence of tick-borne illnesses like Lyme disease.
The chain of events begins with the massive production of acorns during an oak mast year. This abundance of food fuels a population boom for the white-footed mouse, a primary consumer of acorns. The mouse population typically peaks in the summer of the year following the mast event. These mice are also highly efficient hosts for blacklegged ticks, which are the primary vectors for the bacterium that causes Lyme disease.
A larger mouse population supports a larger population of ticks. More importantly, mice are key reservoirs for the Lyme-causing bacterium, Borrelia burgdorferi. As larval ticks feed on the infected mice, they become infected themselves. This leads to a higher number of infected ticks in the environment two years after the initial mast year, increasing the probability that humans will encounter an infected tick and contract Lyme disease.