Hard Mast—Feast or Famine

White oak (Quercus alba) acorn. Some years, white oaks produce a bumper crop of acorns.
Photographer: Matt Jones
Most kinds of oaks are notoriously inconsistent crop producers. During some years, many trees of one oak species or another may produce acorns in huge quantities, and sometimes several species will do so during the same year. American beech does the same with its beechnuts. This yields a bumper crop of acorns and perhaps beechnuts some years, and very little some other years. With a diverse mix of tree, shrub and vine species growing in a forest, if one mast (seed) crop fails, animals can turn to other plants for food, and the impact is not completely devastating. (That’s one reason plant diversity is so important!)

Scientists are still studying the relationship of mast-fruiting (cycles of bumper crops) and animal populations. It is important that after animals feed, there still be enough plant resources (seeds, leaves, twigs, and bark) left for the plants to survive and reproduce. Otherwise, neither the plant nor the animals will be sustained in future years. It seems that years of small crops may decrease animal populations, while high production years ensure that even after the animals have eaten their fill, seeds will remain to sprout and replenish the forest. The interaction between plants and their predators can be a delicate balance. When working properly, this interaction maintains healthy populations in both groups.

Large populations of gypsy moth caterpillars strip the leaves from mature oak trees. Predators such as white-footed mouse are needed to keep them in check.
Photographer: Flickr user Dennis Wilkinson
Another example of how the cyclic nature of bumper crops can sustain the health of a forest is the relationship between acorn production and gypsy moth populations. Gypsy moth caterpillars, which are non-native forest pests in the U.S., consume huge quantities of oak leaves. Each caterpillar eats up to one square foot (roughly 900 square centimeters) of leaves per day! This reduces the energy available to the trees to produce acorns.1

Fewer acorns mean fewer white-footed mice and other small mammals, which are predators of gypsy moth caterpillars. Without plenty of predators, gypsy moth populations explode and do lasting damage to oak forests over a period of years.2 Thankfully, occasional years of bumper crops of acorns resets the balance. The abundance of acorns supports larger populations of small mammals that in turn help control the gypsy moths.