Good mast year for white and bur oaks

Acorns are the most important wildlife food in Midwest deciduous forests ("the ecological equivalent of manna from heaven"; McShea and Healy, 2002; see reference below) Mast is a term used to describe the hard fruit of various trees such as beech or oak. Oaks as a group produce lots of acorns only every three or four years, and a high production year is called a good mast year.

Last year was a good mast year at Pleasant Valley Conservancy for oaks of the red oak group (red, black, and Hill's) and this year is a good mast year for oaks of the white oak group (bur, white). (There were essentially no white or bur oak acorns last fall.) Now is the season when acorns are coming down, and near big oaks the ground is covered with acorns. Kathie and I picked up about pound of acorns under a single large white oak within a few minutes.

The oaks have an interesting life cycle. Soon after the acorns fall they start to germinate, sending out roots but no shoots. Root growth continues until cold weather intervenes. It begins again in the spring, at which time shoot growth also commences. This procedure may speed up tree growth in the spring and at the same time prevent the shoot from being damaged by the cold. According to U.S. Forest Service sources, sound acorns have between 50 and 99% viability.
An interesting aspect of masting is that it is a population phenomenon. In general all the oaks in a stand exhibit high or low acorn production the same year. Animals that make considerable use of mast exhibit population fluctuations that track the changes in mast production.

The masting phenomenon is characteristic of trees with large-sized seeds, and is thought by ecologists to have evolved as a means of ensuring sufficient seeds for the reproduction of the tree species. In high mast years, when large numbers of seeds are produced, there are insufficient animal seed eaters to consume them all, thus leading to successful establishment of new tree seedlings. Because there are more low-mast years than high, populations of acorn eaters are controlled by the low mast condition.

In one study, each species of oak had its own good mast years, which were different from those of other species. Over a five year period, there were three poor mast years, one moderate mast year, and one very good mast year. The cycle of masting will vary not only with the species, but with the location and various environmental variables.

High seed production provides an enormous input of food for seed consumers in forests. High-mast years lead to abundance of body fat in mammals, thus aiding in their survival during harsh winter conditions. Because rodents are short-lived creatures, their populations tend to fluctuate with quantities of mast. On the other hand, deer, which are longer lived, benefit from the high mast situation but do not exhibit the sharp population fluctuations that small mammals do.

Despite extensive research on masting in oaks, there is no single explanation for the phenomenon. Weather is an important factor, but how does it influence masting? Possible environmental factors include late spring freezes that might kill oak flowers, variations in the efficiency of pollination, the initial size of the female flower crop, and conditions that might affect flower survival. Larger trees produce more acorns than smaller ones, although not all large oaks are good acorn producers.

Our observations at Pleasant Valley Conservancy seem to show that the oaks of the same group (red or white) all mast together.


McShea, William J. and Healy, William M. 2002. Oaks and acorns as a foundation for ecosystem management. pp. 1-9 In McShea, William J. and Healy, William M (editors). Oak Forest Ecosystems: Ecology and Management for Wildlife. Johns Hopkins University Press, Baltimore.