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November 21, 2006
It has been obvious to any woodland walker that this has not been a good acorn year; in fact, these fruits of the oak trees have been noticeable in their scarcity. This is of concern because acorns are a very important wildlife food, and many of our creatures may already be feeling the pinch. Birds such as jays and woodpeckers, small mammals like mice and squirrels, and even large animals such as bears and deer depend upon them. It has been estimated that acorns may constitute up to 25% of the diet of some deer in the autumn.
The acorn is a nut usually containing a single seed enclosed in a tough, leathery shell, and attached to a cup-shaped cap. Acorns are important to wildlife because they are large, rich in nutrients, and can be stored for later use. Percentages vary from species to species, but all acorns contain large amounts of protein, carbohydrates and fats, as well as the minerals calcium, phosphorous, and potassium, and the vitamin niacin. It is interesting that red oak acorns are 18-25% fat, while white oak acorns are only 5-10% fat.
Acorns also contain bitter tannins, the amount varying with the species. This is particularly true of the acorns of red oaks that contain 6-10% tannin while white oak acorns have less than 2% tannin. While many insects, birds and mammals seem to metabolize tannins with few ill effects, some reportedly select acorns that contain lesser amounts, while others may store the acorns until sufficient ground water has percolated through them to leach out some of the bitterness. Still other creatures seem to get along by diluting their acorn diet with other foods.
Acorns were a traditional food of many indigenous peoples of North America. They used rock grinders to break open the acorns, and they would soak the acorns in the streams for many days to leach the tannins. Because some species geminate in the fall, the women shelled and pulverized those acorns for immediate use, while spring-germinating types were dried in the sun to discourage mold and then cached in hollow trees or structures on poles to keep them safe from mice and squirrels.
Acorn production typically starts very slowly at around the tree's 25th year, and can eventually reach a yearly crop of about 2000 acorns. Production varies from year to year, as not even the healthiest and largest oak can seem to accumulate enough food and energy to produce strong crops two years in succession. In addition, a late spring frost can damage the flowers, or drought and insect infestations can affect the crop. Curculio weevils bore into immature acorns to deposit their eggs, and some years, up to 90% of the crop can be infested by these insect larvae. Luckily most of these nutritious larvae are eaten along with the acorns, as those that survive eventually destroy the kernels.
Oaks need help to spread their heavy acorns into areas away from their bases, usually depending upon jays and squirrels carry them off. Although these animals remember most cache locations and later return to empty them, a small number of acorns manage to germinate. Red oak acorns don't sprout until spring, but white oak acorns sprout in the autumn soon after they have fallen from the tree. Once they sprout, they are less nutritious, as the seed tissue converts to the indigestible lignins that form the root. Periodically, there is an especially abundant crop of nuts called a "mast year". Europeans have been keeping track of these good seasons for hundreds of years hoping to find a usable pattern, but no reliable predictors have been discovered. It has been suggested that such mast years are necessary for the oak's survival as they supply excess over what the acorn-eaters can destroy.
There are six relatively common species of oak in Wisconsin in two general groups: the red and white. The red type, including the rubrum, the Northern pin, and the black oaks, have leaves with sharp-pointed, shallow lobes. Their acorns contain considerable tannin and they usually require two years to mature. The white type, that includes the alba, swamp white, and bur oaks, have leaves with rounded lobes. Their acorns mature in one year, are not as bitter, and their shells are hairless inside. The white oak is found in well-drained woods, the swamp white oak is usually found on set sites, and the bur oak, a large spreading, long-lived tree typically grows in the open. Its presence in our woodlands indicates that the area was once open grassland. It is also fire resistant, and possesses significant drought resistance by virtue of a long taproot.
Oak trees often live 200 or more years, and for many years
it was thought that the largest was the Wye oak in Maryland. This
tree was believed to be more than 460 years old, and measured
some 32 feet in circumference, 105 feet tall, with a crown spread
of 158 feet. Sadly, this giant was downed in a violent thunderstorm
in 2002, but another certified oak in Louisiana surpassed it,
measuring more than 37 feet in circumference with a crown spread
of 150 feet, and an estimated age of more than 1000 years! Our
oaks are not record-holders, it is true, but most are well past
the century mark and a few bur oaks are magnificent specimens.
We treasure them.