Q. A large red oak in our backyard produces bushels of acorns every year, but this year there are hardly any acorns to be found. Do you know what might have caused this acorn shortage? We're worried our squirrels won't have enough acorns to get them through the winter, especially since cold weather arrived early this year.
A. The lack of acorns this year is extreme and very widespread, extending from Nova Scotia to Virginia and as far west as Kansas, according to a recent article in the Washington Post by Brigid Schulte. It has generated an e-mail discussion among Penn State Cooperative Extension educators in horticulture and forestry, as well as questions from the public.
There are a number of reasons oaks fail to produce acorns, generally a combination of environmental effects and genetics. Forest ecologists have long noted that oaks produce an abundant acorn crop only one out of every three or four years. But they usually produce some acorns, even in bust years. And not all species of oaks -- there are 17 native to Pennsylvania and a few non-native species that have naturalized here -- would have a bust year at the same time.
Unfavorable weather accounts for the lack of acorns in many years, but given the differing climate and weather conditions occurring at any one time across such a large area, it is unlikely we can blame it all on the weather. Late frosts can kill oak flowers before they are pollinated. Of course, 2008's spring was one of the most spectacular in recent memory here in Pittsburgh, so we cannot blame a late frost.
Oaks are wind pollinated, rather than insect pollinated. Although cross-pollination -- the pollen from one tree landing on the female flowers of another tree -- is preferred because it increases genetic diversity, oaks are able to pollinate themselves. It is possible that heavy spring rains at bloom time could wash the pollen out of the air before it reaches female flowers.
But the National Weather Service records for the Pittsburgh area show no unusual rain events when our oaks would have been in bloom.
Another theory is that the oaks are stressed by drought or gypsy moth outbreaks. However, stressed oaks have a tendency to produce a lot of acorns in order to keep the species going.
Oaks are broken down into the white and red groups. The white oak group includes white oak, swamp white oak and chestnut oak. Their leaves do not have bristles on the lobes and their acorns mature in one growing season. The red oak group includes northern red oak, pin oak and black oak. Their leaves have bristles at the tips of the lobes, and their acorns take two seasons to mature. If unfavorable weather in the spring of 2007 interfered with flowering or pollination, there would be few ripe red oak acorns on your tree.
Hard-shelled fruits like acorns are known as hard mast, a critical source of protein, carbohydrates and fats that helps wildlife survive winter when other food sources are minimal. Fortunately, there are other sources of hard mast, including hickory nuts, beechnuts, hazelnuts, walnuts and pine seeds. If you want to help your local squirrels through the winter, put food out for them. Although they love sunflowers and unsalted peanuts, it is healthier for them if you stick with native food sources such as hazelnuts or walnuts in the shell.
Field biologists, naturalists and others will be watching 2009's acorn crop with keen interest to see if this was just an abnormally bad "bust" year or if 2008's scarce acorn crop is a harbinger of things to come.
Black Oak (Quercus velutina) - Acorn ½-1 inch long, ripens over 2 growing seasons, kernel yellow and bitter.
Chestnut Oak (Quercus montana) - Acorn 1-1½ inches long, ripens in 1 growing season, kernels somewhat sweet.
Northern Red Oak (Quercus rubra) - Acorn ¾-1¼ inches long, ripens over 2 growing seasons, kernel is bitter.
Pin Oak (Quercus palustris) - Acorn ½ inch long, ripens over 2 growing seasons.
Scarlet Oak (Quercus coccinea) - Acorn 1 inch long, ripens over 2 growing seasons, kernel white and bitter.
White Oak (Quercus alba) - Acorn ¾-1 inch long, ripens in 1 growing season, kernels were used by Indians for making flour.