Sumatra oak, Quercus sumatrana the southernmost oak, Sumatra, Indonesia. The buttresses and surface roots are typical of many tropical trees. Photograph by Tom Kimmerer

The internet is atwitter with the Case of The Missing Acorns. The Washington Post and National Public Radio, among others, are reporting that oak and other nut-bearing trees have no nuts this year. Reports are concentrated along the east coast from Virginia to Maryland, with scattered reports from elsewhere.

The news reports give the impression that this is a shocking and novel event. It is not. I write this from the perspective of a tree biologist who has studied oaks for more than 25 years. It is fairly common for oaks of a single species to fail to produce acorns in one year. It is less common, but not unheard of, for all the oak and hickory trees in an area to fail to produce nuts.  I have seen 3 years in which there were no acorns over large areas of central and eastern Kentucky, the most recent being 2007 in central Kentucky.

There are several possible causes of the loss of nut crops. The main cause is late season frost that kills the developing flowers. In 2007, there was a cold snap in central Kentucky just as flowers were maturing. Male flowers were completely killed by the freeze, and released no pollen. As a result, there were no acorns on oaks in most of central and eastern Kentucky, except in bur oaks, which flower a few days later than the other species.

Early-season drought can also cause flowers to fail. However, I have never seen this in the eastern US, which nearly always has adequate spring moisture.  Insects can kill flowers, but no large insect outbreak was reported in the area, and oak insects are unlikely to affect hickories.

According to the Washington Post article, there were exceptionally heavy rains in the spring which could have prevented the wind-borne pollen from spreading. Last May, Reagan National Airport reported 10.6 inches of rains, right at the time that oaks and hickories are flowering. Ed Zimmer, regional forester for the Virginia Department of Forestry, doesn’t think that even a large rain event could stop all pollination. However, it is possible, especially if the rain was accompanied by high winds.  Heavy rains over multiple days could wash pollen down to the ground, preventing pollination.  I know of no scientific literature or naturalist observations to support this idea.

Next year will tell the tale. If acorns and hickory nuts return next year, then there is no problem.  Whatever the cause, it is a regional problem, as oaks through most of the east are bearing acorns as usual. The huge oak in my back yard flowered normally this year and produced at least 10 bushels of acorns this fall.

Oaks and hickories never bear heavily every year anyway. These are mast-fruiting species that have heavy crop years followed by one or more light or no-crop years. It is common for heavy mast years and light mast years to by synchronous over many species and a wide area. While that can’t account for the present situation, it may be part of the story: if this was an off-year for mast-fruiting trees, the combination of a light flower crop and heavy rain could account for the lack of nuts.

Mast fruiting is partly due to resource limitations – if a tree puts large amounts of carbohydrates, proteins and fats into a heavy fruit crop, it is unlikely to be able to do so the following year.  There is also an evolutionary explanation – heavy fruit crops every year will promote large populations of seed predators such as squirrels and weevils.  Boom and bust cycles of fruiting at irregular intervals plays havoc with the diet of predators, and keeps their population levels low.

Picture:  White oak, Quercus alba, acorns. Picture by Tom Kimmerer.