Autumn’s bounty-the feast before the famine

acorns
Northern red oak (Quercus rubra) leaf and acorns were abundant this fall. Acorns, hickory and beech nuts are considered hard mast as their hard coating helps protects the nut from damage. Photo: Mike Fargione.

Here in the Hudson Valley, nature’s harvest has been abundant. Nuts and fruits will help wildlife fuel their southern migrations or stock their winter larders. Not every year produces such a bounty; this season’s bumper crop of wild foods will impact local plants and animals for years to come.

We are experiencing what is known as a “mast year” in our region. Mast refers to the seeds of woody plants that are eaten by wildlife. “Soft mast” has seeds that are surrounded by fleshy pulp, and includes true berries, drupes (stone fruit) and pomes (like apple and rose hips). “Hard mast” has seeds that are protected by a hard outer coat, such as acorns, hickory nuts and beechnuts.

“Masting behavior,” or the intermittent production of large seed crops by trees and shrubs, is a common phenomenon, and 2015 looks like a classic year. While white and black oaks, and hickories produced few to modest amounts of seeds, red oaks were prolific. Some forest sites at the Cary Institute are a hazard to walk in due to the volume of acorns underfoot. (Think “walking on marbles!”) The causes of boom-and-bust cycles in acorn production are poorly understood, but big crops of red oak acorns may be linked to warmer spring temperatures during oak flowering.

Hard masts, like acorns, are calorie- and nutrient-rich foods packaged in convenient hard-shelled containers that are easy to store. As you read this, thousands of local mice, chipmunks and squirrels are hording away these rich food packets. By storing hard mast, small mammals and a few birds become important seed dispersers, unwittingly planting the next generation of trees. Many a giant oak owes its existence to the workings of these small creatures. Other mammals, including deer, bears and turkeys, consume great quantities of acorns to produce thick fat layers that will be converted to energy when food is scarce this winter. More than 90 wildlife species are reported to feed on acorns; our forests would have fewer animals in the absence of this resource.

Many native and introduced trees and shrubs also had huge soft mast crops in 2015. Access to fruit promotes late-season reproduction in small mammals, fattens and fuels birds prior to and during their migration south, and helps larger mammals store fat for winter. Soft masts are not equally palatable to wildlife. Fruits of black cherry, native grapes and gray-stemmed dogwood are consumed early compared with those of sumacs, poison ivy and Virginia creeper. Yet even less-palatable fruits perform important ecological functions, as they remain on plants throughout winter when other foods are in short supply. Soft mast-producing plants benefit when their fruit is eaten; animals disperse their seeds far and wide via their droppings.

Wildlife populations respond strongly to the availability of food. Following mast years, we see both an increase in populations of seed-eaters and the predators that feed on seed-eaters. Other creatures’ fates are also linked to mast availability. Cary Institute scientists have found that Lyme disease risk spikes two years after large acorn crops. This is because white-footed mouse populations explode the summer following acorn abundance, increasing the likelihood that newly hatched black-legged ticks will take their first blood meal from a mouse. Mice are very efficient at infecting ticks with the bacterium that causes Lyme disease. More infected ticks raises Lyme disease risk for all of us.

Unlike soft mast, hard mast cannot germinate into a viable plant once it is consumed, so hard mast trees do not benefit if their crops are eaten. Boom-and-bust crop cycles may be one way hard mast trees cope with being the target of so many hungry critters. Reductions in seed-eater populations follow poor mast years. While mouse populations try to catch up, some acorns go uneaten during boom years. Mast years like 2015 have important consequences affecting wildlife populations, which plant species will colonize our forests, and our risk of contracting tick-bone disease. All this because it was a good year for seeds!

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