AMES, Iowa – Brightly-colored leafs, harvest and the busy labors of squirrels burying nuts for the long winter ahead are iconic images of fall in Iowa. This year, however, Iowans may notice fewer scurrying squirrels in their yards, thanks to a late spring frost that negatively impacted hard mast production in many areas throughout the state.
“Hard mast is the botanical name for hard nuts produced by trees and shrubs such as acorns, walnuts, hickories and hazel nuts,” said Gabbi Edwards, urban forestry specialist with Iowa State University Extension and Outreach. “Hard mast provides food for many familiar wildlife species seen in urban and rural environments throughout Iowa.”
Birds including bluejays, ducks and wild turkeys, white-tailed deer, fox squirrels and many other creatures rely on hard mast from trees and shrubs to build up energy reserves for long winter days or fuel their migration to warmer climates. Iowa Department of Natural Resources foresters have noted widespread hard mast failure this year, making for lean times for some animals counting on this seasonal food source.
Mast failure results from frosts when hardwood tree flowers emerge during April and May. In the absence of killing frosts, flowers formed in April and May produce the seed, or mast, that matures and drops from the tree in the fall. A late spring frost after flowers have emerged can damage or kill the flowers, resulting in fewer acorns or nuts being produced. Spring 2016 saw frosts around the state as late as May 18, later than normal and after many trees had already produced the flowers necessary for mast production. The map illustrates the last date this spring where a temperature below 32 degerees F was recorded at each of the Iowa Environmental Mesonet stations. However, local variation in frost severity could result in failures in areas other than what is shown.
Not only has the late spring frost affected mast production for this fall, but there will be the same decrease in mast next year in some trees as well. Red oaks, and other oaks within the red oak family like black oak and shingle oak, take 15 months for acorns to mature. This means the fall of 2017 will have decreased mast due to flowers that were lost in the spring of 2016. White oaks and walnuts produce and mature acorns and nuts within the same growing year.
Failure of mast does not affect all trees in the same area; that depends on the severity of the frost, exposure to the sun or other factors affecting local microclimates, and whether an individual tree had flowered at the time of a late frost. Thus some trees on the same block may have different production patterns this year, but on average many trees have failed to produce mast.
“Large-scale failures of mast crops can have negative impacts on some wildlife species that count on the seasonal food source,” said Adam Janke, assistant professor and extension wildlife specialist at Iowa State. “Studies in some northern states with near-complete failures have shown significant reductions in squirrel populations. However, such a widespread decline is unlikely in Iowa, given that mast failure did not affect all trees and many other food sources, such as waste grain in crop fields, are available for wildlife.”
Motorists and homeowners may expect to see some animals, like deer and squirrels, moving longer distances this fall and winter in search of trees that were spared from the damaging frosts.
Home or property owners can do little to abate the impacts of a late frost on annual mast production. However, the impacts of a late frost on some species is a reminder of the importance of planting or managing for a diversity of trees and shrubs in landscaping and in natural areas to hedge against environmental uncertainties. In this year, having species of red and white oaks in the yard would ensure the impacts of the late frost were spread over two years, safeguarding the sight of busy squirrels in the yard foreshadowing the coming winter days each November.