Acorns feed birds, bears, deer, and many small mammals. But the big oak trees that produce those acorns are also harvested to become timber. In managing hardwood forests, there’s a potential trade-off between harvesting oak trees for their valuable wood and reducing the number of oak trees left to produce acorns.
To help determine a good balance, USDA Forest Service research ecologist Katie Greenberg counted acorns and measured tree crown area and trunk diameter growth rates of five eastern oak species in shelterwoods (where many oaks had been harvested) and mature mixed-oak forests. Results of the 17-year study were published in Forest Ecology and Management.
In uncut, mature forests, oak density and basal area were five times greater than in shelterwoods. However, only twice as many acorns per hectare were produced.
In forests that had been harvested with a shelterwood method, the remaining oaks produced about 70 percent more acorns per unit area of crown than similar-sized trees in mature forest.
In shelterwoods, the remaining oak trees grew quickly, and their crowns expanded more than would be expected due to trunk growth alone. All this growth eventually led to bigger trees and larger tree crowns in shelterwoods.
But larger crowns alone did not explain how acorn production per tree was so much greater in shelterwoods than in mature forests. The oak trees in shelterwoods, though fewer, were able to produce more acorns than their counterparts in dense, mature forest because they had larger crowns and acorns that were more densely packed onto those crowns.
“If you cut trees around an oak tree, it has more sun and more space to grow,” says Greenberg.
Because trees make energy from light, more sun exposure may give them more energy. And with more exposure to wind, they may receive more pollen to fertilize their flowers, that eventually become acorns. That crown expansion and greater abundance of acorns on those crowns compensate partially for removing other oak trees in a shelterwood harvest.
Greenberg’s research shows that retaining 20–40 oaks per hectare in shelterwoods may yield 50–100% of the number of acorns per hectare in mature forests on average.
“It’s not going to happen instantly that you have more acorns,” cautions Greenberg. “You’ve got to give it time for crowns to grow and have all that extra light.”
So, you could harvest oaks for timber and still produce plenty of acorns to feed wildlife and grow new trees.
Yet as forest managers know, acorn production is highly erratic. It varies tremendously among oak species and years. Therefore, these results reflect average acorn production, not how many acorns will be produced in any particular year.
Because of this variation in acorn production, forest managers should consider leaving multiple species of oak to increase the likelihood of at least some acorn production in most years. They also may benefit from selecting poor acorn producers to harvest for timber, leaving the good acorn producers to keep on producing.
For more information, email Katie Greenberg at firstname.lastname@example.org.