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Sunday, October 4, 2015

Great oaks from little acorns

I just learned that one out of 10,000 acorns grows into a mature oak tree. That’s not great reproductive success. A mature oak, fifty years old or more, produces thousands of acorns every year, up to 10,000 in a boom or mast year, which occurs every three to five years in red oaks. One reason that few acorns grow into adult oaks is that so many become food for wildlife.
            An eastern red oak (Quercus rubra) happens to be one of the keystone trees in my yard, more than 100 years old and standing close to the house. I used to resent its shade. 
This red oak, our tallest tree, is older than the house.

Now I see it not just as an individual but as a whole community, including a housing project and grocery store (or maybe a community-supported agriculture cooperative) for wildlife I want to support.
            At this time of year, acorns are starting to ping off the metal roof of the garage, and the ground under the tree is littered with them, making for a lumpy surface underfoot. 
Red oak acorns

Red oak acorns take two growing seasons to mature, so the ones on the ground today sprouted two springs ago. White oak acorns (Quercus alba), the other type common in my area, develop in a year.
            At least ninety-six North American animals feed on oaks, eating acorns, twigs, buds or bark. White oak acorns are sweeter than those of red oaks, and they get gobbled up by squirrels and other creatures as they fall. Red oak acorns are bitter, because they contain more tannin. They retain their food value through the winter; animals can eat them in spring when all the tasty white oak acorns are gone.
            The oaks benefit from providing nuts that animals want to eat. Researchers observing jays found that each bird transported and cached around 110 acorns per day. The acorns the jays hid—they prefer to bury them in soft, moist soil—were more likely to germinate than acorns that fell under the tree. Squirrels dig holes in my garden to hide acorns, and some of them do sprout, although they’re far outnumbered by Norway maple seedlings.
Two white oak seedlings in a neighbor's juniper bed

            Besides feeding the birds and mammals that visit my yard and eat acorns—jays, crows, wild turkeys, squirrels, raccoons, chipmunks, rabbits and opossums—oaks are the best tree hosts for native insects I’d like to attract. Oaks support 534 butterfly and moth species, and there are 600 insect species that use oaks as their only host plants.
            As our red oak loses its lower limbs with age, its shade at garden level becomes more diffuse. In addition to all the ecological services it provides— improving air quality, modulating temperature, and sequestering carbon, to name a few--it’s now supporting a 30-foot climbing hydrangea (Hydrangea anomala subsp. petiolaris). I’ve learned to think of the giant oak as an essential part of my garden’s habitat.
The base of the red oak is hidden by the climbing hydrangea clinging to its bark.

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