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The Mighty Acorn

Oak Tree Seeds Feed the Masses


Tuesday, October 9, 2012

It sounds like a hailstorm at times; the acorns are raining down in great numbers this year. They started a month or more ago, but the pace has quickened just now. Why would a single oak tree produce thousands of seeds every year (barring drought and other natural forces)? Surely it would take only a few germinating each year to keep the population going. It probably took many thousands of years, but the native oaks of California, including the coast live oak in this area, evolved in step with an entire ecosystem that includes fungi, bacteria, insects, mammals, birds, and even humans in all likelihood, for we know that acorns of all the oak species were used by native peoples.

That bumper crop of acorns is feeding a myriad of these symbionts (the rest of the tree, from leaves to roots, is also involved, but that’s another story). Most obvious in our urban areas is the clown-faced acorn woodpecker. These crafty birds bore holes in another tree and stash the acorns for later use. Together with their closely interrelated family group, they harvest the acorns, tend them, and defend them against other acorn-loving birds and rodents. Once they have established a suitable tree, referred to as a granary by biologists, they will maintain it for many generations, only abandoning it if hard times deplete the local acorn crop. While these woodpeckers do sip nectar and sap, as well as eat insects, about half of their diet consists of acorns.

Second and third in line are the scrub jay and gray squirrel (although now, the fox squirrel may be edging both of them out). Both of these species collect and store acorns in sites away from the mother tree. Here lies one of the clues to why an oak would produce such a prodigious amount of acorns. If the seeds fall near the mother tree, there will be more competition to establish. In fact, some seeds may lay dormant for a very long time waiting for the senescence of the mother plant before having the space and resources to succeed. Both jays and squirrels return and consume only a portion of the acorns that they have stashed away, so the remaining acorns may have a chance to germinate and grow. A single scrub jay can collect and transport up to 5,000 acorns in one season but return to eat just one-third of them. Reproductive cycles of squirrels and other rodents have been correlated with the magnitude of available acorns each season, so it is clear that they play a major role in their nutrition. Pocket gophers also sequester acorns in their burrows, and those that don’t get eaten may sprout and grow.

Other diners at the acorn table include mice, rats, foxes, and, in the less densely populated areas, deer (and formerly bear). Occasional meals by flickers and white-breasted nuthatches, as well as band-tailed pigeons, are on offer for these birds that may find enough cover to visit and inhabit urban oaks (many more birds in the wildlands also add acorns to their diet). Insects also drill through the hard outer coat and lay their eggs so that larvae may consume the highly nutritious nut within; as much as half of a tree’s production can be invaded by insect larvae.

What may seem like a nuisance to the humans in a modern urban landscape is still a prized commodity to a host of other inhabitants.

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Virginia Hayes, curator of Ganna Walska Lotusland, will answer your gardening questions. Address them to Gardens, The Independent, 122 W. Figueroa St., Santa Barbara, CA 93101. Send email to vahayes@lotusland.org.

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