The Mystery of the Acorn Bonanza
Penn State is situated in the middle of what used to be vast hardwood forest. Some areas of the county have been more completely deforested than others, and we happen to live in an area in which the developers retained as many oaks as they could when they put up houses in the 1970s. One consequence of this is that we spend many hours raking leaves this time of year.
|Photo: A Buchanan|
Many many leaves, and this year, many many acorns. Masses of them. It's a good year for acorns. I wanted to know why.
Taking Holly's message on the 'scientific method' to heart, I wondered, could I apply the scientific method to this question? Well, unfortunately no, not without having collected data on possibly relevant variables last year as trees were making their acorns -- or even two years ago, since the acorn production of some oak species is a two year affair. And that would have required knowing the relevant variables.
So I did the next best thing. I googled 'variation acorn production', thinking someone must have applied the scientific method to this question and done the requisite hypothesis testing.
|Photo: A Buchanan|
Which of course they have. It turns out that people look at this from various angles, though. Some are thinking of the downstream effects, so advise hunters on how to maximize acorn production, because acorns are a staple food for deer. Others are thinking about sustainable forests, and about how to keep deer from eating the acorns that they hope will go on to produce new trees.
Even so, this approach was not nearly as productive as I'd hoped. In fact, I kept coming up with a similar theme:
The number of seeds produced by a population of woody plants can vary markedly from year to year. Unfortunately, knowledge of the patterns and causes of crop-size variation is limited....and
However, little is known about the proximate factors that control the yearly variation in acorn production in oak species...and
Most acorn production studies note wide and consistent differences in acorn productivity among individuals, but none clearly demonstrate determinants of productivity.
Hm. Well, this site looked more promising -- a list of actual variables!
A number of factors affect acorn production in oaks.
|∙masting cycles||∙acorn predators|
|∙oak species||∙tree age and diameter|
When combined, these factors make acorn production highly variable from year to year, between the different oak species, between trees of the same species, and from one property to another.And further,
[Production is high] once every two to five years. Acorn production during an abundant crop year may be 80 percent higher than in a low production year; the difference to deer can be hundreds of pounds of acorns per acre. Although the exact mechanisms that control masting are not fully understood, biologists believe that oak species, weather, and genetics are important factors that determine how often oaks produce abundant crops.If we knew what masting was, this might be helpful, but probably not to answer my question -- there's that 'not fully understood' thing again. And really, only 'weather' is a variable in this equation, as the species and genes of a tree don't change season by season, so this isn't really very helpful after all.
This was interesting:
LONG-TERM PATTERNS OF ACORN PRODUCTION FOR FIVE OAK SPECIES IN XERIC FLORIDA UPLANDS
We examined long-term patterns of acorn crop sizes for five species of shrubby oaks in three xeric upland vegetative associations of south-central peninsular Florida for evidence of regular fruiting cycles and in relation to winter temperature and precipitation.And potentially rewarding -- by looking at different species in a single area they were able to control for variation in all the possibly relevant factors. What did they find? "[E]vidence that annual acorn production is affected by the interactions of precipitation, which is highly variable seasonally and annually in peninsular Florida, with endogenous reproductive patterns." Oh, so it's rainfall.
Except that, as it turns out, a number of people have studied variation in acorn production in five local species in different areas. There's a report of a study in California and one in Appalachia, and even one in Japan in which sea breeze was a factor, none definitively confirming the rainfall explanation.
In frustration, I emailed a local forestry agent. I haven't heard back. It's possible he's out counting acorns.
Ok, so I accept that there's no simple answer to this simple question. The serious upshot of this little exploration is that here, too, complexity reigns. Despite the list I cite above, who can really say what all the relevant variables are, not to mention measure them at the right time or place? Oak flowers are wind-pollinated -- maybe acorn production depends greatly on wind catching the pollen at just the right time. Which would be essentially unmeasurable. And, perhaps variation in rainfall is a significant factor, but where and when? The roots of mature oak trees run wide and deep, and when are which roots feeding which flowers? And so on.
And how does one construct believable evolutionary (that is, adaptive Darwinian) scenarios for this? There's no acorn gene! (But, of course, it has been tried.)
And think how utterly confusing this must be for any squirrel who's just trying to use his experience to get ahead, to put away a good cache of meals, and wonders if he's going nuts because he's losing his memory.
But one interesting thing caught our eye here as we ventured away from our usual comfort level, scientific literature-wise. Ecological studies, by their very nature, are less prone to reductive thinking than what we're used to. "When combined, these factors make acorn production highly variable from year to year." By and large, these studies accept that the cause of variation in crop production is the result of interactions among various factors.
If only this were so readily accepted in genetics and anthropology.
This year's acorn crop, continued
I did hear back from the forester on the question of why so many acorns this year. He says that oaks are generally sporadic fruit producers, with really good crops every 4 to 7 years. There are several reasons for this, one being the weather and the other an ecological adaptation.
A late spring frost is hard on oak flowers, and will lead to a low yield, he says. And, insects play a role. There are on the order of 30 different species of acorn weevils "that can destroy up to 90% of any given year's production either while it is on the tree developing or after they fall in the autumn." The cyclic nature of fruit production helps keep the insect population down.
And, he says that there are advantages to sporadic fruit production. It keeps predator populations down, which increases the chances that some acorns from a given tree will survive and grow. If not, my informant says, the tree would always be having to produce more and more fruit to stay ahead of the rodents. Similarly, the fluctuation keeps weevil populations down, and thus acorn destruction down. Good for the tree, not so good for the predators.
Both explanations sound plausible. However, regular MT readers won't be surprised if we are a bit reluctant to accept the adaptive explanation right off the shelf. First, an oak tree is lucky if even a few of the acorns it produces in any given year makes its perilous way to treehood. Even in a bad year, oaks way overproduce acorns relative to what will take root, or replacement needs and so on.
However, sporadic fruit production in response to the vagaries of climate or other means of destruction of flowers or developing acorns is completely in keeping with the adaptability or facultativeness that is a core evolutionary principle. Oak trees need to be able to adapt to change, and good and bad fruit production years is one way they do so. It's easier to suggest but a lot more difficult to conceive how a tree 'knows' (genetically evolves) to adjust for variable predator loads in the hypothesized way, when climate itself is unpredictable.