Did you ever notice that some years oak trees produce a seemingly overabundance of acorns? And in other years you are hard pressed to even find a single acorn? There is a reason for oak tree seed boom and bust and it is not just weather and climatic conditions that drive this interesting dynamic.
The production of periodic large seed crops in trees is referred to as masting. This does not occur every year, for it takes an enormous amount of energy to produce an extraordinarily large number of acorns by an oak tree. It is not surprising that weather patterns and general climatic conditions play a role in this natural phenomenon. For example, if the previous year was droughty, an oak tree will form more buds that eventually become vegetative structures (leaves and shoots) and fewer buds that turn into reproductive structures (male and female flowers). Temperature also influences acorn development and untimely weather events such as late frosts, battering winds, and heavy rains during the emergence of spring flowers can virtually obliterate the current year’s acorn crop.
However, the physical environment merely sets the stage upon which populations of oak trees go about their life processes. Variability in weather events and environmental conditions can act as forcing factors, pushing population levels one way or the other, but such exogenous effects are not embedded within the population feedback structure. So, there must be something more to the synchronized masting of oaks. Scientists who study these internal feedback structures (endogenous effects), most notably entomologists, have come up with an additional explanation for the boom and bust of acorn crops.
Like all other living things, oak trees have many natural enemies that are part of normal negative feedback loops that promote stability and prohibit populations from growing exponentially. Defoliators chewing leaves and borers tunneling through wood cause harm, but they usually do not significantly weaken an otherwise healthy tree. Acorn feeders, on the other hand, rob the tree of its reproductive potential and ability to replace old and dying individuals with young seedlings. If natural enemies consumed all acorns every single year, oaks would eventually vanish from the environment. But we know that oaks persist, so we know that some, in fact many, acorns survive to produce the next generation of oak trees.
Deer, mice, squirrels, blue jays, and turkeys favor acorns because they are rich in protein, carbohydrates, and fats, as well as essential minerals. These vertebrates gorge themselves during mast years but during lean acorn years switch to alternate food sources. And they too have numerous natural enemies keeping their populations in check. It is the invertebrates, the insects, which play the most critical role in the cat-and-mouse game of an oak tree’s evolutionary survival strategy. Acorn weevils (Coleoptera: Curculionidae) depend solely on acorns for food and the development of offspring. Their survival is dependent on acorns, they have no alternate food sources. That means in exceptionally low acorn-producing years, weevils have scant food resources on which to grow their population. And during a subsequent heavy mast year the weevil population becomes overwhelmed and cannot reproduce fast enough to significantly impact overall acorn survival. Mast years only occur once every two to five years, thus oak trees effectively escape the weevil’s potential stranglehold on its vital acorn production. Oak trees maintain a slim advantage over their principle and most damaging natural enemy by masting.
But nature always finds a way and weevils are certainly no exception. They have developed a counter strategy to deal with the impediment of host masting. The weevil life cycle from egg to adult can vary from one to three years. Weevils have partially synchronized their development to account for the extreme variation in year-to-year acorn production. But the oaks still maintain an upper hand in the grand scheme of things, remaining one step ahead of a potentially mortal natural enemy. Both species (predator and prey, sensu lato [in a broad sense]) maintain their respective populations by adaptation and counter-adaptation. These types of stabilizing dynamic interactions abound in nature and are the norm, having occurred for as long as there has been life on Earth.
So, the next time you traverse through a Delaware oak forest in the fall and observe a massive number of acorns strewn across the landscape, rest assured that this masting represents oak tree generational security for the future. And the acorn-devouring weevils are still out there, albeit one step behind, playing an indispensable role in the stability of both populations.
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If walnuts come from walnut trees,
And almonds come from almond trees,
Then how come acorns come from oaks?
Can someone explain this, please?
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The Morton Meadow, [just across the road from Abbott’s Mill Nature Center,] was purposely burned in late March.
This carefully planned management activity simulates natural fire that, historically, was part of many local and regional ecosystems. Smokey Bear’s ad campaign of “Only You Can Prevent Forest Fires” and nearby human developments removed fire from many natural environments. Land managers now recognize that low-intensity, controlled burns have many benefits.
By mid-June the Morton Meadow was full of milkweed, butterfly weed, aster, black-eyed susan, queen anne’s lace, four o’clocks, and many other native flowering plants and grasses. Before the burn, the meadow contained tree and shrub seedlings that would have eventually choked out all the flowering plants. Fire returned the meadow back to a tree- and shrub-free environment that is now a haven for pollinators and birds.
Meadow photos and text: Mike Valenti
Closeups: Steve Childers
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Next week: The insidious invasion of Amur peppervine. It's really creepy stuff.
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