Photo by Virginia Barlow
Trees that rapidly colonize forest openings made by recent cutting, fire, or wind throw are often called “pioneers.” Species such as aspen and paper birch, which produce millions of light, wind-disseminated seeds, are usually seen as the standard-bearers for the pioneer strategy. It is with good reason.
Remember, space in the woods is at a premium. Create an opening in the forest, and all kinds of organisms will attempt to occupy it. First come, first served. By getting to the newly available space first, pioneers have a distinct advantage. Other late-coming and typically heavier-seeded tree species, such as American beech or even sugar maple, rely on distinctly different strategies for outcompeting the pioneers. Rather than trying to be first, those late-successional species are built to last. They live a long time and are capable of growing under the shade of other trees. Pioneer species, on the other hand, don’t live as long; they tend to live fast and die young. And so it is all the more important for them to establish themselves quickly, before the shade of other trees overwhelms them.
But if getting there first is a smart way to get a leaf up on the competition, then being there already is downright brilliant. (Especially when the opening is surrounded by maple and beech trees ready to drop their seeds.) Consider then the ecological genius of the heretofore maligned and unheralded pioneer species, pin cherry. Like other pioneers, it too is a relatively short-lived, fast-maturing tree unable to compete and survive in the shade of the forest. But unlike its more-well-known pioneering associates, pin cherry has a heavy, flesh-covered seed that does not travel well in the wind. It has evolved an adaptive strategy relying not on the vagaries of a breeze to blow it into new places but rather on its own investment in a bank of seed long buried in the soil.
For a tree, pin cherry is extremely short lived; it tends to die off by age 30, or even earlier when heavily shaded. Accordingly, once it germinates, it has to grow fast, both to stay ahead of its competitors and also to reach reproductive maturity quickly. To meet its biological imperative for passing on its genes, it is in the best interest of pin cherry to get busy early. Indeed, pin cherry trees can begin fruiting within just two years, and, once they start, they are capable of producing abundant seeds for years. Many plant species do this. The great distinction of the pin cherry is that most of the seeds in their fruits do not germinate for many years, even decades, to come. There’s the brilliance. Immediate germination would likely result in near-immediate failure. Those new seedlings won’t make it under the developing forest canopy. It’s not enough just to pass on the genes; it’s better to give them a fighting chance at survival. So instead, the growing pin cherry just keeps adding seeds to the seed bank, and those seeds remain dormant and do not germinate until the time is right.
When is that? Well, given pin cherry’s inability to survive under shade, the right time would be when the canopy is removed. Although the specific mechanism remains a mystery, forest ecologists agree that when a forest canopy is sufficiently altered (read: removed), some combination of changes in environmental conditions in the forest floor (light, nutrients, temperature, moisture) seems to trigger those seeds to break dormancy and begin germinating – even after all those years. Sure, over decades some seeds do rot and some are eaten by animals, but by sheer numbers, millions make it to see the light of disturbance.
Following this buried seed strategy seems to make good sense for the pin cherry tree and other forest plants as well (raspberries do it, and there is some evidence that elderberries do as well). But it also makes good sense for the forest ecosystem as a whole. It has been suggested that this buried seed strategy is part of a larger feedback mechanism that helps forested ecosystems recover from catastrophic disturbances – the kinds that sever trees from substantial areas. Maybe this is the ultimate genius of the pin cherry: rapid revegetation of a disturbed forest – owing at least in part to a ready supply of buried seeds – minimizes losses of water, soil, and nutrients from the disturbed forest and allows it to regain normal and lasting function. In this way, the great maples and beeches owe much to the pin cherry.
Michael Snyder is the Chittenden (Vermont) County Forester.