Senescence is the decline in vigor that happens to all creatures great and diminutive as they approach their species’ life-expectancy limit. Individual genetics matter, too, as does environment. For us, eating and sleeping well, cultivating gratitude, and laughing a lot can keep us healthier for longer. But at some point, even the best-preserved specimen can’t avoid the end.
Trees go through senescence at different rates, each species having an approximate timeline beyond which no amount of TLC can keep them alive. As a genus, poplars are short-lived: trembling aspen is usually decrepit around fifty, while its cousin the bigtooth aspen might reach eighty. Pin cherry declines in twenty to thirty years, and gray birch rarely see the other side of forty.
At the other end of the spectrum, bur oak is a massive and picturesque tree which can live eight centuries or more. A red oak might survive to be four hundred, and sugar maples and hickories are in the same club. With a few exceptions, short-lived species tend to be shade-intolerant, and are usually pioneer, or early-succession, trees.
Woods go through many phases as they move toward a stable system. Succession is the permutation of forests as they mature from a beaver meadow or pasture to some endpoint (climax) community. Once beavers move on and their pond drains, or a farm field is abandoned, a natural order of plant life begins to shape the landscape like a living symphony.
The first trees on the scene are often the fast-growing poplars, whose cottony seeds fly many kilometres. Birch pips, airy as insect wings, blow in, too. Mice cache pin cherry seeds; deer spread hawthorn and viburnum. As these mature and block out the sun, the forest floor becomes too shady for their own seedlings to survive.
Into our intrepid pioneer forest of birch, poplar, cherry and hawthorn fly the helicopter samaras (winged seeds) of sugar, soft, and striped maples. Grey squirrels bury acorns, and red squirrels hoard conifer seeds. These are the long-lived, shade-tolerant trees. Seeds germinate, and saplings bide their time in the understory, sometimes for decades, waiting for senescence to play out.
Forty years on, many pioneer trees are in decline, succumbing to insects, stem cankers, root rots, or storm damage. Every time a trembling or bigtooth aspen goes down, a patch of sunlight illuminates the forest floor, allowing a patient, shade-tolerant maple, oak or hemlock to quickly stretch for the sky.
Eventually a stable mix of long-lived, relatively shade-tolerant trees develops – a climax community. Species composition will vary depending on elements such as soil type, slope and climate. Elders topple every so often, allowing youngsters a chance at the sun, but the makeup of a given forest stand will tend to stay roughly the same until the next beaver dam or glacier wipes the slate clean.
Neither person nor plant can avoid senescence, which has the same Latin root, senex, as senility. In that sense I envy trees. The decline of individual trees is a critical part of the forest life cycle, and they don’t have to worry about remembering where they left their glasses or car keys.
Paul Hetzler is an arborist and former Cornell Extension Educator. He doesn’t recall ever having misplaced an item.
Photo by Katja Schulz from Washington, D. C., CC BY 2.0, via Wikimedia Commons