Monday, July 26, 2021

When Trees Go over the Hill

bur oak treeSenescence 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

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Paul Hetzler

Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.

You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World


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10 Responses

  1. Philip Terrie Phil Terrie says:

    Good article. Thanks! Reminds me of a book I read a few years ago, How Trees Die: The Past, Present and Future of Our Forests, by Jeff Gillman.

  2. Boreas says:

    Excellent article.

    In my mind, a forest is one organism, not a collection of isolated individual species. Much of a forest is underground, unseen to our eyes. Coordination and cooperation on an immense scale. Soils, plants, invertebrates, microorganisms, and huge fungal webs are all part of a living forest. An analogy is the human body. We are a collection of coordinated microbes, parasites, and human cells in a bag of skin – the human cells being vastly outnumbered by the microbes. Do we celebrate the human body as an organism, or just a collection of isolated individual species that we can manipulate without repercussions?

    Trees growing old and dying in-place unmolested is key to a vital, functioning forest. Diversity within a forest keeps it healthy and robust – able to handle most of what nature can throw at it. We need to start viewing biomes as living organisms and not just look at what they contain. Otherwise, we miss the big picture – and their value.

    • Boreas says:

      Sorry, I left out vertebrates as part of a living forest above!

      • nathan says:

        forgot slime molds.
        Ecology all intertwined, dependant upon each other in a melody of life…mankind has become the sour note disrupting the music of life and all life is paying dearly..
        we talk of preserving the Adirondacks, when the entire environment around the world is dying…
        One scientist explained global warming in a way that i truly understood the issue. He stated “If every person in the world gave up their car, It would not be enough” that made me think of how everyone would fight over not giving up their car and “freedom”. How every store has row upon row of poisons we use indesciminantly, scott’s1,2,3,4 kill insects, kill fungas, kill plants…a sterile poisonous lawn of grass to prove what??? let a few other plants grow, just mow…cleaners to kill germs used in our homes to breath in, touch, ingest constantly…air fresheners full of chemicals( do you know what you are breathing in?)..We only have one home, yet we do nothing but poison and pollute it, yet we think saving a tiny patch in NY will change the world???
        Oh and the joke “electric cars are the solution” are people so foolish as to think we can build 50 million electric cars in the USA alone? think of the extra trillion tons of poisonous waste generated and dumped into air, water, land. the accidents releasing lithium fires onto the enviroment…all the toxic metal oxides.

        • Boreas says:

          Yup – slime molds too.

          Saving the Forest Preserve will certainly not save the world, but perhaps it just may be the last real forest in the continental US. At least until the next glaciation.

          • nathan says:

            saving the adirondacks is not enough, we saw that with acid rain from 1,000 miles away, sterilized the lakes of almost every aquatic life form. we need to focus on a much bigger picture as the heat, weather changes, droughts, floods will hit the adirondacks, with floods and raging forest fires very soon. a huge forest fire would take 100 years to regrow..cant be just local anymore.

  3. Ben says:

    Interested in the lives of trees? I recommend the much acclaimed novel “The Overstory” by Richard Powers.winner of the 2019 Pulitzer Prize for fiction. It was on The New York Times best seller list for one year.

  4. geogymn says:

    You know you are near the end of your cycle when you out live the trees that you planted.

    • nathan says:

      or worse yet see entire species disappear, elm, chestnut, black walnut, butternut to name a few from NY. as the enviroment changes, plant become less strong and more prone to infectation of bugs, fungas, desease….
      I truly miss the large elms and the graceful shapes and shade, the picking of chestnuts, walnuts and buternut as a kid roasting them on the woodstove. or yester-year when walks along roads and railroad tracks has tons of blackberries, raspberries from the brush cutting, now long gone because it’s ok to spray tons of herbicides along roads and RR tracks. just drive along the northway and look at all the brown pine trees sprayed with herbicides for a hundred miles…Thats ok????? it goes into streams, rivers and wells, thinned out but there for 100 years slowing building up concentrations, just like the salts used in winter. 50 plus years ago, they plowed and sanded. no salt, we drove like it was winter and had no problems..but now people drive too fast and cant deal with non-bare roads…by end of winter we were driving on sandy ice and had good traction, just spring melts were annoying as it melted and potholy ice

  5. TomC says:

    Thank you for an enjoyable and succinctly informative article. I have grandkids (7, 6, 4) who are science fact sponges and they frequently educate me on the natural world. This is a reminder to share the story of forests with them the next time we’re in the woods.

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