Seasonal Senescence
As hazy summer days slip away, cool nights creep in, and the air becomes crisp with the anticipation of the autumnal equinox, the lush green that has surrounded our summer excitement begins preparing for winter. But first, there is a show to be had. Flowers fall away, replaced with ripe seeds and berries, the spiders spin in overtime, their work exhibited by the morning dew, and squirrels scurry with more urgency as their bounty arrives and nights lengthen. We are at the cusp of Autumn. Earth tilts and light shifts, as if it knows the time is right to bathe our trees in gold and make their changing leaves glow.
While us humans garner our own benefit from the phenomena, the ecosystem has been evolving with this reliable re-incorporation of leaf matter into its soils since plants began creating them over 400 million years ago. In that time, countless insects and mammals have found shelter in the thick layers of deciduous leaves and rely on them to get through the winter and protect their young. These small creatures are often the base of the food chain as well, meaning their success indicates the success of their predators. Then there are decomposing creatures, animals, bacteria, and mycorrhiza alike use them as fuel, leaving behind soft loam. This decomposition cycle is what keeps us from drowning in detritus and creates healthy soils for future life to dig in its roots and start the process again.
The process that creates this future life is started by a controlled death set in motion by environmental changes. The cooler nights, shorter days, and lower intensity of the sun create a combination of indicators to the trees that change is in the air, their growing days are limited. These factors trigger the tree to produce hormones that set senescence in motion. Senescence is the aging process, a breaking down of cells that leads to death. Deciduous trees are ultimately selectively killing off their leafy organs and it just happens that humans find it beautiful.
Abscission is the specific process by which a plant intentionally sheds a part of itself in a systematic way, this includes the release of flowers, seeds, and leaves. In autumn, the mass-abscission event that we know as foliage season, is a process made up of three distinct parts within the leaf and stem: reabsorption, the formation of a protective layer, and then detachment. First, the reabsorption of nutrients left in the leaf involves the breaking down of chlorophyll to take advantage of the nitrogen and other nutrients that are still available there while leaving behind unneeded sugars. When the chlorophyll has finished breaking down, what is left is the true color of the leaves, ranging from yellows all the way to deep purples, each with their own chemical makeup and purpose.
The second part of abscission is the creation of a protective barrier at the site of division, called the abscission zone. When nutrients are done flowing back into the tree, the cells on either side of the zone form into thicker layers of parenchyma cells, essentially creating a waterproof cork-like layer at the base of the leaf stem and the branch where it has been connected. Once this layer has formed, the final step in the process is simply detachment. While this also can differ in mechanics from tree to tree, the short of it is once the barriers are formed, the cells attaching the leaf to the stem experience a controlled death and allow for it to fall with only the force of its weight pulling it down. This is what allows for them to be swept so easily and beautifully in the breeze.
While it is sometimes hard to pin down all the reasons why a tree does something, and it may even be a fallacy to believe everything has a pure purpose and isn’t just chance that it has been allowed to flourish, the main purpose behind the process is to protect the tree. In parts of the world where there is a mass senescence, it is more likely than not, followed by a period of inclement weather. Here in New England, the loss of deciduous leaves is a means of protection from winter winds, snow, and ice that could cause damage to the tree. With no leaves, the wind is allowed to whip through the empty branches rather than catch on thousands of tiny sales, and snow and ice can only get their footing on the stems instead of getting caught and piling up to weights too great for the trunk to handle.
As the trees in the ground begin their senesce, as do the ones in pots at the Heritage Tree Center. The growing season is done, except for a few stubborn parrotias (Parrotia persica) and redbuds (Cercis canadensis), and that comes with changes in care as well. The trees that couldn’t get watered enough in the dry sunny days of August have slowed down to a more reasonable need, with nights getting cooler and days shorter, they’ve gotten the message to take it easy, winter is coming. The most prominent colors so far are on the sweetgums, seedlings from the grand tree at Morton Park. They have shifted to show their velvety dark reds and purples in a stunning display. The red maples are on their way as well, one of the most notable trees in the fall. Our gordlinias (xGordlinia grandiflora) put on a unique display of the brightest red leaves accompanied by their flowers that will continue to bloom almost until frost. Yellow hues are popping up in our Morton Park river birch (Betula nigra) grafts, the native calycanthus shrubs (Calycanthus floridus), and the Hiroshima Ginkgo seedling.
While some trees in town have begun to senesce already, the best is still on its way. A variety of bright reds will begin to streak through our native red maples (Acer rubrum) and tupelos (Nyssa sylvatica) along with the variety Japanese maples (Acer palmatum) through town that also can be found in shades of yellow and orange. Sweetgums (Liquidambar styraciflua) follow behind with a full range of yellow to purple lighting up their leaves. Notably, the large sweetgum in Morton Park is one to see, with red to burgundy leaves, a trait it has passed down to its progeny growing at our Heritage Tree Center. Sargent’s cherries exhibit a lovely array of orange, bronze, and red, a display that can be seen in a trio on Ruggles and a set in Aquidneck Park. Another stunning native that is, featured in Touro Park, is the sugar maple (Acer saccharum), featuring a blazing orange that is always striking against the clear blue skies that autumn brings. An unusual and beautiful tree to watch for is a deciduous conifer, the dawn redwood (Metasequoia glyptostroboides). Miantonomi Park hosts a small grove of dawn redwoods near the southeast entrance that is worth admiring when their burnt orange needles begin to shower the path. Lastly, a stunning yellow to not miss this season is found in large ginkgo specimens (Ginkgo biloba) scattered along Bellevue Ave, and more prominently featured in Touro Synagogue Cemetery where it towers over the street.
It is not difficult to see why foliage season is a multi-billion-dollar business for New England, with visitors flocking from all over the globe to experience the unique full spectrum of Fall color provided by our trees. What we may have come to take for granted is the array of color that we experience here in New England, being a lucky part of the world with prominent species that turn red in the autumn to accompany the more common oranges and yellows. All the more reason to continue our planting of diverse species throughout Newport, ensuring that there are trees to enhance our city and lives in every season, now and into the future.
By Morgan Palmer — Plant Production Manager
September 2024
