Left Hand Valley Courier - All Local, All The Time

By Josh Morin
Special to the Courier 

Holding on and letting go… of leaves


November 29, 2018

Holiday traditions with evergreens

I look out and see patches of snow still frozen hiding in the northern shadows of plants and homes. We’ve been covered up several times in the last month by a wintery white layer of snow and even a little freezing rain. Ahhhhh…. a collective sigh of relief, seeing the mountains covered in white, knowing that we’ve been given the gift of moisture, a precious thing on the high plains. I’m grateful for that snow. Perhaps the El Nino phenomenon will settle in and bring us an abundance of precipitation this winter. I know all of our plants would appreciate a snowy winter.

There is a maple in the yard still holding onto its dried leaves outside of my home, an unusual occurrence for this species of tree. Something happened this fall with the quick freezes that interrupted its normal leaf senescence and abscission process.

There is no shortage of complex Latin words in the world of plants and arboriculture. Let’s stop for a moment to define a few of them.

Senescence describes the process of deterioration or aging of biological organisms. In deciduous trees that drop their leaves in the fall, it describes the last stage of leaf development when chlorophyll breaks down and colors change. This is typically followed by the leaves dropping from the tree, or abscission. Abscission refers to the shedding of a part of an organism, in this case the dropping of leaves from the tree. Typically, in deciduous trees, abscission occurs as the last part of senescence.

During this process, a thin layer of cells forms at the base of the leaf. This layer of cells is called the abscission layer or abscission zone. Some of these cells have weak walls and when the cell walls break the leaf falls from the tree. The breaking of the cell walls can be triggered by hormones like auxin, compounds like ethylene and often times physical force such as wind.

A Norway maple in marcescence

This year, however, my maple tree has joined the ranks of trees like pin oaks and white oaks that experience marcescence. These plants hold onto their dead leaves rather than letting them go like most other trees. It’s not entirely clear why trees experience marcescence, but there are many theories. Typically, juvenile trees are marcescent and then start dropping their leaves in the fall when they become mature. One theory is that the dead leaves protect the succulent buds of young trees from being eaten by herbivores who don’t like the taste of dead leaves.

Some trees like pin oak stay marcescent their entire lives and won’t drop their dead leaves until the buds begin to open in the spring.

So, this winter if you see your trees covered in brown leaves, know that it’s just holding on and being marcescent.

Perhaps you’ll take a moment to slow down and listen. You might experience psithurism-- the sound of wind moving through the trees.

Josh Morin is an ISA Board Certified Master Arborist and co-owner of Taddiken Tree Company, a locally owned and operated company.


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