Anyone who has spent October in the Northland has seen the amazing changes that happen during this autumn month. The trees of September gave us plenty of color and delight as we went through the days of summer to early fall.

Though wearing quite a dazzling display of foliage, most of the leaves were still on the branches as we left the month of the solstice and got further into fall. Early October continued the arboreal show, but by the time we get to the end of this month, we look out on a forest flora mostly devoid of leaves.

Most of us have seen this before, but it is worth a closer look at what has happened and why.

During my observations of this annual leafy change, I have noted what I call four phases in the colorful scene.

Phase 1 happens in September and it is during this time that we see reds blending with yellows. Red maple, sumac, dogwood and Virginia creeper are quick to take on the scarlet appearance. Joining these bright reds are the yellows of basswood, birch, sugar maple and ash. We can watch this colorful bouquet for much of the month.

As we go into October, we see Phase 2. As we approach mid-month, many of the trees in the forest have dropped their leaves, but two trees persist with their yellow: various willows of the wetlands and the abundant aspens. These clonal trees often form groups of yellow surrounded by trees that are mostly bare.

Sometime at about the middle of the month to Oct. 20, the bulk of the deciduous trees drop their foliage, often accompanied by winds and rains. This sets the stage for Phase 3. Unlike the first two phases, this phase takes place in the swamps and bogs with a yellow-gold glow of the tamaracks — the only local conifer to drop all of its leaves (needles) at one time.

The last week to 10 days of the month are filled with this tamarack show. They serve as an encore to the colorful foliage earlier from the deciduous trees.

Autumn leaves cover the surface of a puddle. (Photo by Larry Weber)
Autumn leaves cover the surface of a puddle. (Photo by Larry Weber)

Finally, by the end of the month, even these conifers will be without leaves. Some trees, like silver maple, weeping willow, lilac and buckthorn, will stubbornly hold leaves well into November. They constitute Phase 4.

We see this phenomenon each year and we may have an understanding of how it happens, but we usually do not look at why. At first glance, it seems like the trees could hurt themselves by dropping their only food source. The green leaves in the sunlight are the sites of photosynthesis — the making of food for the tree. Why would they dispose of their only food-making organs?

Yes, the leaves are where food is made, but here, too, is where much water is lost from the trees, evaporated off the surface of the leaves in a process known as transpiration. In summer, with adequate rain and replenishment, the trees can afford to lose this moisture and still survive. But during the arid chilly days of winter, such a loss of moisture could be fatal.

Also, the broad tree leaves would hold weight of snow and likely break down. (We sometimes see this during an early-season snowfall.) And so, the trees drop their food-making leaves and stand bare all winter, but not before they prepare for the next spring by forming the beginnings of new leaves inside the buds on their twigs.

And now in October, we see this huge leaf drop.

Larry Weber
Larry Weber