Nature’s Depths

DECIDUOUS TREES THROUGH THE SEASONS

Walking through a deciduous forest is a startlingly different experience from one time of the year to another. The trees leaf out in the spring, form a dense canopy during the summer, release their leaves to fall to the ground in the autumn, stand naked during the cold of winter, and leaf out once again when the next spring comes.

Today in Nature’s Depths we look at these seasonal transformations a little more carefully. Let’s start in the spring, though we will soon see that any particular starting point is arbitrary.

The spring blush of the trees is delicate, with colors ranging from yellowish-green to pink depending on the proportion of the main pigments— chlorophylls, carotenes, and anthocyanins—in the cells of their buds and emerging leaves. These are the same pigments we have encountered before in the context of photosynthesis and fall coloration.

Spring in the wetlands of the Earth Sanctuary on Whidbey Island, WA.

In the summertime, the woods surround us with an intense green. The leaves are using their chlorophyll to capture photons of light as efficiently as they can, and this energy they then use to make the sugar that powers metabolism and for the assembly of the myriad molecules needed to create the structures of new life. This is photosynthesis, the process that directly or indirectly drives most of life on Earth.

Summer in the wetlands of the Earth Sanctuary on Whidbey Island, WA.

In the fall another big shift starts to happen. One by one, and then in accelerating numbers, the green leaves lose their chlorophyll and thus stop photosynthesizing. This loss of green often reveals the presence of the other pigments—the red anthocyanins and orange or yellow carotenes—and gives the forest its glorious autumn foliage.

Early autumn in the wetlands of the Earth Sanctuary on Whidbey Island, WA.

Autumn colors of cottonwoods (Populus deltoides) along the Yellowstone River in central Montana.

At some point the trees release these no-longer-photosynthesizing leaves to fall to the ground.

A single leaf of sugar maple (Acer saccharum) glowing against a dark trunk as it falls. It looks larger and slightly out of focus because it is closer than the tree trunks are.

There, the leaves accumulate to form a natural mulch that replenishes the soil.

Fallen maple leaves accumulate against the trunk of a sugar maple.

During the winter most deciduous trees are naked of leaves and thus less susceptible to the ice, snow, and winter winds that are capable of breaking limbs and snapping or uprooting whole trunks.

A deciduous forest in the Minnesota winter.

Some species, however, like this northern pin oak (Quercus ellipsoidalis), stay clad in leaves despite the extra weight, wind resistance, and snow-catching surfaces that the leaves provide.

A northern pin oak (Acer ellipsoidalis) during the winter in Minnesota. Unlike most deciduous trees, this species keeps most of its dead leaves until spring.

PREPARATION FOR SPRING RENEWAL

In the spring the cycle starts all over again. The trees prepare for this well in advance. Even before the leaves fall, buds from which the next year’s growth will burst forth have been generated—not only leaf buds but also the rudiments of flowers, catkins, cones, and other reproductive structures.

Here is how the common red alders of the Pacific Northwest (Alnus rubra) prepare for the spring. If you look carefully in the angles (axils) between the branches and the leaf stalks (petioles), you will find in many a small, elongated bud. These so-called axillary buds form well before the leaves turn color and fall; they persist through the winter; and from them in the spring come the fresh stems and leaves of the trees’ new growth.

A branch of a red alder in Washington in the fall, showing the base of a leaf that is still green (notice the pattern of veins), its petiole, and the associated axillary bud.

Looking further, especially near the tips of the branches, you will also find male and female flower buds developing in the autumn and ready to burst forth in the spring. The incipient male flowers are arranged in elongated catkins like these.

Four recently-formed male catkins of a red alder in early autumn, just as some of the leaves are falling.

The female flowers in the form of cones develop in the fall, preparing to receive pollen in the spring. Here they are glowing in the afternoon sun of September.

A cluster of young female cones of a red alder in the fall, all set to mature in the spring.

If you look at an alder carefully, you may be able to see several generations of leaves, catkins and cones all at the same time—multiple phases of the ongoing cycle of life captured in a single view. In the picture below this year’s leaves have been shed, but there is a leaf bud at the top. Then, going clockwise, we see new male catkins, last year’s female cones still remaining on the tree, and new female cones prepared for the spring.

A red alder in the fall, with buds all set for the following spring and a cluster of female cones remaining from the previous spring.

LIFE GOES ON IN WINTER

We often think of the winter as a time of dormancy for a deciduous tree, of a rest from the frenetic processes of life. And so it is. The leaves fall so there is no photosynthesis and thus no fresh sugar to fuel metabolic processes. Only the tree’s stored reserves, found primarily in the roots, provide the energy for life. The temperature is cold. Life slows down. It does not stop, however. Individual cells continue to perform the biochemical reactions that constitute the living state, and the whole tree is poised to take full advantage of the nourishing conditions of the next spring.

In the next post we will explore the mechanism by which a tree actively drops its leaves at the appropriate time. In the future we will ask how trees recognize what season it is, and by what processes their activities are regulated so that they will be in harmony with both the prevailing season and with the season which is approaching.

Now I invite you to contemplate the annual cycle of a tree’s life as you walk through the autumn woods and revel in the almost daily changes you can see and smell around you.