(parts of this week’s post were first presented in a September 14, 2014 blog!)
During the last week of July the red maples and the yellow poplars along the Roaring Run Trail began to shed their leaves. On a few of my early morning bike rides breezes would sweep across the tree canopy and send down clouds of yellow and red leaves. A few spots on the trail were well covered with leaves and took on the appearance of autumn.
The trees that usually shed their leaves during a summer drought are the cherries and the locusts. The two, tall, skinny black locusts out on the back edge of my field often lose half of their leaves in a typical July. Sudden breezes send swirling clouds of yellow leaflets down onto the lush, green grass, and the black locusts, which are typically one of the last trees to leaf out in the spring, stand mostly denuded but increasingly watertight against the summer drought. My cherry trees (black and sweet cherries) have the same response to drought but don’t shed their leaves quite as extensively as the locusts. This year both species retained their leaves in spite of the hot, dry summer weather! Soil moisture from our spring rains must have been sufficient for their summer water needs.
Leaf loss is a purely “economic” decision for a tree. Leaves are the organs for photosynthesis and energy acquisition, but they also lose incredible quantities of water via transpiration. In the summer many trees balance their needs for energy (for growth, reproduction, repair etc.) with the necessity of maintaining an acceptable water balance in their tissues and cells. In wet summers these trees can keep all of their leaves, fix abundant energy, and transpire water without damage. In dry summers, the limiting factor of water availability makes the tree give up some of its photosynthetic potential in order to maintain its water balance.
With the approaching winter the leaves for all deciduous trees are shed primarily to help the trees withstand the dry conditions of winter (also, the freezing of the water in the leaves would destroy their cellular structures and render the leaves useless as photosynthetic organs!). The types of trees that keep their leaves (the coniferous, or “evergreen” trees) do so by making tougher, more water tight “leaves” (often tightly pored needles that are wrapped in layers of waxes) and by some elegant physiological adaptations that go on inside the cells of the needles. This winter acclimation adaptation includes altering the chemical nature of the lipid molecules inside the cells (making the lipids more “unsaturated” and, therefore, more twisted and bent and thus less able to join together in a solid form (this significantly reduces the freezing temperature of the cells!). The cells also increase the cytoplasmic concentrations of these freeze-resistant lipids to amplify this antifreeze effect. The cells also add other solutes to their cytoplasm and break up some of their intracellular proteins into many smaller pieces. Both of these responses act to further decrease their freezing points.
The cells in these conifer needles also alter their plasma membranes to allow water to move across the membrane more freely. Then, as ice begins to form in the spaces around the cells, the water of cytoplasm is drawn out into the surrounding ice crystals and away from triggering possible freeze events inside the cell itself! An interesting side note here is that the freezing of this surrounding liquid water to form ice releases a small amount of heat energy (the “heat of fusion”) and the cells of the leaf take advantage of this added heat to help maintain their internal liquidity!
When the deciduous trees get ready to shed their leaves in the fall, they undergo several well defined stages of change. First, in response to the duration of the dark period of the day reaching a critical length, the leaves begin to generate large numbers of cells right at the junction of the leaf’s stem and its branch. These cells greatly increase in number but not, at first, in their individual sizes. This layer of cells (the “abscission layer”) slowly starts to interfere with the flow of sugars out of the leaf and nutrients into the leaf. The lack of nutrients entering the leaf stops the synthesis of the new chlorophyll molecules that are needed to replace the ones that wear out in the ongoing process of photosynthesis. Chlorophylls are, of course, the pigments that give plants their characteristic green colors. Initial cessation of chlorophyll production makes the leaves appear a bit paler and less intensely green than they were during the height of summer. Continued breakdown of the chlorophylls then starts to unmask the other pigments (the “accessory” pigments of photosynthesis: the carotinoids and xanthophylls) that had been present in the leaves all summer long). As these pigments are “revealed” the leaves then “turn” orange (from the carotinoids) or yellow (from the xanthophylls) before they finally fall from the tree. The accumulation of the sugars in the leaves also has an effect on eventual leaf color. These sugars stimulate the synthesis of anthocyanin pigments in the leaf. These pigments generate purple or bright red colors in the leaf and are thought (by W. D. Hamilton, the famous “Bill Hamilton” of biology!) to protect the leaf (and particularly next year’s delicate leaf buds) from insect damage.
The deciduous trees in our area will be turning their autumnal colors over the next month or so. The breakdown of the chlorophyll and the revealing of the accessory pigments is inevitable in our climate zone. In some years, though, the intensity of the reveled colors is much more extreme than in other years. The weather patterns of the fall and of the preceding spring and summer all contribute to the magnitude of the final color response.
Good, healthy abundant leaves are favored if the previous spring had adequate rainfall. A normal to wet summer will then insure that these leaves persisted intact through their active photosynthetic seasons. Warm, sunny autumn days combined with cool but not freezing autumn nights will maximize sugar production and anthocyanin synthesis in the leaves. These accumulating anthocyanins then give the leaves their brilliant red and crimson colors that seem to define a “good” color year in the forest!
The way this year is working out, we should have some very spectacular colors around us, and that is almost everyone’s favorite Sign of Fall!