Signs of Fall #4: Leaf Changes

Photo by D. Sillman

Photo by D. Sillman

During most summers we hit a dry spell and several types of trees respond to the lack of water with leaf loss. The two, tall, skinny black locusts out on the back edge of my field 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. Cherry trees (black and sweet cherries) have the same response to drought but don’t shed their leaves quite as extensively as the locusts. I do remember, though, back in 2010 that the cherry trees along the Baker Trail lost at least thirty percent of their canopies in the dry summer. The trail surface was littered with bright, yellow leaflets all through July and August.

This summer both the locusts and the cherries have kept their leaves and are only just now starting to show any color changes or leaf losses. The abundant and remarkably steady rainfall this year (May through August we were three and half inches above average) is probably the reason: no water stress, no leaf changes or leaf losses until the seasonal cues kick in that push the deciduous trees into their winter physiologies.

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 the black locusts and the cherries 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.

Photo by D. Sillman

Photo by D. Sillman

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 a tougher, more water tight “leaf” (often very 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 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!

Photo by D. Sillman

Photo by D. Sillman

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 flows of sugars out of the leaf and nutrients into the leaf. The lack of nutrients entering the leaf stops the synthesis of 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. 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 possibly 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 very soon. 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!

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One Response to Signs of Fall #4: Leaf Changes

  1. mary mcnavage says:

    In addition to glorious weather, it sounds like we will have a feast for the eyes this coming October–what a gift!

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