I am waiting for the leaves to fall en mass from my trees. It is an event that occurs about the same time each year (sometime after Columbus Day but before Halloween), but every year it seems late in coming. I am not sure why I am always so eager to get on with the Fall, it just means that winter is closing in on us and that the color green is going away for five months!
Leaf loss is a purely “economic” decision for a tree. Leaves are a tree’s organs for photosynthesis and energy acquisition, but leaves also lose incredible quantities of water via transpiration. In the summer many tree species (like 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, like this year, these trees can keep all of their leaves, fix abundant energy, and transpire water without damage. In dry summers, though, the limiting factor of water availability makes the tree give up some of its photosynthetic potential in order to maintain its water balance. In these dry years “Fall” actually starts in June or July!
With the approaching winter the leaves for all deciduous trees are shed primarily to help the trees to withstand the incredibly dry conditions of winter. This seasonal leaf loss also recognizes that the freezing of leaf cells’ cytoplasm and interstitial fluids would cause such widespread damage to the cellular structures of the leaves that they would be incapable of any future photosynthesis.
When deciduous trees get ready to shed their leaves, they undergo several well defined stages of change. First, in response to the length of the dark period of the day reaching a critical length, they begin to generate large numbers of cells right at the junction of the leaf’s petiole 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 the flow of nutrients into the leaf, and, so, sugar levels rise in the leaf and nutrient levels fall. The lack of nutrients causes the leaf to stop synthesizing new chlorophyll molecules. Chlorophyll is, of course, the functional pigment of photosynthesis and also the pigment that gives plants their characteristic green color. 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 loss of the chlorophyll 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 to possibly protect the leaf (and particularly next year’s delicate leaf buds) from insect damage.
The deciduous trees in our area are beginning to turn their autumnal colors. 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 insures that a large number of leaves will persist 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 are so important in defining 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!
After the leaves fall from my trees, I usually rake them up into several large, strategically located piles around my yard and leave them to nourish the worms and beetles and other invertebrates that will shred and grind them up into food for fungi and bacteria. In the old days my kids and I would jump in the piles and further accelerate their fragmentation. Now I just rely on the worms to do the job with less noise and vigor. Through the next spring and summer birds (especially the robins and the cardinals) peck at and dig around in the leaf piles looking for insect larvae and earthworms. These leaf piles are a great source of nutrition for these hunters and gleaners. By the time the next fall rolls around, the piles are remarkably reduced in size and are ready to be renewed by freshly raked up leaves. One pile down in my orchard was kept in this yearly equilibrium for over twenty years. I eventually dug up the rich, humus that accumulated at the bottom of the pile and added it to the soil of my tomato patch.
In a forest, the fallen leaves spread out in a thin layer over a broad area. Earthworms start working on these leaves right away, pulling them into their middens and burrows, grinding them up with their muscular mouth-parts and gizzards, mixing them up with ingested soil, and defecating them out in nutrient rich, erosion resistant pellets. In soils without earthworms, numerous arthropods of many sizes begin to slowly chew away the leaf materials making a fine powder of organic residues enriched with bacteria. Both the worms and the arthropods are setting the table for the bacteria and fungi that then steadily work away at the less resistant molecules in the leaves. Like in my leaf piles, by the time the next fall comes around what’s left of the old leaves serves as a base for the new and the decomposition process grinds on.
So, enjoy the coming events of the Fall and make sure you use your leaves for the biological health of your yard!