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| Fall leaves left to right: huckleberry, Rocky Mountain maple, red-osier dogwood |
Leaves are widely recognized as disposable solar panels. But did you know that some leaves are secret agents, too?
You probably know that summer leaves look green because they contain chloroplasts. Chlorophyll in these chloroplasts absorb daylight in blue, yellow and red wavelengths - leaving green light reflecting back to our eyes. And you may know that most northern plants change colors each fall because they stop producing chlorophyll in late summer. As the leaf's chlorophyll gets used up, the orange and yellow pigments - which are always present in some leaves - begin to show through. But do you know that many plants spend lots of energy in fall to initiate the production of red pigments?
Without getting mucked down in chemistry, the short explanation is that the production of red pigments (anthocyanins) helps some plants in several ways. Red pigments allow plants to efficiently resorb nutrients from the dying leaf, attract some seed-eaters that spread the seeds while repelling other leaf-eating insects, and serve to reduce competition from other plants.
A plant is more likely to survive winter dormancy if it reclaims more of the valuable chemicals from each leaf before shedding them in the fall. The plant itself doesn't necessarily need protection from sunlight. But during cold weather, continuing with photosynthesis would chemically reduce the plants' ability to resorb nutrients from the leaves, like nitrogen and phosphate. By shielding the chloroplasts with red pigments, the plant becomes more efficient at shutting down for the winter.
Some plants also use red leaves to either attract or deter animals. Lots of plant species use red berries to attract fruit-eating animals, who then spread the plant seeds in their manure, or repel animals by advertising bitter-tasting or toxic fruits. But some plants also use red leaves in fall to attract animals to their seeds. One example is poison ivy, which has red leaves and greenish-white berries in fall.
Some plants also use red leaves as a signal to deter insects, which lowers their parasite load. Spending energy to produce red leaves is thought to be an honest signal - one that can't be faked by less-healthy plants - of how much energy the plant has to produce chemical defenses against insects, like aphids, helping to convince some insect pests to lay their autumn eggs elsewhere. Red leaves also tend to make the insects more visible to insect predators, like birds.
Finally, the red pigments produced by some plant species can affect the growth of different plant species. As the red leaves are shed and become part of the surrounding leaf litter, the chemicals that are released can affect what does or doesn't grow in that soil. Most of the time, this slows the growth of other species to reduce competition for sunlight, water and soil nutrients. In some tree species, this also limits the growth of nearby saplings of the same species.
One organism producing chemicals (from leaves, roots, leachates, etc.) that affect another organism is known as, "allelopathy." This secret chemical warfare appears to be common among plants, and it's led to a lot of research on potential uses to reduce weeds and/or increase growth of crop plants. One study showed that 25 out of 33 noxious weed species had allelopathic properties. Back in 2001, two Montana high school students from Sunburst showed that a noxious weed, burning bush (Bassia scopari), growing ahead of spring wheat delayed the wheat's emergence and reduced its growth.
Farmers using plants as cover crops and mulch are often employing the secret allelopathic activity of one plant species against that of another species. It's a relatively new skirmish in a hidden battle that wild plants have been have been waging for eons.
