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Falling Leaves

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Many calendar photos on display this month show brilliantly coloured leaves from eastern North America. Our trees and shrubs are perhaps a bit easy going in this regard—they are after all west coasters. However, similar processes are underway here too. The subdued beauty of the yellow and brown maple, willow and cottonwood leaves interspersed with reds from dogwood leaves and contrasted by the green of the conifers is still a sight of great aesthetic value. This transformation in leaf colour in deciduous trees is also of great practical value because during this time nutrients are recycled from the leaf to other parts of the plant to support growth until next spring.

Leaf drop is the final stage in leaf development, but the leaf has prepared for this since the spring. The junction between the leaf and stem has a special layer of cells through which pass tiny tubes that carry water to the leaf and food back to the stems and roots. In the autumn, these special cells start to swell and slowly cut off the connection between the tree and the leaf, until the leaf falls.

Leaves contain multiple chemical compounds which give them colour; however during the growing season, these colours are overpowered by green chlorophyll. When chlorophyll production slows or is stopped by decreasing daylight (and this year by the drought), the gold, yellow and browns of the leaf become visible. Red colours are a little different. They are produced by anthocyanins in the autumn. (The anthocyanin is an antioxidant such as found in beets or purple grapes.) The reason for this is not understood. Production of anthocyanin requires carbohydrates and nutrients. Why a plant would commit energy to change leaf colour shortly before leaf drop has puzzled plant physiologists. Some think that the red acts to reduce sun damage. This may allow the leaf to stay on for a few weeks longer and thus recycle more of the nutrients in the leaf. Another idea is that when the leaves drop, the anthocyanin in them inhibits the growth of other plants around the tree, thus reducing competition. Chemical defences are not uncommon in the plant kingdom—walnuts are known to inhibit the germination of other seeds via the chemical juglone in the leaves (and tissues). (I had a teacher once tell our class that when you can’t think of a reason why a plant is not doing well think about allelochemical interactions.)

Interestingly some species which have the ability to turn leaves red seem to also do this for stress of other sorts. The Pacific dogwood frequently does this early in summer in response to disease.

Many herbaceous plants follow the similar strategy to deciduous trees and shrubs: allow the photosynthetic tissues to die and store energy in the roots. Annuals prefer to put their efforts to seed growth and let the adults die each winter. Seeds have very low metabolic requirements and if the conditions are right, they can live many years in this dormant state.

Evergreen trees have gone a different route. They keep their needles or leaves green all year and have had to develop strategies that minimize damage to leaf tissues from the cold and wind. Even on sunny days in winter, little photosynthesis occurs as many metabolic processes are hindered by low temperatures and, if soils are frozen, by lack of available water. The waxy needles (or leaves) have resins, which act like anti-freeze. They also have the ability to close the stomata (pores that allow air and moisture exchange) on the underside of the needles to prevent water loss.

During early summer, both evergreen and deciduous trees and shrubs prepare for dormancy with the production of winter buds. These buds contain all the components for the next year’s new growth and sit mostly dormant for the winter.

As I watch the leaves change colour this autumn (and every autumn), I am amazed at the variety not just between species but within a species. Sometimes even when growing side by side in what seems like uniform growing conditions, different individuals will have leaves of different colours.

This morning I walked by four red-osier dogwood in a patch and collected a few leaves from each. A close up inspection revealed a tremendous variety: some yellow, others brown, red and some completely green. Most had some specks of brown or yellow. One yellowing leaf had a dime-sized patch of green—a sign that a small fungal colony had established itself and was using the plant’s chloroplasts for its benefit. A couple of the leaves were rolled up into an over-wintering home for an insect. In this small patch of plants was a world of wonder and discovery.


Jay Rastogi is a naturalist, horticulturist and educator living in Yellow Point. Reach him at ecoforestry [at] gmail [dot] com

About the author: Angie

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