Saturday, December 08, 2007

Winter Gardening

November slid by, cold and wet and windy until all the leaves came off the trees and lay in golden swathes, calling to me to come with my rake. I know I ought to rake them up and store them in wire baskets until they rot down for perfect compost, but really, I can't be bothered. And if we are honest with ourselves, no-one wants half-filled black bin liners leaking slowly rotting cellulose hanging around their garden for a couple of years, either. So throughout autumn I rake the moss from the lawns, gather the leaves, then stuff the lot under the hedge in the kitchen garden. I like to think of this process as creating warm bedding for the hedgehogs' hibernation in this relatively quiet part of the garden.

What I'm actually doing is intervening in the Carbon Cycle. The element Carbon is a basic constituent of all living organisms. Its atoms combine easily with other atoms to form a huge variety of molecules. Some of these, Carbon Dioxide CO2 and carbohydrates C6H12O6 have names which are clearly Carbon based, whilst others, fats and proteins for example, are not so obvious. All cells, whether animal, plant or bacteria, contain Carbon in the form of fats, proteins and carbohydrates. Plant cell walls are made of cellulose, a form of carbohydrate.

Carbon cycles through ecosystems, moving repeatedly from one organism to another, and between organisms and the environment. The Carbon cycle is a key factor in maintaining the balance of an ecosystem, and works thus:

Plants photosynthesise, taking Carbon in the form of Carbon Dioxide from the atmosphere and locking it into the carbohydrate glucose: Carbon Dioxide + Water = Glucose + Oxygen, or 6CO2 + 6H2O (+Light) = C6H12O6 + 6O2 Chlorophyll is the green pigment that enables plants capture light energy.

Animals get their Carbon from eating either plants (carbohydrates) or other animals (proteins and fats). They respire, releasing Carbon Dioxide to the environment. Plants also respire, taking Oxygen from the atmosphere or the by-products of photosynthesis: C6H12O6 + 6O2 > 6CO2 + 6H2O (+ released energy)

Waste Carbon-based material is excreted by animals, and is digested by decomposers, mainly microbes and fungi. The decomposers also respire, releasing Carbon Dioxide.

When animals die, their remains may be either eaten as carrion by scavengers / roadkill by you-know-who, or digested by decomposers. Both scavengers and decomposers respire, giving off more Carbon Dioxide. Here's a diagram:

Plant leaves therefore have two primary functions; to act as solar panels for the plant, enabling the sunlight falling onto the leaves convert into carbohydrates by the process of photosynthesis; and to enable gas exchange via the lenticels. Of course, Jan doesn't know this because she didn't bother reading this far, did you Jan? Naughty blogger.

The growth cycle of deciduous trees and shrubs is linked to day length. Most have a relatively short period of annual growth. New stems begin to grow from overwintering buds when the days lengthen and temperatures are warm enough to support growth. For most trees, growth is usually completed by late June in the Northern Hemisphere. The following year's leaf buds are set at this time and will not open until they experience the chill and short days of winter followed by the warmth and increasing daylight of spring. Once the leaves are fully expanded and the buds are set, the work of manufacturing and storing carbohydrates to support the following season's growth accelerates. These carbohydrates are stored in the branches, roots, and buds throughout the growing season to support next year's growth. In late summer or early autumn, the days begin to get shorter, and nights lengthen. Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. During the growing season, chlorophyll is replaced constantly in the leaves. Chlorophyll breaks down with exposure to light in the same way that colored paper fades in sunlight; the leaves must manufacture new chlorophyll to replace chlorophyll that is lost in this way. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. When this happens, the leaf falls. It retains little nutrient value, is almost wholly cellulose, and thus takes at least two years to rot down. It makes a good soil conditioner, and mulch, and that's about it. So under the hedge go the fallen leaves.


Jan said...

Very scientific. I'm impressed. But I have to admit I didn't read it all. I space out at the periodic table, partly because my husband has his PhD. in organic chemistry.

April said...

Very cool, Amalee. I didn't know this.