How do green plants cope with 24 hours of darkness for long periods? Does this affect oxygen production, and in turn cause any problems for local wildlife?
鈥� During the Arctic winter, for example in December in Lapland, there are 24 hours of almost darkness. Santa and Rudolph tolerate it, and so do the vast areas of conifers that grow there, up to 70掳 north, well above the Arctic circle. During this period, trees can鈥檛 photosynthesise, so they produce no oxygen. However, this dark spell coincides with a cold period. The average December temperature in Lapland is about -10掳C (or -25掳C for coniferous forests at similar latitudes in Yukon, Canada, and Siberia, Russia).
At such low temperatures, all metabolic processes grind to a virtual standstill. This includes respiration, the main activity that might otherwise have caused the trees to use up atmospheric oxygen. Like hibernating bears, the forests become dormant 鈥� almost 鈥渄ead wood鈥�.
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In temperate regions, in comparison, deciduous trees such as the oak and ash effectively enter a period of self-imposed darkness in winter. With their leaves having fallen, they might as well be in 24-hour darkness as they can鈥檛 carry out any photosynthesis.
They do continue to respire at low levels, especially in their roots, fuelled by reserves such as the starch manufactured the previous summer. But when this is happening in the northern winter, it is offset by deciduous trees in the southern hemisphere photosynthesising during their summer. Any effect on local oxygen concentrations is negligible because winds ensure the global circulation of our shared oxygen supplies. So don鈥檛 worry!
Stephen Fry, University of Edinburgh, UK
鈥� A lack of oxygen isn鈥檛 of immediate importance where this element can be freely exchanged with the atmosphere. The main exception is under water because oxygen is so poorly soluble that it must be continually supplemented by photosynthesis and convection if active animals are to function.
Where banks of plants such as or water lilies prevent the wind from circulating oxygenated water and block out light, inhibiting photosynthesis at lower levels, the water beneath the plants deteriorates into a smelly dead zone. Fish die if they cannot escape and mosquitoes flourish on the rotting material.
Jon Richfield, Somerset West, South Africa
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