The Two Direct Effects Of Co On Plants Photosynthesis And Water Balance
Carbon dioxide may affect plants by changing the climate, but it can have another more subtle and quite separate influence, through its direct effects on plant physiology. Since CO: is fundamental to photosynthesis, it makes sense that increasing the amount of CO; in the atmosphere will tend to allow plants to photosynthesize faster. This then is one-half of the direct C02 effect on plants. But there is also another less straightforward direct effect of C02 on the water balance of plants. Why should this be?
Ever since plants first came out of the sea to live on land, they have faced a dilemma. They must prevent themselves from losing too much water in the drying air. but they also need to take in C02 in order to photosynthesize. A plant could easily almost eliminate water loss by coating itself in some sort of thick waxv layer that c1 J J
water cannot pass through. But, at the same time this would almost totally prevent C02 from getting into its leaves, and it would be unable to grow. So, plants have to balance a "trade-off" between gathering enough C02 in order to photosynthesize, and avoiding death by desiccation. Vascular plants (those with roots, stems and leaves) have solved the problem in a satisfactory way by using tiny pores in their leaves—called stomata—which can open and close. When a plant has plenty of water, the stomata let C02 in to the moist interior of the leaf and the plant tolerates the evaporation of water through the stomata for the benefits of photosynthesis. When the plant has enough carbon, or when it begins to run short of water, it partially or totally closes these pores to prevent further water loss.
Much of the time, plants only open their stomata part-way, or keep them shut altogether, which limits the amount of C02 they can take up and the amount by which they can grow. If you add more water around the roots of the plants, they will open their stomata more fully and keep them open for longer, take up more C03 and grow more. If instead you add more C02 to the air around the plants, very often they do the opposite, keeping their stomata only part-way open or closing them after a short time. T his is because at high enough concentrations C02 veritably pours into a leaf, even through partially closed stomata. So, without keeping stomata fully open for long, the plant has soon got all the CO: it needs, and has synthesized all the sugar that it can use for the time being. Having got enough C'02, the leaf then shuts the stomata to prevent any further loss of water. Evolution has selected plants that take this conservative path, avoiding "spending" water around their roots that they might need for another day, as soon as they have enough sugars to keep them going. Thus, a plant that has more C02 may not actually do more photosynthesis, but instead it may avoid dying of drought because the supply of water around its roots lasts longer.
All in all, C02 and water arc interchangeable; they are part of a trade-off for plants. More C02 means that a plant has more water. Giving a plant more water means that it can open its stomata and take in more C02, which allows more photosynthesis. So, more C02 can benefit plants in two ways: it can mean that they get more growing done because they can do more photosynthesis, and it can also mean that they are less susceptible to drought. An increase in C02 to the sort of levels that will be reached in the next ccntury will affect plants everywhere in the world, altering their growth rate and their water balance. The only question is how large these effects will be, and what long-term consequences they will have for ecosystems and communities.
If the amount of C02 in the atmosphere increases, in a general way we can expect it to benefit plant activity on land. Plants can photosynthesi/e more, and also suffer less risk of dying of dehydration. Over the next few centuries, this "direct C02 effect" might well turn out to be ecologically more important than the greenhouse effect of C02 and other greenhouse gases. However, there is a lot of uncertainty and indeed quite a bit of mystery associated with the direct C02 effect. There are some good reasons for thinking that it could be very important in altering vegetation, but a frustrating lack of evidence to show whether such suspicions are right or wrong. Within the small amount of evidence that we do have, there are quite a few contradictions and paradoxes.
8.2 INCREASED C02 EFFECTS AT THE SCALE OF A LEAF
Some tentative clues to the effects of increased C02 on plants come from short-term observations of individual leaves exposed to artificially high C02 concentrations. It is possible to estimate how last a leaf is photosynthesizing by measuring the uptake of CO-, labeled with radioactive l4C. The more radioactive the leaf is at the end of the mm experiment, the more carbon it has managed to fix by photosynthesis. Such small-scale experiments on raised C02 have tended to involve a doubling of C02 from about 350 ppm—the approximate "background" level of C02 during the past couple of decades—to 700 ppm, a level that C02 is likely to reach well before the end of this century. Short-term exposure to high C02 tends to result in a major increase in the amount of sugars fixed—a typical sort of change observed would be a doubling or experiments with raised C02, and then taking a C02 fertilization factor for the increase in growth rate, known as "beta".
What sort of things do these models predict for the coming decades and centuries? Essentially, if everything else in the world stays the same cxccpt for C02 increasing, the models all agree on two things. First, there will be an increase in net primary productivity (the growth rate of plants) all around the world as the supply of carbon for photosynthesis increases. Second, plants will be using the water supply around their roots more efficiently because they do not have to open the stomata in their leaves quite as much. This will be rather like an increase in rainfall as far as the plants arc concerned.
Different models predict different degrees of response to any given increase in C02, depending on subtle details in the assumptions that they are based upon. A review of models over the past ten years or so showed that on average for a world with 580 ppm C02 (which will probably occur around 2050 given the current rate of increase in C02) compared with a baseline of 350 ppm, the models predict a 22% increase in plant productivity around the world. However, the range of estimates amongst them extends from 10 to 33%. And, of course, given that they are only models, if they have overlooked or misjudged some important factor, they might all be wrong.
If plants increase their productivity and also make better use of the water available to them, we can expect that there will be some changes in the structure of vegetation around the world. The distributions of biomes are often determined by water availability, and an increase in C02 that allows plants to use water more effectively should allow wetter-climate biomes to spread. Models which combine a biome vegetation scheme (Chapter 2 in this book) with a C02 fertilization model predict that there will, for example, be an increase in tropical rainforest, allowing it to spread out into zones which get less rainfall and currently have dry forest or savanna vegetation. In arid areas, plants that are able to get by on less water because of higher C02 will be able to spread—the deserts will become greener. There is also predicted to be a general shift around the world towards C3 plants (see Box 8.1, p. 206), and away from C4 plants which do not benefit so much from increased C02. The speed with which these changes in vegetation actually occur depends on many different factors. Even though tropical rainforest might be capable of spreading into savanna regions, it will likely take hundreds or even thousands of years for the forest trees to disperse out and grow up into dense forest in these new areas. In the meantime, more subtle shifts in the structure and composition of vegetation are also likely to occur as some of the plants that were already in place grow bigger, and shade out other species around them.
Continue reading here: Adding Climate Change To The C02 Fertilization Effect
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