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 C02 is fundamental to photosynthesis, it makes sense that increasing the amount of C02 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 waxy layer that 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 C02 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. This 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 C02 it needs, and has synthesized all the sugar that it can use for the time being. Having got enough C02, 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 are 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 century 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 photosynthesize more, and also suffer less risk of dying of dehydration. 0ver 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.
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