Many plants in arid environments decrease the problem of water loss through stomata by chemical tricks that help them take up CO? with less water loss. These are known as C4 and CAM plants.
Most plants are known as C3 plants. They take CO? up into leaf cells which handle the whole photosynthetic reaction in the same cells. The C02 gets fixed into a three-carbon chain (hence the name C3), and then in the same cell the water-splitting part of photosynthesis gives the hydrogen needed to tack on to carbon.
C3 plants C4 plants
C3 plants C4 plants
A C4 plant, on the other hand, avoids photorespiration because it shuttles the fixed carbon to have the final sugar-making reaction occur in special cells deep within its leaf that aren't producing any oxygen (which is the thing that "spoils" the reaction). And concentrating C02 at high levels relative to oxygen also helps suppress photorespiration. Having special "CO?-gathering" cells that take up C02 without producing any C02 through photorespiration helps to ensure maximum efficiency in C02 uptake (it is like a vacuum cleaner for C02), in terms of "stomatal opening time" and water loss. Hence, in a C4 plant stomata need not be open for as long to take up a unit of carbon, and for this reason too water use is more efficient. Thus, the C4 plant loses less water per unit time per unit of carbon fixed. This should help it to do better in dry environments.
Note that, because photorespiration also occurs especially fast in warm climates and at high light intensity (which causes high temperatures in the leaf), the C4 system is also directly advantageous for avoiding wasting solar energy, irrespective of water balance. Of course, dry environments also tend to be sunny and hot, so in this respect (avoiding water loss, and allowing effective utilization of high light intensities) they doubly favor C4 plants.
Not surprisingly, then, C4 plants tend to be most abundant in warm, fairly dry environments. The C4 system is especially often found in grasses in semi-arid environments such as the western and especially the southwestern parts of the North American prairies (no trees have the C4 metabolism). There is a gradient in C4 abundance going from cool to warm, and from wet to dry. However, the pattern is not always quite as expected. Surely, one should see C4 plants totally dominating in the most arid environments, the deserts and semi deserts? Yet, in the hot semi-deserts of North America, one or more species of C3 plants (e.g., creosote bush, Larrea tridentatd) usually dominates the plant communities. According to their general physiological characteristics, C3 plants should be the least adapted to hot desert environments because they are less water-efficient than C4 plants and have a lower optimum temperature and lower rates of photosynthesis, and C3 plants also reach their maximum response to sunlight at low light intensities. Clearly, there are other factors which we don't quite understand that can contribute to the success of plants which have a "wrong" photosynthetic system for the climate.
Given that C4 plants are so effective at gathering in CO?, they do not have so much to gain from an increase in C02 concentrations in the atmosphere. So, it is generally expected that they will not respond as much to a higher C02 world. In fact, they might end up being pushed out, as C3 plants do better in response to C02 fertilization. It is to be expected that the reason C4 plants do not occur everywhere is that there is a "cost" in maintaining this complex photosynthetic system, and in places where it is not really needed they lose out in competition to C3 plants. On the other hand, if climates grow warmer due to the greenhouse effect, photorespiration will tend to increase and C4 plants might still find themselves favored by this factor, because they suffer less from photorespiration.
CAM plants can be expected to show even less response than C4 plants to increased C02, because they are so good at taking in C02 and they also do it at night when they do not lose so much water by evaporation. Experiments show that they are essentially unaffected by doubled or tripled C02 levels.
Among other effects noted in arid-land plants exposed to increased C02, it appears that increasing the atmospheric C02 concentration can reduce the impact of salinity on plant growth. This could improve crop growth in desert-marginal areas which tend to have salty soils, and perhaps increase productivity and biomass of natural desert vegetation.
From the limited amount of experimental information on responses of desert and arid-land plants to increased C02, it seems that most of the preconceptions have to some extent been supported and to some extent challenged. Some experiments suggest that either because of nutrient limitations or their innately low growth rate, desert and semi-desert plants may hardly be able to respond to high C02 in terms of growth rate and biomass. Other experiments suggest a strong response by these very same spccies of drought-tolcrators. In certain experiments, there is a disproportionate response to C02 by particular plant "types" or even by certain individual species which apparently arbitrarily show a very large response when most others barely respond. The general expectation that increased C'02 will favor a stronger response of C3 plants over C4 species is tentatively supported, but it is subject to uncertainty given the contradictory results from prairie species. The amount of idiosyncrasy in responses seen in all of these various experiments seems to make the prediction of C02 effects on any particular arid region (or arid regions in general) a rather risky business, for it may vary greatly with the detailed community assemblage and perhaps other local factors such as soil variations and hcrbivory.
Another factor which should be borne in mind is that many of the free air C02 experiments that have been run in moister climate biomes (e.g., tundra and some forest systems) for more than a few years show a decline or even a disappearance of the effects of CO: on plant growth rates. It is unclear what this might mean in terms of biomass and species composition as the plant community reaches a rough equilibrium in the longer term. The Nevada desert C02 experiment has not been run for as long as some other FACE experiments, and because desert plants tend to be slow-growing. the time taken for the ecosystem to reach a balance in response to higher C02 levels may be even longer. Even if growth rates arc initially boosted in arid lands with raised C02 (as some chamber experiments suggest), there is no certainty that this will translate into greater vegetation biomass beyond a boost in the earliest years, because shortage of nutrients may begin to dominate.
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