Forests have been studied quite closely over time in relation to raised C02, using the FACE method. Where stands of young temperate deciduous and conifer trees growing in forest soils have been exposed to raised C02 over a year or two, they tend to show a strong initial increase in growth rate. However, after a few years, there is often a major decline in the effect of C02. I'll take one particular case study to look more closely at the complex and contradictory results of such experiments. At a FACE experiment in Tennessee on young sweetgum (Liquidambar) forest, wood growth was 35% greater in C02-enriched plots than in control plots in the first year of the experiment. In the second year, the growth response was reduced to 15% and was no longer statistically significant, with further reductions in the third year and so on (Figure 8.4). However, some other longer lasting changes have been found. For example, below ground the rate of production and turnover of fine roots has remained higher throughout the experiment. In fact, there has been so much increase in growth rate and turnover of fine roots that the rate of primary production at the site has stayed about constant; it has simply shifted underground. Bear in mind, though, that the trees that have been exposed to increased C02 remain a bit larger than the untreated ones because they got a "head start'' a few years back, even if they are not responding much any more. The increase in NPP is just about at the average of what several different models of C02 fertilization have predicted, at around 23% (the models predict a 22% increase, though the range amongst them is quite broad). However, the form of the increase in NPP is not quite what the modelers would expect; it goes into making small roots which grow and then rot quickly, and not into long-lived wood. Some plant physiological modelers have found ways to try to explain all of this, in the context of familiar expectations of the C02 fertilization models. However, others such as the redoubtable Christian Koerner point out that these arguments, and the selection of parts of the data, involve a fair amount of gymnastics to make the results fit with the expectations of the models. My own view is that the confusing and conflicting results of C02 fertilization experiments put the grand global models in doubt. I am not saying that the models are necessarily wrong: it is just that we have no strong reasons at present for thinking that they are right.
A drop-off in the increased wood production at high C02 does not always occur. A strong response in terms of NPP (about as much as at the Tennessee site over the other side of the Smokey Mountains) was found at a FACE experiment on pine forest in North Carolina. After 10 years there is still a growth enhancement of around 20%, but in this case much of it seems to be going into extra wood and not fine roots. Among the various C02-fertilized plots of forest at the North Carolina site, those that are on nitrogen-rich patches of soil tend to be responding more than those on nitrogen-poor soil. In fact, most of the strong, sustained response to C02 comes from a single plot where the soil is very rich in nitrogen.
In the experiment in Switzerland, the mature mixed oak forest exposed to raised C02 (530 ppm) at first grew several percent more than an untreated area of forest adjacent to it, but then the effect diminished rapidly. By the fourth year of the experiment, the raised C02 forest was not putting on additional wood any more rapidly than the forest exposed to ambient C02 levels. In this case, root changes were not studied, so it is not known if the increased C02 instead affected roots as in the Tennessee experiment.
So far no FACE experiments have been run for tropical forest, which reflects the logistical and monetary challenges of setting up such a complex piece of work. The closest thing to a tropical FACE experiment so far has been the very localized and short-timescale release of extra C02 onto branches of various tropical tree species, using a canopy crane for access. These experiments showed that an expected "benefit" of C02 for tropical trees, reduced evaporation of water through the stomata, did not materialize. Instead, the stomata stayed stubbornly open for as long as usual, suggesting that the trees wouldn't economize on water under enhanced C02 (Koerner, 2004). This particular piece of knowledge looks like bad news for the accuracy of global change models, such as that used by Peter Cox and colleagues, to predict the loss of water recycling from tropical trees and a collapse of part of the rainfall system of the Amazon Basin. It is, however, perhaps good news for the Amazon rainforest, which would not then be expected to suffer as a result of the loss of this waterrecycling effect.
Because the FACE experiments have not yet been run for several decades, we do not know whether the raised C02 response will produce any effects that only emerge as the trees grow bigger. 0nly the "natural" experiment at the hot springs in Italy has so far run for this length of time. In this particular study, the evergreen oaks were studied after they were cut and started regenerating from stumps (as coppice). The oaks close to the C02 source initially grew much faster, but they do not now grow any quicker than the nearby "control" individuals under normal ambient C02 levels that were cut at the same time. However, because they got a head start during its first few years of regrowth from stumps, the high C02 trees are quite a bit bigger than the normal C02 area.
The overall conclusion—from the various temperate forest types that have been studied using C02 fertilization—is essentially that there is "no conclusion". In some experiments there is a lasting response, but in other cases the response seems to have vanished. In some the response is mainly above ground, in others it is below ground in the roots. At some sites the total increase in primary productivity is much as global
C02 fertilization models would predict, but the nature of the response is rather odd (e.g., going into fine roots, not wood). It is difficult to know whether to take such results as supporting the models, or refuting them. Also, it is important to bear in mind that there are many forest types in the world that have not yet been studied using FACE experiments, including boreal conifer and tropical rainforests. For all we know, they might respond quite differently.
Semi-desert and dry grassland vegetation is generally forecast to respond especially strongly to increased C02 levels, because it is so limited by water. Since adding C02 means that the plants can make use of water more efficiently, this should surely offer a massive boost to them. In one study using C02 fertilization models, Jerry Mellilo and colleagues forecast an increase in primary productivity in the world's semi-desert regions of between 50% and 70% if the C02 concentration gets to be double what it was 200 years ago. This amount is much greater than the sort of productivity increase forecast for wetter ecosystem types such as the world's forests, which is typically around 20-25%.
How does the experimental evidence match up with this prediction of a big boost for plant growth in deserts? Probably the most realistic study of desert vegetation under increased C02 is a FACE experiment that was set up in the Nevada desert of the southwestern USA with a ring of miniature "towers" to match the low vegetation. This experiment increased the C02 concentration by 52% above the "background" level across the desert. In some ways the initial response of the C02-fertilized plots (compared with the controls at normal C02 levels) was dramatic, much as the models would predict. There was a big increase in photosynthesis by 80-100%, and water expenditure through evaporation by the desert plants was only about half of what it would normally be per unit of photosynthetic production. Yet, strangely this did not translate into any increase in shoot or root growth rates of the commonest two desert shrubs creosote bush (Larrea) and Ambrosia.
However, in contrast to this, closed-chamber experiments with creosote bush and mesquite (Prosopis) shrubs grown under doubled C02 showed a significant growth response of the shrubs, with an increase in biomass of these species by 69% and 55%, respectively. Quite what is so different between the open air and closed-chamber experiments is not known!
0ne closed-chamber experiment found that when C02 levels were increased there was an increase in survival rates of seedlings under droughty conditions, which is what would be expected since the seedlings would be able to make better use of the water they had available amongst their roots. In a different short-term chamber experiment on various southwestern US semi-desert species, there was a doubling in root nitrogen (N) and phosphorus (P) uptake under high C02 by the grass Bouteloua, and yet a major decrease in N uptake by the creosote bush Larrea— perhaps due to the competition. Because nutrient limitation on plant growth is thought to be important in deserts, this unequal response by different species might tend to bring about longer-term changes in plant communities.
The inconsistency in results between closed-chamber and free air fertilization studies, and between different species, presents a confusing picture for what might happen to semi-desert vegetation in the future. 0ne may regard free air and relatively undisturbed communities at the FACE site as more representative of what will actually happen as ambient C02 increases, although some authors have argued that chamber experiments can actually sometimes be more representative than free air studies. The upshot is that it is too early to say with any confidence how even the most intensively studied desert shrub communities of the southwestern USA will respond to rising C02, let alone all the other desert areas of the world.
Another interesting observation from the Nevada Desert FACE site is that the non-native invasive grass cheatgrass (Bromus tectorum) responds to C02 such that it is far more productive than native plants during wet years. Cheatgrass invasion of the southwestern US deserts has been found to greatly increase the frequency of fires, from a 75-100 year cycle to a 4-7 year cycle. These fires are also far more intense than those in native vegetation and usually result in a loss of native shrubs. A further change from shrubs to grasses under increased C02 would have a dramatic effect on desert water cycles and wildlife habitat, as well as the suitability of the lands for cattle-ranching.
The results so far from the FACE experiment in Nevada indicate that both desert shrubs and wet-season herbaceous plants such as cheatgrass respond especially strongly to increased C02 during the occasional wet years that correspond to El Nino events. There is more year-to-year variation in growth rate at elevated C02, suggesting that the whole ecosystem may become even more episodic and thus, in this sense, more desert-like in a future high C02 world.
In a study of desert margin species from the semi-desert environment of the Negev Desert (Israel), transplanted into closed chambers, species-rich assemblages of winter annual grasses and herbs showed very little biomass response to doubled C02 but significant changes in tissue quality and species dominance. However, these changes were solely the result of the response of a single species of legume (a member of the pea family) which became much more vigorous and abundant. Had this particular species not been included, overall responses would have been minute. The general lack of response to C02 for most of the desert species in this system was rather unexpected, since C02 fertilization models predict an especially strong effect in arid vegetation.
A FACE experiment on semi-arid Mediterranean-type grassland in California likewise confounded all the expectations of models. Right from its first year at increased C02 levels, to the third year when results were reported, there was no significant enhancement of net primary productivity (growth rate) of the plants. This was true across a whole range of treatments, some of which involved increasing nutrient supply and water supply as well as C02.
8.8.3 Will C4 plants lose out in an increased CO2 world?
It is often expected that plants which use the more water-efficient and C02-efficient C4 photosynthetic system (see Box 8.1) will respond less strongly to raised C02 than plants using the conventional C3 system. Because desert and semi-desert ecosystems contain a high proportion of C4 species, one might expect those species to decrease as a proportion of the vegetation, relative to increased growth of C3 species. Closed-chamber experiments with C4 and C3 species growing in competition have often supported this view. In a chamber experiment with various southwestern US semi-desert species, the C4 grass Bouteloua responded with only about half as much increase in biomass (a 25% increase) as the C3 shrubs creosote and mesquite, which is the sort of response that might be expected. However, the grass also greatly increased its nitrogen content, which might seem to suggest that it was also doing better than would be expected from growth rate alone, despite being a C4 species.
In the semi-arid grasslands of the central US that contain a mixture of C4 and C3 plants, the picture of C02 response is not at all as models predict. When intact pieces of prairie grassland turf containing both C4 and C3 plants were studied in elevated C02 in the greenhouse, the greater response forecast for C3 species was not found and both types responded about equally. A field experiment in open-top chambers on the prairie actually showed the opposite trend: there was no response in the most important C3 grass (Poa spp.) but significant growth stimulation of C4 prairie grasses! Whether such a situation will "carry over'' into other grasslands around the world and into drier environments such as semi-deserts is a moot point, but these results should be considered as a further uncertainty in predicting arid-land vegetation responses to C02.
Box 8.1 C4, C3 and CAM plants
Many plants in arid environments decrease the problem of water loss through stomata by chemical tricks that help them take up C02 with less water loss. These are known as C4 and CAM plants.
Most plants are known as C3 plants. They take C02 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 watersplitting part of photosynthesis gives the hydrogen needed to tack on to carbon. The hydrogen and the carbon are then combined in that same cell, to make sugars. C4 and CAM plants do something a bit different.
The most straightforward alternative is in plants that have the Crassulacean Acid Metabolism or CAM system, including the cactus family Cactaceae. These plants open their stomata at night when it's cool (so evaporation loss is low) and soak up C02 and store it chemically. The photosynthetic cells then release the stored C02 for fixing by photosynthesis during the day. The storage chemical is an organic acid; the plant tissues become acidic during the night as the storage product accumulates, then less so during the day as it's released to yield C02. CAM plants tend to live in the most arid environments. They are all succulents: fleshy leaved or with fleshy stems. In addition to occurring in deserts, CAM is often found in plants growing in salt marshes and on seashores, and this shows how significant drought is for these seaside plants, due to salt in the soil exerting an osmotic effect, preventing water from being taken up by their roots.
C4 is a bit less obvious as a trick. Throughout the day a C4 plant captures C02 in special "C02-fixing" cells on the outer parts of its leaf tissue, and concentrates it into the center of its leaves. The outer C02-fixing cells are also busy photosynthe-sizing, but they are only doing part of the photosynthetic reaction, the part that yields oxygen: 2H20 ^ 4H + 02. The H atoms are stored on special intermediate molecules, ready to help form sugars later on.
At the same time (as I mentioned above) those cells are taking up C02, and fixing it into special carrier molecules (which consist of a four-carbon chain, hence the name C4) that are moved to the innermost part of the leaf where there are other special cells which are also photosynthesising, but using the light energy to combine the stored C02 and stored H into sugar molecules, which is how the plant wants them.
So, in summary, there are three parts to this process in a C4 plant: in the outer photosynthetic cells: (1) H and C02 are taken up and fixed (and oxygen produced). Then (2) the H and C02 are transported and (3) using more sunlight are made into sugars in other photosynthetic cells deeper within the leaf (Figure 8.7).
C3 plants C4 plants
C3 plants C4 plants
sugars, starch sugars, starch
sugars, starch sugars, starch
Figure 8.7. The sequence of reactions in a C4 leaf. In a "normal" C3 plant all these reactions take place in the same cell.
Why does the C4 plant do all this? In a "conventional" C3 plant, something called photorespiration is going on continually in the photosynthesizing cells around the stomata that are also exchanging C02 with the atmosphere. Oxygen gets tangled up in the photosynthetic reaction and "spoils" the molecule that has fixed the carbon, which has to be burnt back to C02 because it can't be used. This spoiling reaction is known as photorespiration. The burning of the useless by-product of photorespiration spits C02 back out within the leaf and many of those C02 molecules are lost again as they leak back out of the stomata. To make up for this lost C02 the conventional plant has to keep its stomata open for longer. This presents problems: in a dry environment, opening stomata is something that the plant needs to avoid doing because it risks dying from drought.
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 "C02-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. 0f 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 tridentata) 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 C02, 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. As with the C4 plants, they can be expected to lose their advantage over C3 plants in at least some situations in nature as C02 levels increase.
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. 0ther similar growth studies suggest a strong response by these very same species of drought-tolerators. In certain experiments on mixtures of desert species growing together, 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 around them barely respond. 0verall, the experiments bolster the general expectation that increased C02 will favor a stronger response of C3 plants over C4 species, but the contradictory results from prairie species add an element of uncertainty to all of this. 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 herbivory.
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 C02 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 are 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 after a while.
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