Temperature Ani C02 ResponsewS Interacting
Over the next century or so, C02 itself will not be the only environmental factor changing. Climate is likely to become substantially warmer in most parts of the world, partly because of the greenhouse effect of the C02 itself. So, to know how-vegetation will really change, it may be helpful to try altering both factors simultaneously and see how they interact.
There have been a few short-term (one or two year) open-top chamber experiments in the temperate zone which artificially warmed plants growing in natural settings, while they were being fertilized by C02. Usually, this was done with either under-soil heating, or with infra-red lamps. For example, a study carried out in Tennessee by Rich Norby and colleagues studied young red maples (Acer rubrum) under this combination of conditions. The results were compared with controls at normal temperatures and C02. or under either only increased C02 or only increased temperatures. What studies such as this have tended to find is that there can be an additive effect in terms of growth rate of increasing both temperature and C02: the warming increases growth somewhat, and C02 also increases growth some more. Some studies seem to show a synergistic effect: the acceleration of growth under both C02 and warmer temperatures is greater than the sum of the two separately. This might be. for example, because the benefit of high C02 in reducing photorespiration is greater at higher temperatures, so there is more to gain. In the warmer greenhouse world, this might then mean that biota will respond strongly to C02.
8.8 A FEW EXAMPLES OF WHAT IS FOUND IN FACE EXPERIMENTS
FACE studies (above) have now been carried out on a fairly diverse array of ccosystcms. Rather than try to give a full account of the results from every one in turn, 1 will focus mostly on two very different types of ecosystems—forests and arid lands—to give some idea of the sorts of complexities that are found in the results.
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. 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 line roots that the rate of primary production at the site has stayed about constant; it has simply shifted underground. Also, 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.
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 Carolin. 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.
Because the FACE experiments have not yet been run for several decades, we do not know whether the raised C02 response will producc any cffccts that only emerge as the trees grow bigger. Only 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 CO: levels that were cut at the same time. IIowever, because they got a head start during its first few years of rcgrowth from stumps, the high C02 trees arc 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.
8.8.2 Semi-desert and dry grassland vegetation
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 semi-desert regions of 50 70% if the CO-, 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 desert productivity? 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. This experiment increased the C02 concentration by 52% above the "background" level across the desert. In some ways the initial response of the C02-fcrtilizcd plots (compared with the controls at normal C02 levels) was dramatic, much as the models would predict. There was an 80-100% increase in photosynthesis, and water expenditure 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!
One closed-chamber experiment found an increase in seedling survival rates 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 another 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 lend 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. One 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 CO,, let alone all the other desert areas o I the world.
Another interesting observation from the Nevada 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 Niño events. There is greater 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 Negcv 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 spccies 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 CO? 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.
8.8.3 Will C4 plants lose out in an increased CO? world?
It is often expcctcd that plants which use the more water-efficient and C02-cfficicnt C4 photosynthetic system (sec Box 8.1) will respond less strongly to raised CO: than plants using the conventional C3 system. Because desert and semi-desert ccosystcms contain a high proportion of C4 species, one might expect those species 10 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 CO? response is not at all as models predict. When intact picces of prairie grassland turf containing both C4 and C3 plants were studied in elevated C02 in the greenhouse, the greater response forecast for C3 specics 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 (Poo spp.) but significant grow th 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 CO?.
Continue reading here: Box 81 Q C3 and CAM plants
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