A leaf studied in high C02 concentration over a few minutes is not necessarily at all representative of nature. This statement might seem obvious, but modelers have not always been prepared to acknowledge it! Because there are many factors that could potentially change plants' responsiveness to C02, a good way to get a firmer idea of how wild or crop plants will behave is to do experiments on whole plants grown over weeks, months or years. For about 20 years now, plant biologists have been
experimentally raising C02 levels, growing plants in small-scale systems to see what effect future increases in C02 might have. While they are no more than isolated snapshots of the future world, these experiments at least have the advantage that they are based on actual whole plants, often growing under at least plausibly complex combinations of influences.
The earliest and simplest experiments were in closed chambers with plants growing either in compost or natural soil, adding C02 to the air beyond the atmospheric concentration (Figure 8.2a). This would be compared with a "control" chamber where air with the C02 concentration of normal outside air (ambient air) was piped in. The trouble with these closed chambers was that they always seemed rather artificial. There was not the exchange with the outside world that might allow insects, herbivores and fungi to move in and out: the plants were effectively living in a "sterile" environment. And you couldn't grow big trees in these chambers, only small plants.
To help deal with these limitations, more sophisticated experiments were developed to look at the effects of raised C02. 0pen-top chambers (Figure 8.2b) of translucent plastic were used out in fields and natural vegetation; C02 was piped in to make a "double-C02" atmosphere. Because C02 continually leaked out of the top, you had to use more C02 than in a closed chamber, making them rather more expensive to maintain (the cost of all the pure C02, maintained over months or years, adds up to quite a lot). The trouble with these open-topped chambers has been that they tend to be warmer inside, which complicates the conclusions (in an experiment it is always best to start by holding all things constant except for the one factor you are studying the effects of). Nevertheless, if their internal temperature is controlled with some sort of cooling system, such chambers do not necessarily have to be very different from the outside. Another problem with open-top chambers is that herbivores like deer cannot get in to nibble the plants, making the situation a bit artificial in ecological terms; although at many of the more ambitious C02 experiments mentioned below, grazing may also be very limited simply because of low densities of large herbivores in the areas studied. For studying how trees respond to increased C02, they have also been limited to seedlings or very young trees, because it is so difficult to build a chamber for a big tree.
The latest generation of raised C02 experiments uses open air release of C02 in fields or natural vegetation—an apparently more "realistic" situation which does not involve artificial enclosures (Figure 8.2c). These are called "Free Air C02 Experiments" or FACE. Some of these FACE set-ups are very large-scale, involving areas of forest. 0ther FACE experiments on desert scrub or agricultural grassland are much smaller, scaled down to correspond to the smaller size of the plants. Generally, the FACE experiments use a ring of towers that reach just above the height of the local vegetation, and release C02 at various points along their height, at rates carefully calculated to produce an atmosphere with double the normal amount of C02 (Figures 8.3, 8.4). Each individual tower only releases C02 part of the time, when the wind is blowing past it towards the plants within the ring. When the wind is blowing in the other direction it switches off, to avoid wasting C02. Nevertheless, a lot of C02 must be thrown around in such an experiment, much more than in open-topped chambers, and this adds greatly to the costs. Regular deliveries of tankers loaded with pure liquid C02 are necessary to keep the supply up. Because the experimental equipment and running costs for simulating future C02 concentrations are so high, few countries outside the USA, Europe and Japan have conducted any such work.
The classic FACE set-up of a ring of C02 release towers does not seem to be suitable for all vegetation types. Mature forests of heavy trees with thick branches do not tend to be very amenable to C02 release from towers, because the trees themselves block the movement of the gas and create too much turbulence that mixes the C02-enriched air in with normal air, in rather unpredictable ways. For this reason young forests of thin, straight trees have normally been studied using the FACE system. The only exception is an experiment by Christian Koerner and colleagues in Switzerland, on a mature mixed oak forest (Figures 8.5, 8.6*). They used a branching series of pipes that released C02 into the crowns of the trees in precisely calculated ways, simulating a uniformly C02-enriched atmosphere.
In addition, there is one interesting example of a "natural experiment" in the form of a local C02-rich atmosphere near some hot springs in central Italy. The springs release carbon dioxide from within the earth, much of it derived from heating of ancient carbonate rocks. The growth of nearby clumps of evergreen oaks that are exposed to C02 levels two or three times the background level is compared with other clumps further away that experience normal background levels of C02. These observations have continued for more than 35 years, much longer than any of the FACE experiments.
* See also color section.
8.6.1 The sort of results that are found in CO2 enrichment experiments
Looking at whole plants rather than just isolated leaves, we find that the actual growth response is generally weaker with whole plants. The strongest initial responses to raised C02 in terms of growth rate are usually found in well-fertilized crop plants with the "normal" C3 photosynthetic metabolism (see Box 8.1, p. 238). C4 plants— like corn and sugar cane—tend to show only a weak response to C02 levels, and even this only when water is in especially short supply. When they are kept well-watered, the response is negligible.
In "wild" C3 plants grown at "natural" low soil nutrient levels, there is often at least a temporary increase in plant growth by about 30-100% as C02 is doubled from the present concentration. C02 fertilization initially has more or less the same effect
Figure 8.6. Scientists at the Swiss FACE site inspect the forest canopy for direct C02 effects using a crane. Source: Christian Koerner.
as watering more in most ecosystems, since watering also enables greater carbon uptake through the opened stomata.
The wide range of open-topped chamber C02 fertilization experiments in ecosystems around the world suggest a broad range of responses, varying widely from one species and locality to another. When several slightly different treatments are used on the same species, it is often found that the growth response of the plants varies according to precise nutrient conditions in the soils. For example, take a look at the range of responses in tropical forests. Experiments with tropical tree seedlings—grown to two meters in height under increased C02 (around 550 ppm)— showed no response when grown out in full sunlight in the normal tropical forest soil. Yet there was a dramatic response in growth rate when nutrients were added to the soil (Koerner, 2004). What this suggests, then, is that in tropical forest there is not going to be much effect of increasing C02 because nutrient shortage prevents the plants growing any faster. 0n the other hand, when tropical rainforest seedlings grown in shade are fertilized with C02, there can sometimes be a quite dramatic response. Tree seedlings in the shade of the forest floor grew up to 50% faster over more than a year, an interesting and important finding because survival in the seedling stage is key to determining which tree species make it to the canopy (Koerner, 2004). Tropical forest vines seem to respond much more strongly than forest trees to C02, sometimes doubling their growth rate, which might explain why 0liver Philips and colleagues are noting an increase in the abundance of vines in tropical forest plots across Amazonia during the last few decades (although, strangely, at levels above 550 ppm of C02 the vines seem to grow slower again— something hard to explain). The fast-growing "secondary" forest trees, which normally grow in high-light conditions in disturbed areas of forest, also show a stronger response to C02 than the canopy trees of older undisturbed forest—unsurprisingly perhaps because they are normally growing in more nutrient-rich soils when they are fed additional C02.
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