Figure 8.2. The three types of increased C02 experiment: (a) closed-chamber, (b) open-top chamber, (c) free air release.

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 arc called "Free Air C0: Experiments*' or FACE. Some of these FACE set-ups are very large-scale, involving areas of forest. Other 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.

Figure 8.3. The Tennesec FACE site showing the towers used to release CO> into the forest Source: Rich Norby/ORNL FACE.

The classic FACE set-up of a ring of CO2 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-enrichcd 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.

* See also color section.

Figure 8.4. Aerial view of the Tennessee FACE experiment showing rings of towers (see arrows). Source: Rich Norby/ORNL FACE.

Figure 8.4. Aerial view of the Tennessee FACE experiment showing rings of towers (see arrows). Source: Rich Norby/ORNL FACE.

Gas control C02 ♦ 13C tracer

Figure 8.5. The Swiss FACE site on mature mixed temperate forest uses a network of tubes twisting up the branches to deliver C02 in the right places and right quantities to simulate a higher C()2 atmosphere. Source: Christian Koerner.

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 as watering more in most ecosystems, since watering also enables greater carbon uptake through the opened stomata.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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