Most models for predicting the broad-scale effects of increased C02 on vegetation tend to be based on some basic components of the photosynthetic process (from the Farquhar model, above): how fast C02 can diffuse into the leaf at higher concentrations, and how fast the photosynthetic chemistry in the cells of the leaf can potentially take it up at higher concentrations. Some models take a more empirical approach, seeing just how much faster on average plants tend to grow in closed-chamber experiments with raised C02, and then taking a C02 fertilization factor for the increase in growth rate, known as "beta".
What sort of things do these models predict for the coming decades and centuries? Essentially, if everything else in the world stays the same except for C02 increasing, the models all agree on two things. First, there will be an increase in net primary productivity (the growth rate of plants) all around the world as the supply of carbon for photosynthesis increases. Second, plants will be using the water supply around their roots more efficiently because they do not have to open the stomata in their leaves quite as much. This will be rather like an increase in rainfall as far as the plants are concerned.
Different models predict different degrees of response to any given increase in C02, depending on subtle details in the assumptions that they are based upon. A review of models over the past ten years or so showed that on average for a world with 580 ppm C02 (which will probably occur around 2050 given the current rate of increase in C02) compared with a baseline of 350 ppm, the models predict a 22% increase in plant productivity around the world. However, the range of estimates amongst them extends from 10 to 33%. The higher C02 levels go, the higher the effect on plant growth is predicted to be. In a recent review, the well-known vegetation-climate modeler Gordon Bonan suggests that above 700 ppm of C02, the increase in the world's plant productivity will be somewhere between 12% and 76%, averaging at 48%. If this is correct—and remember this forecast is only based on models—it would represent a transformation of the energy flow through the world's land ecosystems. And, of course, given that they are only models, if they have overlooked or misjudged some important factor, they might all be wrong.
If plants increase their productivity and also make better use of the water available to them, we can expect that there will be some changes in the structure of vegetation around the world. The distributions of biomes are often determined by water availability, and an increase in C02 that allows plants to use water more effectively should allow wetter-climate biomes to spread. For example, models which combine a biome vegetation scheme (Chapter 2 in this book) with a C02 fertilization model usually predict that there will, for example, be an increase in tropical rainforest, allowing it to spread out into zones which get less rainfall and currently have dry forest or savanna vegetation. In arid areas, plants that are able to get by on less water because of higher C02 will be able to spread—the deserts will become greener. There is also predicted to be a general shift around the world towards C3 plants (see Box 8.1, p. 238), and away from C4 plants which do not benefit so much from increased C02. The speed with which these changes in vegetation actually occur depends on many different factors. Even though tropical rainforest might be capable of spreading into savanna regions, it will likely take hundreds or even thousands of years for the forest trees to disperse out and grow up into dense forest in these new areas. In the meantime, more subtle shifts in the structure and composition of vegetation are also likely to occur as some of the plants that were already in place grow bigger, and shade out other species around them.
Was this article helpful?