Modeling Direct C02 Effects

How can we predict how plants around the world will respond to raised C02 levels? Plant physiologists and global ecosystem modelers have put a great deal of emphasis on short-term observations of the effects of increased C02 on photosynthesis. They have also tended to make a lot of use of a set of principles together known as the "Farquahar Model", put together by Graham Farquahar at the Australian National University. This model reduces the complex process of assimilation of C02 into the plant to certain simple key components that act as bottlenecks (Figure 8.1): first, the C02 diffuses into the leaf through a stomatal pore and basic gas physics shows how the rate depends of the concentration of C02. Then, the C02 gets incorporated by an enzyme (known as rubisco) into organic form in the cell, at a rate that can be predicted pretty much exactly from the way a mix of the relevant components in a beaker would behave. So, C02 fertilization of photosynthesis comes down simply to gas diffusion, and then a chemical reaction. And, hey presto, here is a universal model to predict C02 fertilization effects on plants. This model has been taken up with great enthusiasm by some global modelers who have essentially used it as the core of a series of other more ambitious models, extrapolating the future effects of C02 on all the world's vegetation for the next several hundred years.

We will examine some of these predictions in more detail later on, but it is important to be aware that many ecolo-gists are intuitively skeptical of any broad-scale model built upwards from biochemical principles. Ecologists are constantly reminded of the sheer com

Figure 8.1. Key steps in photosynthesis which are altered by C02 concentrations.

(a) Diffusion of C02 into the leaf.

(b) Uptake of C02 by rubisco enzyme into carbon-containing molecules.

Figure 8.1. Key steps in photosynthesis which are altered by C02 concentrations.

(a) Diffusion of C02 into the leaf.

(b) Uptake of C02 by rubisco enzyme into carbon-containing molecules.

plexity of nature, and its tendency to do the opposite of whatever is expected. Considering the often frustrating experiences that ecologists have in working with the natural world, their skepticism about a model built up from molecules is understandable. In fact, it is now looking rather like their intuition is well founded: experiments which have been set up to provide a more direct indication of how plant communities will respond to increased C02 have shown some very complex and often unexpected results. The question of C02 fertilization is a classic example of the problems of scaling up in ecological systems. How things work at the smallest scale and in the short term in either a test tube or a leaf does not necessarily indicate what will happen to the whole plant over weeks or months. And what happens to the whole plant does not necessarily predict what will happen to a community of species over several years. Furthermore, what happens in a few years to the community does not necessarily indicate what will happen to the entire ecosystem over decades or centuries.

In this sense, modeling of processes that get right inside the plants, into their core metabolism and growth, is much more difficult than modeling the purely physical processes of heat exchange, turbulence and evaporation by which plants affect climate. In climate modeling, extrapolating up from the most local level (even from the individual leaf) tends to be a fairly good basis for understanding how vegetation interacts with the climate system on even the broadest scale (Chapters 5 and 6). We can tell that it can be done, from the various instances where vegetation-climate models have been tested against real changes in vegetation and found to predict climate changes more or less correctly. Modeling how living processes will be affected by C02 is much, much more difficult.

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