Watching Forests Take Up Carbon

Because of the present and future importance of forests affecting the time course of the rise in atmospheric C02, there is presently a lot of work going on to understand whether—and how fast—they are taking up carbon in particular parts of the world, and how they respond to climate fluctuation. When modern ecosystem ecology first began in the 1960s, studies of forest growth concentrated on estimating the amount of wood added to all the trees throughout the forest, from the width of tree rings or the increased girth of the trunks. From this it was possible to infer the approximate amount of carbon by which a growing young forest increased its carbon storage each year, or in an old forest in equilibrium that rate at which carbon was flowing through the ecosystem (balanced by death of old trees and loss of branches). This approach has been used to estimate that the eastern USA forests are taking up carbon at the rate of several tens of millions of tonnes per year. However, it is a very broad-brush approach. It would be better if we had more details about exactly how much carbon is being taken up where, as this would enable us to predict better what will happen in the future.

In the 1980s ecologists began to consider a more ambitious and detailed approach to understanding where and how fast forests take up carbon. This relied on taking very comprehensive and precise measurements of the C02 concentration around the trees. The idea is, that if a forest is photosynthesizing and sucking up carbon from the atmosphere around it, this should show up as a localized depletion of C02 in the air just above, around and inside the forest canopy. Using large numbers of well-placed sensors to measure C02 concentration, it is in theory possible to estimate just how much net photosynthesis is going on during the day, and thus how fast the forest is accumulating carbon. Even though the forest ecosystem is also respiring during the day, in daytime there will normally be more photosynthetic uptake of carbon than carbon released from respiration. This estimate has to be balanced against the amount of carbon lost from the forest at night, when there is only respiration and no photosynthesis. Again this night-time assessment can be done using the sensors to measure how much the C02 concentration around the forest has been raised at night relative to the background level in the atmosphere. To make these estimates properly, it is necessary to estimate how fast more C02 is getting delivered (during the day) or taken away (during the night) by air movement. This involves a lot of complex physics and calculation. If the measurements are continued month after month, year after year, then it may be possible to infer how the balance of carbon in a sample patch of forest is changing over time.

This approach, known as the eddy flux covariance method, is compelling but also very ambitious. It requires a huge investment of labor and money to put in place the complex measuring equipment and maintain it, and analyze the data that comes out. At an intuitive level, it is easy to see that if a slight portion of the carbon loss or gain each day was not included in the accounting (e.g., because a sensor missed it) this error would accumulate over many months and might give a totally misleading picture of the direction in which the forest's carbon balance was changing. One big problem that this method has run into is that it is difficult to summarize the amount and direction of air movement over the forest during day and night. At night, especially, air movement from the forest canopy is very sensitive to local conditions and it may stay stable (stratify), or become turbulent carrying C02 away from the sensors and giving the impression that there is less respiration than is actually the case.

Perhaps because of these problems, eddy llux covariance has sometimes given strange results that do not seem to tally with previous knowledge of ecosystem processes built up during the 1960s and 1970s. A study site in pristine Amazon rainforest seemed to be inexplicably gaining carbon so rapidly that it was set to double in carbon mass within 60 years. In Europe, forests and their soils in the north seemed to be accumulating carbon more slowly than those in the south, not because trees were growing slower in the cooler climate but because the northerly soils were breaking down carbon more rapidly. This seemed to contradict decades of knowledge about how soil respiration responds to temperature. In certain other areas such as the eastern USA and Japan, the measurements seem to make much more sense in terms of previous knowledge of how climate and forest age affect carbon balance. However, there is the nagging question of whether these studies too might contain errors which are too small to clash with previous understanding of ecosystems, but scientifically important nevertheless.

Some ecologists have pointed out that, in addition to the measurement problems, the fact that important processes which affect forests such as tree falls, land slides, droughts and fires occur in an occasional and unpredictable way means that intensive measurements of small patches of forests may not give a particularly relevant picture of long-term trends on a broad scalc. which is the sort of question these studies are basically attempting to answer.

While the eddy flux covariance method is a considerable achievement of ensin-

* w eering and scientific collaboration, it remains an open question as to how much it can really teach us, compared with more old-fashioned methods of looking at the carbon balance of ecosystems.

7.9.1 Predicting changes in global carbon balance under global warming

From the year-to-year variability in the amount by which C02 builds up in the atmosphere, it looks as if the amount of C02 taken up or released by the world's vegetation responds quite a lot to changes in the climate from one year to the next. Such small responses to year-to-year climate variation might give us clues to longer term trends that will emerge as the global climate warms due to the greenhouse effect.

Since C02 responds to climate, and climate responds to C02, there is the potential here for some important feedbacks. It could be that as the world gets warmer it will favor more carbon being stored in vegetation and soils, slowing the warming by-taking C02 out of the atmosphere. This would be a negative feedback loop, tending to act against the main cause of the warming. On the other hand, in a warmer world, vegetation and soils might actually respond by releasing C02, adding further to the warming in a positive feedback loop.

Given what we know of the responses of forest carbon balance to year-to-year climate fluctuation, the effect that global warming might have on CO: uptake or release by forests is complex. It depends on the particular region, and the detailed nature of the climate shift: how big it is, and whether rainfall changes as well as temperature. The whole task of predicting what will happen tests the limits of understanding of both climate and the global carbon cycle. Attempting to model the whole system over the coming centuries requires inter-disciplinary teams of experts using some of the fastest computers available. One study by Peter Cox and his colleagues based at the lladlcy Center in the UK predicted that, as the world warms, carbon will gush out of the world's ecosystems into the atmosphere, amplifying the warming in a positive feedback (Figure 7.18). In the model, a large part of the positive feedback occurs due to more frequent and more severe El Niño events affecting the carbon balance in the tropical forests of South America, South-East Asia and Africa. Drying

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