Long-term experiments under realistic conditions are beginning to deliver data on how forests — or at least some forests — will react to increasing levels of CO2 in the atmosphere.
The global experiment of increasing atmospheric CO2 concentrations by burning fossil fuels has neither a control nor replicates. So it is difficult to quantify how much faster the world's forests might be growing under high CO2 conditions. Higher levels of CO2 can clearly make plants grow better. But will Earth's vegetation absorb from the atmosphere, and retain, much of the CO2 pouring out of our exhaust pipes and smoke stacks? If it does, then the threat of global warming from increasing CO2 would be less severe. Current estimates of annual terrestrial plant uptake of carbon attributable to 'CO2 fertilization' are within the range 0.5–2.0 × 1015 g, which is about 8–33% of annual fossil-fuel emissions1. What are needed of course, are controlled, replicated experiments under realistic field conditions to help reduce the uncertainty of these figures.
Papers in this issue by Oren et al.2 and Schlesinger and Lichter3 describe just that — well-designed studies to test the effects of CO2 fertilization, in these cases at the scale of a forest stand in the United States. The data show only modest carbon storage in the soil and leaf-litter pools, and also provide preliminary evidence that shortage of nutrients and water may limit the long-term response of trees to increased levels of CO2.
Well-watered and fertilized citrus trees clearly grow better when exposed to high CO2 (ref. 4). But the effect on forest trees is uncertain because most forests have limited supplies of other resources needed for growth. Equally important is how trees allocate their photosynthetic output. As described in Box 1, if the increased carbohydrate is used to produce more wood, then that carbon is likely to reside in the forest for many decades. But if it goes instead to leaves, which die and decompose within a few years, the carbon will quickly be released back to the atmosphere. Allocation of carbon to roots and associated fungi could create either a short- or long-term sink, depending on whether the carbon is stabilized in the soil or is rapidly decomposed to CO2 after the roots die5.
Both papers2,3 describe results from the 'Free air CO2 enrichment' (FACE) experiment at the Duke forest of Duke University, North Carolina. The experiment began with the construction of a prototype 700-m2 ring-shaped fumigation structure in a young loblolly pine plantation in 1993. This was followed by construction of three treatment and three control rings in 1996. The treatment rings are fumigated with high-CO2 air to maintain concentrations 200 μl l−1 above ambient; control plots are fumigated with ambient air (CO2 concentration 365 μl l−1). Previous CO2 enrichment experiments used chambers containing at most a few saplings, but each of the Duke FACE rings contains several dozen trees that are now about 15 m tall.
Oren et al.2 (page 469) report enhanced wood growth in the prototype FACE ring during the first three years of CO2 fumigation, but then a return to pre-fumigation growth rates. To see why increased growth stopped, they fertilized half of the ring with nitrogen while maintaining the high-CO2 treatment. Trees on the nitrogen-fertilized half resumed increased growth, while those on the other half did not. Outside the FACE ring, nitrogen fertilization also increased tree growth relative to controls, but the combination of nitrogen and CO2 fertilization caused a larger response than either treatment alone. This synergistic nitrogen– CO2 response was greater in a rainy year than a drought year, although possible time lags between drought and its effect on wood growth complicate this interpretation. These results are the clearest evidence yet that nutrients, and perhaps water, may constrain the trees' response to CO2 fertilization in real forests.
Meanwhile, Schlesinger and Lichter3 (page 466) report on the fate of the leaf and root litter in the replicated FACE rings. The amount of forest-floor carbon has increased significantly. But given the rapid turnover of this pool (mean residence time is only about three years), the authors calculate that a new steady state will soon be established with only a slightly higher mass of litter. They also found no significant increases in mineral soil carbon attributable to higher concentrations of CO2, supporting other evidence that increased soil CO2 production is balancing larger carbon input into the soil6.
If the current trends7 persist in the replicated FACE rings, an extra 4.3 kg C m−2 would be sequestered after 40 years because of CO2 fertilization (Box 1), almost entirely in wood. But if nutrients limit increased wood growth to only three years, as apparently happened in the prototype FACE ring, then this extra carbon sequestration will cease in at most ten years, at a cumulative sum of less than 1 kg C m−2.
Human activities are also increasing the deposition of nitrogen, a component of 'acid rain', and this might lead to the synergism between nitrogen and CO2 fertilization reported by Oren et al.2. But this response is also complex. Soils immobilize most of the nitrogen inputs8, and large amounts of extra nitrogen can create nutrient imbalances and soil acidification that reduce plant growth9. There was little evidence of either nitrogen or CO2 fertilization in an analysis10 of the long-term forest inventory records of the US Forest Service, with nearly all of the forest growth being attributed to tree recovery from previous forest harvesting or following cessation of agriculture. Inventory analyses address larger spatial and temporal scales than experiments such as FACE, and both approaches will be needed to disentangle the complex interactions affecting carbon storage.
Obviously, this fast-growing pine plantation in North Carolina does not represent all forests of the world. Other species grow and allocate carbon at different rates, and soils with different mineralogy and texture, and under different climatic conditions, might sequester more or less carbon11 than reported here2,3. Nevertheless, we now have some hard data from well-designed experiments in a realistic forest setting. Information such as this will be essential in using models to make global extrapolations of the effects of increasing atmospheric CO2 on the carbon cycle as a whole.