What sort of effects might increasing C02 have on broader community structure? How will the animals and the fungi that feed off living plants respond to changes in the growth and composition of plants that are C02-fertilized?
Most of the work on how increased C02 can affect such interactions has focused on crop plant systems, although the findings might also apply to more natural communities. Several studies of herbivory on C02-fertilized crops have suggested there might be an increase in insect or fungal attack on plants at higher C02, which will "take back'' part of the gain from C02 fertilization. Some work has shown that at raised C02 levels insects increase their rate of feeding, perhaps because the leaves have a lower protein content when they are C02-fertilized. The insect simply has to eat more leaf in order to get the protein it needs to grow. It has been suggested that this means that in the future high-C02 world, insects will cause more damage.
However, it is important to bear in mind that the plants themselves are generally bigger when they are C02-fertilized, and the extra amount lost to hungry insects in these experiments often works out to be less as a percentage of the total leaf area. Also, insects which have to eat more leaf material to extract enough protein are generally placed in a difficult situation: it takes a lot of work for the insect to digest the extra material, and the insect may also have to take in extra amounts of poisons the host plant produces in the process of consuming more leaf. The insect may also have to spend more time feeding out on the leaf exposed to enemies when it cannot get enough protein. It seems that insects on C02-fertilized plants not only consume a smaller proportion of leaf tissue, they grow more slowly and die more often. In fact, the evidence is that overall with C02 fertilization the advantage is tipped in favor of the plant, against the insect. However, not all the experiments have shown this, so there is a definite need for more studies on direct C02 effects on herbivore activity.
Most species in the world are herbivorous insects, and it is rather frightening to consider what effects any large change in insect-plant interactions might have on insect biodiversity in the tropics and elsewhere. It is quite possible that a large change in nutrient content will push many insect herbivore species over the edge into extinction. It is widely considered by ecologists that a large part of the reason so many species of tropical trees can coexist in the tropical rainforests is that selective insect herbivores prevent each tree species from becoming too abundant. If we start to see these specialized herbivores dropping out of existence because of a direct C02 effect, many tropical trees may go extinct because the most competitive species among them are no longer so closely density-limited and can now push the others out.
Even though plants may benefit from C02 fertilization, humans who also want to eat them may suffer from some of the same problems as insects do. Experiments on wheat and rice suggest that with C02 fertilization their grain contains proportionally more starch and less protein than when they are grown at background C02 levels. This may mean poorer nutrition for human populations in some parts of the world where protein intake is already very limited. Analogous problems might also come up for mammalian herbivores that feed off wild plant materials. If the decline in nutrient content is severe enough, some may go extinct.
8.13.1 Looking for signs of a CO2 fertilization effect in agriculture
It is always good to back up models and experiments with unfettered observations, showing that what we expect to be happening is actually happening. How about agricultural systems, which we can expect to respond particularly strongly to increasing C02? If there is indeed going to be a strong future response of crops to increased C02, we might expect that the 40% increase in C02 that has occurred over the last
250 years has already had some effect on yields. Is there any direct evidence for this? It is certainly true that there has been a massive increase in crop productivity over that time period. Even in the last several decades in the USA, yields have gone up by 50-100% in many areas. 0ne might take this as indicating that the direct C02 effect is at work here. However, such a conclusion would be far too simplistic. 0ver time, many different factors have changed agricultural yields, including crop-breeding, fertilizer use and pesticide use. Because so many of these other things have changed too, it is basically impossible to "extract" the trend of increasing yields from C02 in order to "test" or "prove" models of C02 fertilization. It is a reasonable guess that the direct C02 fertilization effect is in there somewhere, but we really cannot be sure how large it is.
Crop plants are not the only components of agricultural ecosystems. Weeds are always present too, and they too can be expected to be benefiting from increased C02. Some interesting chamber experiments with growing common temperate field weeds in pre-industrial (280 ppm) C02 levels showed that they grew 8% slower compared with present-day (around 350 ppm) C02 levels. If the weeds in fields are growing faster under increasing C02 levels, they can be expected to "take back'' some of the gain from increased C02 that would otherwise go into more vigorous growth of the crop plants.
8.13.2 Looking for signs of a C02 fertilization effect in natural plant communities
Likewise, if there is an increasing C02 fertilization effect in natural and semi-natural vegetation, we might expect to see signs of it already. After all, C02 has been increasing for many decades now, so if we have good records of how the vegetation was 100 or 200 years ago, we should be able to compare the "before" and "after".
Are there any inexplicable increases in tree growth, for example? Tree rings go back hundreds of years in old trees, so we can compare growth rates now by looking at ring widths: more rapid growth should produce wider rings. In northern and central Europe there has been an increase in tree ring widths—allowing for the growth stage of the tree—over the past couple of hundred years, which shows that trees are growing faster. This is consistent with what might be expected from a C02 fertilization effect, but it could equally be due to other factors. Climate has warmed substantially over the last two centuries throughout Europe, perhaps due to natural climate fluctuation and perhaps due to an increasing greenhouse effect. Trees may well be growing faster in response to the warmer climate. Even though climate-warming due to an increase in the greenhouse effect is mostly due to C02, this is not the same mechanism as direct C02 fertilization that we are talking about here. Another possible explanation for the increase in tree growth is a different form of pollution, from the nitrogen and sulfur oxides produced by power plants, factories and car engines. Although these acidic gases are usually thought of as destroying ecosystems, in small quantities they may act as fertilizers. Many forest soils are very low in nitrogen and sulfur, and experiments suggest that adding traces of sulfate and nitrate salts often promotes tree growth.
There is not even a noticeable positive trend in tree growth in most other parts of the mid-latitudes. For example, a trend towards faster tree growth is not seen in southern Europe. There, growth actually seems to be slowing down despite the ongoing increase in atmospheric C02. My friend Gianluca Piovesan of the University of Tuscia has implicated a drying of climate as the main factor behind this trend. In the temperate deciduous forests of the northern USA, Pacala and colleagues have looked for the anomalous increase in ring widths of trees that might indicate a C02 fertilization effect over the past century, and found none. In fact, they actually found a decline in growth rate (adjusted for the age of the trees), that might be due to too much air pollution with nitric and sulfuric acids or ozone.
In a wide-ranging tree ring study of high-latitude conifer forests, Schweingruber and colleagues reported that there is no sign of a response of the boreal forest to atmospheric C02 growth. In certain parts of the boreal zone, tree ring widths have actually decreased over the past century or so. Northern Siberia is one such region where trees seem to be growing more slowly. This is not at all what we would expect from C02 fertilization, and is probably due to a change in climate towards drier conditions.
How about the tropics? In the early 1990s, 0liver Phillips and Alwyn Gentry looked at inventories of the girth of tropical trees taken by generations of foresters, and announced that they had found a clear trend of increasing growth rates throughout the tropics during the previous 60 years. This caused a ripple of excitement through the world of ecology; surely here at last was clear, systematically gathered evidence of a C02 fertilization effect. Because it occurred all across the world's tropical forest regions, it was unlikely that any regional climate effect was the cause. Such a widespread trend seemed to leave only C02 as the driving factor. However, the trend turned out to be an example of what can go wrong if one does not meticulously check one's sources of data. Someone pointed out that the frequency with which forest managers returned to each tree to check its girth increment had changed over the decades. Nowadays, they were measuring girth less often and this meant that each tree had put on more growth between the measurements. Philips and Gentry had assumed that the interval between measurements had stayed the same throughout, so naturally they found what looked like an increase in tree growth rate. Another "false alert'' occurred when a carbon balance study of old-growth tropical forest in the Amazon Basin—using the eddy flux covariance method mentioned in Chapter 7— suggested that the forest was putting on a remarkable burst of growth and was set to double its biomass in another 60 years. This was initially suggested as being a response to direct C02 fertilization. This trend turned out to be the result of some combination of problems in the use of the equipment, plus short-term variability in forest processes.
Some years later, there was similar widespread interest when data gathered across the Amazon Basin suggested an ongoing trend in forest turnover—the rate of birth, growth and death of trees—and an increase in the abundance of vines over the last few decades. Most recently, a similar trend has been found in the rainforests of central Africa. In both regions, the forests are also apparently increasing in carbon storage as the trees grow larger, taking up some of the C02 that is added to the atmosphere each year by human activities (and agreeing more with those new studies of C02 concentration in the atmosphere over the tropics—see Chapter 7—that suggest they are sucking up much of the C02 added by human activities). Philips and other authors have suggested this might be evidence of a direct C02 effect on the growth of the forests. Whether this is a genuine C02 fertilization effect is a matter of dispute, but the increase in vine growth does seem to agree with the expectations of closed-chamber C02 fertilization experiments. The trend towards faster turnover and increased carbon accumulation is not occurring on every single sample plot: some of the plots in Africa and the Amazon Basin, plus other forest plots in Central America and Peninsular Malaysia, seem to have been slowing their turnover during the same time. And as Christian Koerner has cautioned (Koerner, 2004) a recent increase in carbon uptake and tree turnover does not necessarily mean that the tropical forest will end up with more carbon overall. For all we know, eventually the new faster growing trees might just fall over and rot, releasing much of the extra carbon they had taken up.
I suspect that a lot of the trend that is seen towards increased carbon storage in the forests of Amazonia and Africa is a result of them recovering from pervasive selective logging by local people over the past centuries and especially the last few decades, rather than a real C02 effect. Sandra Brown and her colleagues have found evidence of such a past timber extraction of really big trees by comparing relatively accessible and more inaccessible areas of tropical forest: the more accessible places seem depleted in big trees. There is also a long history of shifting cultivation—now largely abandoned—in many of these forests, that would have involved clearing patches of forest which would then have to slowly recover after being abandoned. Among other possible reasons for the turnover trend in the Amazon is an increase in cloud cover in recent decades, which might be expected to allow faster growth in trees that are exposed to less roasting in direct sunshine. In my opinion, as things stand now, there is still no overwhelming evidence of any direct C02 fertilization response in tropical rainforest.
Many firmly established changes in vegetation are being documented in the coldest regions of the world. For example, in the Canadian Arctic islands and in northern Alaska, shrub cover has expanded over the last several decades. 0n mountains in many parts of the world, the treeline is creeping upwards (Chapter 3). These trends are generally attributed to global warming produced by C02 and other greenhouse gases, but it is also possible that the direct C02 effect is playing a significant role in promoting the growth of plants. Unfortunately, it is presently impossible to disentangle these two factors, and ecologists rely on their gut instincts (plus the results of experiments in Arctic ecosystems showing that C02 fertilization effects are often short-lived and minor) when they attribute these changes to climate rather than C02 fertilization.
0ne indirect effect of increasing C02 could be a decrease in the amount of evaporation through stomata, which are expected to stay closed more of the time when the plants get enough C02 for their needs. This could have indirect effects on other phenomena in the environment; for example, the amount of water running off land surfaces. With less evaporation, due to C02 effects on stomata, a greater proportion of the rain that falls on land surfaces should simply run off. Gedney and colleagues have pointed out that indeed there has been a widespread trend around the world towards increased river runoff in the last few decades, and they suggest that this is best explained as a result of the direct C02 effect. These authors find that the pattern and amount of the increase in runoff cannot simply be explained as a result of factors like increased rainfall, or deforestation, leaving it to be best explained as a result of C02. However, other authors who work on water runoff have cautioned that this approach is rather simplistic, and that many subtle interacting factors could explain such trends. I also wonder why, if extra C02 is letting plants get by with less water, the vegetation doesn't simply get lusher and taller as a result of its improved water supply—and then end up evaporating just as much water as before until the available supply is used up. At least that seems to be how vegetation behaves on a global scale, its leaf area strongly limited by the water available to the vegetation (Chapter 2). 0verall, the intruiging trend towards more runoff is worth considering as possible evidence for a direct C02 effect showing up in nature, even if we should treat the idea with a certain amount of caution, given the complexity of the systems we are dealing with.
So far, then, there does not seem to be anything in terms of natural vegetation change that is unambiguously a sign of C02 fertilization. In such a complex world in which vegetation is buffeted by many different factors, almost anything we see that looks like a direct C02 effect could in fact be due to a multiplicity of other causes. Perhaps, as C02 levels continue to climb, more striking changes in vegetation that can only be attributed to this effect will begin to show up. Even so, it is very likely that climate will also be warming in parallel with C02, always leaving open the possibility that any given change in vegetation will be due to temperature increase, not C02 effects on physiology.
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