There Are Other Effects Of Enhanced C02 On Plants Apart From Growth Rate

Plants grown at higher C02 levels generally have a higher carbon-to-nitrogen ratio. It seems that because they have more carbon to work with, they end up producing more of the carbon-containing structural molecules of cell walls, such as cellulose and lignin. They may also store up excess carbon as starch reserves inside their cells. It is uncertain what implications these changes might have for ecosystem functioning; for example, they might decrease the suitability of the plant as food for herbivores, and the decomposition rate of dead plant tissues when they end up in the soil. There are also concerns that crops might bccomc less nutritious since they have less protein; people could simply fill up on starch.

Plants grown at high C02 also tend to have a greater mass of roots relative to shoots. Alternatively, the rate of growth and turnover of the small roots that gather nutrients may increase (as in the sweetgum plots in Tennessee). From the plant's point of view, having more carbon as a result of growing in high C02 means that nutrients from the soil are more limiting to its growth, so investing in roots is a good s


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280 300 320 340 360

Atmospheric C02 level (ppmv)

Figure 8.8. Stomatal index vs CO: concentration in the clubmoss Selaginella selagineilaides. At high concentrations, there are fewer stomata. After David Beerling.

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Atmospheric C02 level (ppmv)

Figure 8.8. Stomatal index vs CO: concentration in the clubmoss Selaginella selagineilaides. At high concentrations, there are fewer stomata. After David Beerling.

way to gather morcnutrients. Getting more nutrients can then mean that it puts on the maximum amount of growth, and produces more offspring.

Another effect, detectable only at the microscopic level, is that plants grown at higher C02 levels have fewer stomata on their leaves, presumably because fewer are needed to allow C02 in to the plant when C02 levels are higher. Perhaps stomata leak water a bit, or use up energy unnecessarily, so that it is advantageous for the plant to have no more stomata than it really needs (Figure 8.8).

One rather strange effect of increased C02 levels is that respiration rates of plant tissues tend to be lower. If wasteful burning of carbon was being decreased, then this could be a good thing, allowing the plant to accumulate more carbohydrate for useful tasks. I lowever. much of the respiration that goes on in a plant has a purpose, such as the building and repair of tissues. If this sort of respiration is cut down, plants might not be repairing tissues as thoroughly, with possible long-term conscqucnces.


If working out what will happen to vegetation due to dircct C02 fertilization is a challenge, figuring out its effects on soil carbon is even harder to do. Soil organic


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 arc C02-fertilizcd?

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 contcnt when they arc C02-fcrtilized. 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-fcrtilizcd, and the extra amount lost to hungry insects in these experiments actually 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. In fact, the evidence is that overall with C02 fertilization the advantage is tipped in favor of the plant, against the insect. 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.

Most species in the world are herbivorous insects, and it is rather frightening to consider what effects this sort of change might have on insect biodiversity in the tropics and elsewhere. It is quite possible that a large change in nutrient content will push many species over the edge into extinction. It is widely considered by ccologists 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.

Fvcn though plants may benefit from C02 fertilization, humans who also want to cat 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 arc 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 contcnt 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 CO,? 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. One might take this as indicating that the direct CO, effect is at work here. However, such a conclusion would be far too simplistic. Over 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 CO: in order to "test" or "prove" models of C02 fertilization. It is a reasonable guess that the direct C()2 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 350ppm) 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 CO, fertilization effect in natural and semi-natural vegetation, we might expect to see signs of it already. After all, CO, 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 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 expcctcd from a C02 fertilization eficct, 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 eficct. Trees may well be growing faster in response to the warmer climate. Even though climate-warming due to an increase in the greenhouse eficct is mostly due to CO,, 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 arc 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. In the temperate deciduous forests of the northern tJSA, 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. Schwcingruber and colleagues reported that there is no sign of a response of the boreal forest to atmospheric C02 growth. In certain other 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, Phillips and 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 rales 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 eifect. 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 tlux 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 C'02 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. As things stand now, there is presently no convincing 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. On mountains in many parts of the world, the treeline is creeping upwards (Chapter 3). These trends are generally attributed to global warming produced by CO: and other greenhouse gases, but it is also possible that the direct C02 cffcct is playing a significant role in promoting the growth of plants. Unfortunately, it is presently impossible to disentangle these two factors, and ccologists 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.

So far, then, there does not seem to be anything in terms of natural vegetation change that is unambiguously a sign of C02 fertilization. Perhaps, as C02 levels continue to climb, more striking changes in vegetation that can only be attributed to this ciYcct will begin to show up. liven 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.

8.13.3 Ilie changing seasonal amplitude of C()2

The C02 concentration in the northern hemisphere fluctuates with the seasons: it goes up during the winter when decay dominates (releasing C02) and decreases during the summer when photosynthesis is taking up C02 and temporarily building it into leaves (Chapter 7). When autumn comes, most leaves in the mid-latitude forests and also the tundra are shed and they decay releasing C02.

The seasonal fluctuation in C02 in certain places in the northern latitudes has been increasing over the past several decades. 'This trend towards wider seasonal swings is strongest in northern Alaska, at a C02-measuring station located at Barrow. A weaker trend towards more seasonal fluctuation is also found at the Mauna Loa measuring station in Hawaii that ultimately gets a lot of air coming down from the Arctic. However, the trend is absent from other stations around the world.

What is one to make of this trend in the seasonal wiggle in the far north? The first explanation put forward when it was discovered was that it was due to increasing C02 fertilization. More C02 might be giving greater summer leaf mass in shrubs and herbaceous plants in the far north: more leaves sucked in more C02 each growing season, and then this was released by decay after the leaves were dropped at the end of summer. This picture seemed to be reinforced by satellite data showing that the Arctic latitudes had become increasingly greener over the last 10 years. Perhaps this is evidence of an increasingly strong C02 fertilization eiVect?

However, if the trend in the seasonal wiggle is due to C02 fertilization, why is it only noticeable in one part of the world? After all, vegetation everywhere should have at least some potential to respond to C02 fertilization. And. from what little experimental work has been done on exploring direct C'02 responses in tundra, it seems to be particularly unresponsive after a few years due to severe nutrient limitation. Also, there are other straightforward explanations as to why the seasonal amplitude of C02 in the high latitudes might be increasing. Plants arc known to respond strongly to temperature, and greater warmth in the north (where summer temperatures tend to limit the amount of growth that plants can put on) could be allowing the plants to carry more summer leaf mass. Climate records show that indeed there has been a strong warming trend in the Arctic, especially in the parts of Alaska and northeastern Siberia that also show the strongest trend in both C02 seasonal fluctuation and in greenness measured from satellites. This neatly explains why the trend in CO: seasonality has only occurred in that general part of the world, and there seems no particular need to invoke the poorly understood role of direct C02 fertilization. Why temperatures are increasing is another question altogether, and it could be due to CO: and other greenhouse gases acting upon climate (Chapter 1).

Continue reading here: C02 Levels And Stomata Out In Nature

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