Direct C02 Effects And The Ecology Of The Past

There are some fairly good indications that the C02 concentration of the atmosphere has undergone natural variations in the past, before humans began to affect it. The best substantiated changes in C'02 were those that occurred between glacial and interglacial periods during the last 650.000 years, where bubbles trapped in ice caps preserve samples of the ancient atmosphere that can be analyzed (Chapter 7).

The evidence of such fluctuations in CO: has set ecologists wondering what these might have been doing to plants in the past. During glacial phases, with C02 concentrations more than 30% lower than at present, plants may have suffered more drought due to the need to open their stomata more to get enough C02 to grow, indeed, the glacial world was much drier with widespread deserts and scrub vegetation, and far less forest than the present-day world (Chapter 3). For instance, large parts of the central African and Asian rainforests were replaced by arid scrub and grassland. The Sahara Desert extended hundreds of kilometers farther south, compressing the vegetation zones on its margin down towards the equator. And Siberia, which is now covered by forests, was a dry cold semi-desert at that time. There are good climatological reasons to expect that the glacial world would have been drier

(a more arid world is predicted by GCMs for the glacials, due to less evaporation from colder oceans transporting less water vapor around), but it is possible that the appearance of aridity was intensified by low C02 concentrations. Climate models so far seem to have trouble getting the world dry enough to match the indications from the plant fossil record, and it is possible that the additional "missing" factor in the aridity is the direct effect of low C'02 on the plants.

Some other rather strange aspects of the vegetation of the glacial world might have been due to the effects of CO: on plant physiology. For instance, the open, arid steppe tundra vegetation (sec Chapter 3) that covered Siberia and northern Europe during ice ages combined species of plants that do not normally grow together nowadays. There were typical tundra spccies such as dwarf willows (Salt.v) and sedges (Cypcraccac) alongside steppe species such as Russian thistle (Kochia) and wormwoods {Artemisia), leading to the name "steppe-tundra" to describe this vegetation type. Also present in the steppe-tundra were plants that are now confined to seashores (e.g. slag's horn plantain, Plantago maritima), and genera that are commonly found on builders' rubble (e.g., dock, Rumex). Some ecologists who have studied ice age vegetation suggest that the steppe tundra was a product of low C02 levels, bending the niche requirements of plants that cannot normally survive in the same placc. However, it could also have been due to the very different climates of the icc age world, with combinations of climatic attributes that do not occur today allowing these plants from different biomes to grow together.

The vegetation on mountains may also have responded particularly strongly to low C02 during glacials, because the availability of C02 was already limited by the thinner air at high altitude. While ice age climate was too cold for vegetation to grow far up mountains in the high latitudes, tropical mountains did have vegetation growing up to several thousand meters. Even so, vegetation zones on tropical mountains had moved downslope a long way during ice ages. This was certainly in part due to the worldwide cooling of climate at the time, but the vegetation change on mountains seems to correspond to temperature changes that are greater than nonliving indicators such as glacier limits would indicate. It has been suggested that this discrepancy is due to the cffcct of the low C02 on mountain plants: the temperature limits at which their growth was viable shifted under reduced C02, pushing them still farther down the mountains.

One thing we might expect during the low-C02 glacial phases is a greater relative abundance of C4 plants, because experiments show that they are better at making the most of low concentrations of C02. C4 plant carbon has a characteristic isotopic composition, because the C4 photosynthetic system distinguishes more strongly between the various stable isotopes of carbon than the C3 system docs. Buried soil carbon—made originally from dead parts of plants—reflects this isotopic composi-

tion. If it is composed more of C4 plants, it has less of the lighter ~C, more of the heavier L,C isotope. It turns out that at least in some places in tropical Africa that arc now grasslands, preserved grassland soils from the last glacial have more of the isotopic composition of C4 plants, suggesting that C4 plants were doing better in the glacial environment. This has been interpreted as a sign of the influence of low C02 levels, favoring C4 grasses in the tropical grasslands at that time (although a trend towards aridity could produce the same effect as C4 plants do better in more arid climates). However, the trend is not a consistent one; in other places the soil carbon record shows that C4 plants were less abundant in grasslands of the glacial times. It could be that in such cases cooling during the ice age is dominating; C4 plants don't do so well in cooler climates. Because temperature, aridity and CO: could all affect the relative abundance of C4 plants, it is very difficult to disentangle them from one another to say what was really more important during the glacials.

How about an environment which we know stayed wet during the last glacial, because the plants were growing in waterlogged soils throughout? There is a swamp site in the tropical mountains of Burundi that has stayed moist and laying down peat throughout the time since the last glacial. Here at least we can say aridity is removed as a factor, bccausc the plants have had their roots in water all the time. In this site, during the last glacial the peat composition indicates that there was a big shift towards C4 plants during the glacial, compared with the present. The climate was also cooler at this site during the glacial, and that would be expected to favor C? types of plant, and yet the shift was still towards C4. The only factor left in this case seems to be CO, favoring C4 species that can photosynthesize faster where C02 is in very short supply. Perhaps here then, we see one example where the direct C02 effect of lower glacial C02 levels really does show up unambiguously.

As one would expect, the stomatal index of fossil leaves that grew during the low-C02 glacial phases is higher than those of the same species from the interglacials. The lower C02 triggered a growth reaction to increase the supply of C02 into the leaf, by making more stomatal pores. This again is nice confirmation that the change in C02 did affect the plants in at least some way, though how important it actually was in altering vegetation structure and composition is still an unknown. Some clues can be had from chamber experiments which grow plants under lower than present C02 levels, similar to those during the last glacial (at 200 ppm). Many herbaceous plants grown under such conditions turned out much lower growth rates (often around 50% less) than when they were grown at present-day early 21st century (360ppm) C02 levels. However, it is not clear if evolutionary selection of low-C02 tolerant variants would close this gap in a real world low-C02 situation.

8.15.1 Direct CO2 effects on longer geological timescales

It is uncertain how far the CO: level has fluctuated on longer geological timescales. over tens and hundreds of millions of years. Some calculations based on balancing the rate at which volcanoes push up C02, and the rate at which C02 is taken up into weathering reactions with rocks, suggest that there must have been some rather large fluctuations in the C02 level of the atmosphere over geological time, perhaps by a factor of 20 or more (Chapter 7). Another way to estimate past C02 levels comes from looking at the stomatal indices of leaves that lived during each phase of geological history. By looking at the surface of very well-preserved fossil leaves under a microscope, one can compare the abundance of stomata with similar leaves in the modern-day world, and perhaps deduce the concentration of C02 in the ancient atmosphere. For instance, well-preserved magnolia leaves from late Miocene

(7 million year old) deposits in the Netherlands have a stomatal index that suggests that they grew under CO: levels around twice those existing today. The general picture from stomatal densities seems to agree fairly well with that from balancing volcanoes and rock reactions, supporting the idea that there have been several tenfold fluctuations in the C02 concentration over the last few hundred million years. Some other independent indicators of C02 levels in the chemistry of rocks seem to corroborate these estimates, while others disagree, so the picture is perhaps not totally clear overall. However, most geologists who have studied the evidence seem to be convinced that there really were major fluctuations in atmospheric C02 levels over geological time.

It is unclear what these fluctuations in C02 would have been capable of doing to the ecology of plants. Presumably, the higher C02 levels in the distant geological past made plants less drought-susceptible, becausc they would not have needed to open their stomata quite as much to get the carbon they needed. So, they would be better able to eke out whatever supply of water they had around their roots. It has been suggested that the rise to dominance of the flowering plants (angiosperms) between 120 and 60 million years ago was caused by a large decrease in atmospheric C02 levels during the same period. Various features that are common in flowering plants, but rare in other types of plants, seem to be favorable for getting water up from the roots quickly in an environment where leaves are often short of water. For example, the elaborate branching networks of veins in the leaves of flowering plants, and the long open vessels that conduct water up through their stems, may allow better movement of water. It has been suggested, then, that the low C02 world exposed leaves to greater drought stress as they had to keep their stomata open for longer to bring in enough C02. The flowering plants, having the correct features for keeping leaves supplied with water, were able to flourish under these conditions while other older groups of plants were pushed out. The trouble with such assertions is that the rise of the flowering plants was a one-off event that we cannot re-run under different circumstances: there could in fact be many other reasons why the angiosperms did so well after they first appeared in the lower Cretaceous.

8.15.2 Ancient moist climates or high C()2 effects?

High C02 levels would tend to produce more luxuriant vegetation, for a given level of rainfall, than we would normally see in the present-day world. This does seem to tally-in a general way with some aspects of the plant fossil record; for instance, moist climates with tropical and temperate rainforest seem to have dominated the land surfaces around 55 million years ago during the early Tertiary, at a time when gcochcmical calculations and stomatal indices suggest that C02 levels might have been several times higher than at present. It is possible then that the climates were not really as moist as would appear, and that high C02 preserving the water balancc of plants enabled lush vegetation to thrive under less rainfall than would be needed nowadays. However, further back in time during the Cretaceous and Jurassic periods, rock chemistry calculations suggest that C02 levels were as much as 20 times higher than at present, yet semi-arid environments seem to have been fairly widespread.

During the 50 million years that followed the super-moist world of the early Tertiary, plant fossils of drier climate vegetation such as scrub, grassland and semi-desert became progressively more common. From indicators in the rocks, and changing stomatal indices in fossils, gcochcmists suggest that C02 levels were declining during this time. It is certainly tempting to put the shift in vegetation down to lack of C02 making it harder for plants to maintain their water balance, so that in many places forests could no longer survive. Grasses that use the C4 photosynthetic system appeared and have become widespread only during the last 7 million years or so, leaving a characteristic isotopic trace in the fossilized carbon and soil carbonates they leave behind. Because C4 grasses arc very good at sucking in C02 without losing much water, it has been suggested that the progressively lower C02 concentrations favored their spread. The spread of grasses in general, and C4 grasses in particular, during the last 20 million years, has been linked to the evolution of a range of animals adapted mainly to grazing off grasses. Their existence may owe something to the decrease in C02 bringing about the dramatic spread of grasses.

However, it is unlikely that C02 effects on plant water balance arc the entire story in this global shift in vegetation in the last 50 million years. The change in vegetation seems too dramatic for a direct C02 effect alone, and surely requires at least some genuine decline in rainfall. C4 grasses, while using C02 more efficiently, also nowadays inhabit arid environments, and any decline in rainfall would also have favored them. The moist world 55 million years ago was also very warm (perhaps because of

613Cpdb (parts per thousand)

Figure 8.9. The shift in nC in sediments in North America, indicating a "take-over" by CA plants.

613Cpdb (parts per thousand)

Figure 8.9. The shift in nC in sediments in North America, indicating a "take-over" by CA plants.

an increased "greenhouse effect", itself partly caused by higher C02 or CH4) and climate models predict that a warm world should produce more rainfall. In addition, the rain-blocking eficct of some mountain ranges such as the Himalayas and the Rockies was probably not as strong as it is today. These mountains were just beginning to grow and were still much lower, and this would have allowed rains to reach into the interior of the continents to areas that are now very dry. Once again, it is hard to assign any particular part of the shift in vegetation to C02 alone, because so many things changed in parallel.

Continue reading here: Other Direct C02 Effects In The Oceans

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