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 C02 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 C02 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 C02 on the plants.
Some other rather strange aspects of the vegetation of the glacial world might have been due to the effects of C02 on plant physiology. For instance, the open, arid steppe-tundra vegetation (see 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 species such as dwarf willows (Salix) and sedges (Cyperaceae) 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. stag'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 place. However, it could also have been due to the very different climates of the ice 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 effect 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.
0ne 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 does. Buried soil carbon—made originally from dead parts of plants—reflects this isotopic composition. If it is composed more of C4 plants, it has less of the lighter 12C, more of the heavier 13 C isotope. It turns out that at least in some places in tropical Africa that are 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 C02 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, because 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 C3 types of plant, and yet the shift was still towards C4. The only factor left in this case seems to be C02 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 to have much lower growth rates (often around 50% less) than when they were grown at present-day early 21st century (360 ppm) 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.
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