Science in Society Archive

I-SIS Special Miniseries - Life of Gaia

This miniseries is dedicated to our planet earth, so we may better appreciate how she lives and sustains all creatures large and small, that we may learn to dance to the complex rhythms of her life music without stopping her in her tracks.

Space scientist and inventor Jim Lovelock first proposed in the 1970s that the entire earth is a self-organizing, self-regulating entity, rather like an organism. He named the earth Gaia, after the Greek earth goddess.

The idea that Gaia is alive and has a life of her own immediately caught fire. It inspired many earth scientists to look for the dynamic processes that organize and regulate the currents of the earth, to make a congenial home for all her inhabitants. These scientists are richly rewarded.

Records from ice and deep sea cores show detailed globally correlated changes going back at least 800 000 years, leaving us in no doubt that the earth behaves from moment to moment as one coherent whole, just like an organism.

Not only can we can read Gaia’s life-history from her deep memory stores, we can also tune in to her life-force pulsing as she is living today.

Gaia spinning in her perpetual dance around the sun, her mighty breath tumbling from hot belly to the poles, swirling across the continents, bringing welcome rain to forests, grasslands and crops, or torrential downpours, floods and hurricanes. Vast slow vortices of water connect her oceans from the furthest northern reaches to the southernmost haunts, from the shimmering sea surfaces to the dark deep beds, distributing warmth and nutrients, sustaining life with life.

Gaia’s breath is our breath, her water our water. Let Gaia live that we may live.

More CO2 Could Mean Less Biodiversity and Worse

More carbon dioxide doesn’t just make the earth warmer. It is an entire conglomerate of correlated changes of global dimensions in the earth’s climate, water, land, and not the least of all, her living inhabitants. Dr. Mae-Wan Ho reports.

Complex response of plants to CO2

The most significant, quantifiable indicator of climate change is the accumulation of carbon dioxide and other green house gases resulting from excessive burning of fossil fuel and industrial chemical emissions. The current rates of change in the chemical composition of our atmosphere are without geological precedent. The increase in CO2, in particular, will have important consequences on photosynthesis, the process whereby green plants create carbon compounds from carbon dioxide to feed human beings and much of the living world. CO2 concentrations were as low as 180ppm only 18 000 years ago, at the peak of the last glaciation. Current CO2 concentrations are double that, and are predicted to exceed 550ppm during the second half of the present century, i.e., double the pre-industrial concentrations.

The rate of photosynthesis depends on CO2 concentration. For most plants, the rate of photosynthesis is still not saturated at current CO2 concentrations in the atmosphere, and so there is room for more increase in carbon dioxide fixation. In the early days, this response was considered everything there is to understand about the effects of CO2 on photosynthesis. But things are actually more complicated, because apart from photosynthesis, the plant carries out a host of other metabolic reactions, all interconnected, which have to be balanced. A greater abundance of some chemical does not necessarily enhance the availability of other chemicals. Furthermore, CO2 concentration affects the plant’s water budget, which will impact on the photosynthesis. Finally, plants interact with animals, and an increase in CO2 will have impacts on the animals.

Response to CO2 depends on environmental conditions

There are very few relevant observations on the impacts of increased CO2 on plants and the associated ecosystems, especially forest ecosystems, which account for close to 90% of the carbon pool. Over short periods of time, plants can grow faster under elevated CO2 as long as the roots and mycorrhiza (beneficial fungi that grow in association with plant roots) have not fully exhausted the available nutrients in the soil. But sooner or later, elevated CO2 will have negative impacts on nutrient cycling.

Similarly, isolated tree seedlings, or orchard trees receiving an optimal resource supply including light from almost all directions, and horticultural plants supplemented with fertilizers, can all show increased growth in response to a 200-300ppm increase in CO2 concentration. But these effects disappear under more realistic conditions. Very few or no such responses are seen in unfertilised grassland and in dense tree assemblages on unfertilised ground. In some experiments, plants did not even grow more in elevated CO2 despite being supplemented with mineral fertilizers. A clear nutrient-dependence of the CO2 growth response was found for tropical trees grown in ample light on either unfertilised or fertile ground.

The type of soil also matters. In one experiment, two contrasting forest soil types were used with young beech and spruce grown jointly (as they do in nature) on acid or calcareous forest soil from the Swiss central plains. The results "must be a shock" to anybody involved in CO2 research, says Christian Korner the Institute of Botany, University of Basel, Switzerland. The responses were in opposite directions depending on the soil type. In a calcareous soil, beech grows better, whereas in acidic soil, birch predominates. Neither elevated CO2 nor fertilizers alters the basic picture.

Much of the response to CO2 comes during the early stages of growth, when resources (nutrients, space and light) are plentiful, but drop off in the later stages. Often it is not a single nutrient, but the interaction between nutrients that determines the CO2 response. For example, legumes with nitrogen-fixing symbionts are often particularly responsive to CO2 enrichment, but only when supplemented with phosphate. Under some conditions, CO2 enrichment may even induce symptoms of nutrient deficiency. It appears that a carbon-rich diet can lead to the export of soluble carbon compounds from the roots, which in turn may cause the food web around the plant roots to tie up free nitrates. Several years of in situ CO2 enrichment of calcareous grassland caused a drastic reduction of free nitrate in the soil solution; and the more diverse the plant communities, the more pronounced the effect, possibly due to the more effective exploitation of the carbon substrates by soil mycorrhiza.

Response to CO2 affects water budget in a species-specific manner and impacts on the ecosytem

It is widely known that CO2 enrichment tends to reduce the opening of stomata (pores) on the leave surfaces, thereby restricting consumption of water. This is so for grassland species and crops, as well as for young trees grown in open-top chambers.

However, when tested in situ on tall trees, no such response was found in conifers, and in important broad-leaved species such as European beech. Other species such as hornbeam, showed a significant 20% reduction, while other species are intermediate. In other words, much depends on the species involved.

Soil moisture tends to be higher under vegetation that close up their stomata as CO2 increases, thereby favouring species that are not drought resistant over those that are. In calcareous grassland, this moisture-saving response induced a significant stimulation of species such as Carex flacca and Lotus corniculatus. An unexpected side effect of this was to stimulate the activity of earthworms by 30%. The current evidence for grassland responses to elevated CO2 suggest that most if not all the biomass increases are due to such indirect effects on moisture.

All of this makes predictions very difficult, because how forests, grasslands and crops will respond to increase in CO2 will depend on the species present and the state of the soil.

Changes in live tissue composition species-specific and impacts on animals

There are many examples in which elevated CO2 leads to sustained changes in live tissue composition, with carbohydrates commonly increasing, proteins decreasing and secondary compounds varying in response. Carbohydrate/protein ratios were also significantly increased in plants growing for many generations around natural CO2 springs, also in leaves of tropical trees experiencing elevated CO2 levels in situ either in deep shade or in the fully sunlit forest canopy, and at the Swiss Canopy Crane site, where a mature forest has been continuously exposed to increase CO2 atmosphere for two years.

The shoots and leaves of the forest trees are found to have more carbohydrates, and insects feeding on such leaves show significant differences in growth rates, dependent on the species of trees. Thus, caterpillars of the moth Lymantria dispar showed a 23% reduction in growth rate on oak exposed to elevated CO2 compared to those on control oak trees; but on hornbeam trees, the precise opposite was found, a 28% increase in growth rate on trees exposed to elevated CO2 compared to controls.

An earlier test found that caterpillars of Lymantria monacha grew more and consumed proportionately less per unit body mass when fed on high quality, nitrogen-rich spruce needles produced under decreasing CO2. Leaf chewers like Lymantria can compensate for diminished food quality to some extent by increasing the amount of leaf consumed. Other species like leaf miners, apparently, don’t have this option, and will suffer more as a result. These observations point to a broad spectrum of effects on biodiversity across the trophic levels.

Impacts on decomposers

Rates of decomposition of leaf litter is another important factor affecting ecological health. This has mostly been found to remain unchanged. However, when different litter species were fed to specific species, it became obvious again that the results depend on the species. The isopod Oniscus asellus clearly shifted its preference from Fagus to Acer under CO2 enhancement, with no change on Quercus.

Some major consequences observed

All the responses to elevated CO2 described so far are species-specific. The effects may be direct or mediated via effects on moisture and metabolism. Similar species-specific responses of far-ranging ecosystem consequences are found when nutrient availability increases, or when temperature rises, two other key facts of global change. Thus, major changes in biodiversity can result from global climate change.

In forests, climbing vines or lianas can take particular advantage of CO2 enrichment in deep shade, partly because of the shift of the light-compensation point of photosynthesis to low light intensities. This increases the likelihood of lianas reaching the forest canopy. Given that the dynamics of natural forests, tropical ones in particular, are strongly influenced by the vigour of lianas, this biodiversity effect can overrun the direct growth effects of CO2 enrichment on canopy trees.

In the temperate zone, Hedera helix can become a serious forest threat when severe winters become less frequent, as is happening right now. As long as the forest canopy was open – as was the case in 1995 - there was little stimulation of Hedera under CO2 enrichment. However, by 1998, the forest canopy closed up, and the under storey light was reduced to 1% of the above canopy sunlight. The biomass of Hedera increased four to five fold, and increased by another 30 to 40% under CO2 enrichment, irrespective of nitrogen supply. At the same time, the initial strong biomass response of the whole tree assemblage was reduced to nearly zero.

Similarly, tropical lianas took enormous advantage of CO2 enrichment in very dim light, overgrowing and driving tropical forests into faster rotation and reduced carbon storage. When grown on native soil in a simulated typical Yucatan under storey climate, three native lianas exhibited strong responses under the current range of atmospheric CO2 enrichment (280 to 420ppm). At higher concentrations, responses became diminished and even reversed, highlighting the nonlinear responses to CO2.

The best data currently available are for grasslands, which are highly disturbed systems, commonly requiring cutting, grazing or burning to be maintained. ‘Pulsed canopy expansion’ refers to reoccupation of ‘empty’ space; and is a situation where CO2 enrichment can be most effective. The two natural grasslands in this comparison, the alpine and the semi-desert grassland, show very little or no growth response to CO2 enrichment. Remarkably, the small, insignificant semi-desert assemblage’s response is driven by a single species out of about 25 species, in fact, one out of the 5 legumes species in this community. This kind of response may also hold for complex forest ecosystems as well. It is only their slow development that has so far prevented us from detecting such clear-cut biodiversity effects.

To summarise, four main messages emerge from current findings.

  1. Plant species respond differently to CO2 enrichment (irrespective of the type of response involve), and these biodiversity effects translate into ecosystem responses.
  2. The responses depend on soil type, nutrition, light, water and age.
  3. The quality of plant tissue and exudates from roots change (more carbon, less of other elements), so consumers of plant products are affected.
  4. Responses to CO2 concentration are nonlinear, with the strongest relative effects under way right now, and few additional effects beyond about 500 ppm.

Given the globally uniform enrichment of the atmosphere with CO2, all regions should be affected in some way or other. There is no ground for complacency. Increase in CO2 does not translate into an increased in carbon fixation in photosynthesis; no increase is likely in the longer term. On the contrary, biodiversity may decrease, while the carbon cycle may speed up, making forests and other ecosystems less effective in sequestering carbon dioxide, thereby exacerbating global climate change.

Article first published 08/10/03


Korner C. Ecological impacts of atmospheric CO2 enrichment on terrestrial ecosystems. Proceedings of the Royal Society conference, Abrupt climate change: evidence, mechanisms and implications, September 2003.

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