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ISIS Special Miniseries - More CO2 Could Mean Less Biodiversity and

Worse

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The Institute of Science in Society

Science Society Sustainability

http://www.i-sis.org.uk

 

General Enquiries sam

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ISIS Director m.w.ho

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ISIS Special Miniseries

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More CO2 Could Mean Less Biodiversity and Worse

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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.

 

Diagrams and sources for this article are posted on ISIS Members’ website.

www.i-sis.org.uk

 

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 CO2depends 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 onthe 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.

 

 

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This article can be found on the I-SIS website at http://www.i-sis.org.uk/

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