Recent researches have made two discoveries - one that excess carbon dioxide, the gas that green plants use to make food, can jeopardise their survival. The other, that mildly rising nitrogen levels reduces the variety of plant species in an ecosystem, which levies its toll on the surviving species.

CO2 and C-4 grasses

A study in the University of Lausanne, Switzerland, has found that the major food plants that support human life, like corn, wheat, sugarcane, sorghum and millets evolved about 30 million years ago, in a reaction to scarcity of carbon dioxide in the atmosphere. The list of this class, known as C-4 grasses, includes plants used as biofuel and many components of the world's grasslands.

The Lausanne scientists used the molecular clock technique to date the evolution of C-4 grasses. The molecular clock method compares the genetic structure of a species as it evolves over a period of time whose length is known, for example, by geological data or by carbon dating. The level of genetic change then provides a fair measure of how fast the changes occur, over time. Now, if samples of plant species at unknown times are examined, the extent of genetic change could provide an idea of the time taken.

In this way, C-4 grasses were found to have arisen when carbon dioxide levels on the earth were low. Plants normally harvest energy through photosynthesis, using sunlight and carbon dioxide. But when carbon dioxide levels are low, for example, when the leaves close their pores in hot and arid conditions, plants fall back on an alternative process called photorespiration, which uses oxygen in place of CO2 , but the process is energy-inefficient. The C-4 grasses evolved to avoid this alternative procedure, by making use of CO2 within the plant in times of CO2 scarcity. Grasses not so evolved are the C-3 grasses, desert shrubs, not so useful to support animal populations.

As C-4 grasses evolved to combat reduced CO2 levels, major increases in CO2 concentration could set off ecological changes that would alter the distribution of C-4 grasses. Reversal of the conditions that gave rise to C-4 plants may lead to their extinction - a grim prospect, in times of increasing CO2 levels and the substantial reliance of human population on this category of plants.

Plant diversity

Studies have shown that having many species living together is good for all of them, rather than detrimental. Although many species vie for the same nutritional resources, the diversity of species evolves so that each supports the other, in controlling predators or maintaining the nutrient mix. Specialised plant species grown in isolation not only need custom-made fertilisers, but are also sitting ducks for infection and cannot resist minor changes in conditions.

Scientists at the University of Minnesota, USA, in a 23-year-long experiment in neighbouring grasslands, have found that low levels of nitrogen for extended periods seriously reduce plant species numbers in sample plots. Happily, they have also found that diversity reappears when the earlier nitrogen levels are restored.

The use of fossil fuels and chemical fertilisers have increased nitrogen levels to 2 to 7 times the pre-industrialised levels in developed countries. A similar rise is expected in the industrialised areas of Asia and South America in the next 50 years. The effect of such high nitrogen levels on plants have been studied and are documented. What has not been studied is the effect of low level, but chronic increase of nitrogen, which is found in large underdeveloped parts of the globe.

The 2-decade study in Minnesota shows that while higher nitrogen levels result in lower numbers of species co-existing, it is when the nitrogen addition is at low levels that the species number depletion is the highest. This is perhaps because the C-4 grasses dominated prairie has the highest uptake of nitrogen at low nitrogen levels.

The added nitrogen aids specific species to prosper, at the cost of other species, without the automatic controls that develop over centuries in natural ecosystems.

But when nitrogen levels are higher, the C-3 dominated, species-poor plant community loses extra nitrogen and is not affected to the same degree. Large tracts of agricultural land receiving low doses of fertiliser may thus be modified to their disadvantage.

(The writer can be contacted at simplescience@gmail.com)