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CO2 influences transpiration and biodiversity

Updated on February 4, 2016

It is common knowledge that plants uptake carbon dioxide for photosynthesis and growth. It is also common knowledge that during this process plants lose water through the stomata on the leaves. But fewer people know that the amount of carbon dioxide in the atmosphere, or the concentration of atmospheric CO2, can also effect plant growth, transpiration and even the water balance of entire ecosystems and catchments. Even fewer people know that carbon dioxide can affect biodiversity and which plant species are more common than other species.

Let's leave aside climate change for now

Yes we all know that human activities are increasing the amount of CO2 in the atmosphere and this, in turn, is affecting global processes such as the climate system. In this article I want to step away from this debate and focus on the physiological effects an increase in atmospheric CO2 is causing to plants.

From a purely academic or scientific viewpoint, this is an interesting exercise as CO2 is fundamental to plant functioning. Without CO2 there would be no photosynthesis and hence no plant growth. So it is critical to know what the effects of increase CO2 concentrations, or even decreasing them, has on plants. It is the same as manipulating any other variable that is critical to plant growth - such as fertilizers, water or the amount of light.

But in case you're curious to know the role of CO2 in climate change, here is everything you need to know explained in 60 seconds:

Experiments show how CO2 influences plants

How do we know that CO2 has any influence on plants at all? It is quite easy to determine the influence of adding or withholding water or nutrients to plants. It is something you can do easily at home just by adding a bit more water or nutrients to the soil of some plants more than others and then watch the effects over the subsequent weeks or months. Or you could simply stop watering your plants to see how crucial this element is to plant survival!

Atmospheric CO2 is a little more difficult to manipulate mainly because it is everywhere around us in the atmosphere. CO2 in the atmosphere is not uniform and moves around in clouds. CO2 near certain outlets may be higher than other areas. CO2 in the atmosphere can even change throughout the day and throughout the year.

One approach is to take advantage of areas occurring naturally high in CO2. Scientists in Italy have used this to great effect by undertaking CO2 experiments around natural geothermal CO2 springs. Scientists measure various morphological and physiological traits of plants occurring near the geothermal springs and compare these traits to the same species of plants growing away from the springs. These natural, observational experiments are an excellent approach to determining how CO2 influences plants. But it may also be difficult to control confounding factors, such as soil moisture or temperature, in these natural experiments.

Glasshouses and Whole Tree Chamber Experiments

Scientists have also manipulated CO2 in enclosed areas such as glasshouses and growth cabinets. These experiments examine how CO2 affects plants primarily growing in pots. They offer great insights but, again, plants growing in pots are in an artificial environment and may not truly reflect the reality of plants growing in nature.

Stepping up from glasshouses are experiments called Whole Tree Chamber (WTC) experiments. These experiments place chambers, or specialised glasshouses/vinyl houses, around mature trees. Or young trees are plants in the chambers and then grown over subsequent years. WTC experiments can examine larger trees than those growing in pots and also growing on naturally occurring soils. The chamber can be compartmentalized so that above-ground and below-ground processes can be examined in isolation. That is, the effect of increasing atmospheric CO2 on leaves versus the roots can be determined.

Whole Tree Chamber Experiment at the Hawkesbury Institue of the Environment, University of Western Sydney, Australia

Eucalyptus trees grow in whole tree chamber experiments in Sydney, Australia
Eucalyptus trees grow in whole tree chamber experiments in Sydney, Australia | Source

FACE Experiments

Another approach is to manipulate atmospheric CO2 around entire ecosystems or forests. These experiments are called FACE – or Free Air Carbon Enrichment – where massive towers gorge out copious amounts of CO2 around trees and ecosystems.

The FACE experiment at Duke University, North Carolina, USA
The FACE experiment at Duke University, North Carolina, USA | Source

Plants use less and more water

What these experiments have generally shown is that plants actually like it when there is more carbon dioxide in the atmosphere. Plants generally grow larger and faster with more CO2. The process is similar to when you add fertilizer to the soil to assist the plant’s growth. Instead, we are adding fertilizer to the atmosphere in the form of more carbon dioxide. As there is more carbon dioxide in the atmosphere, it is generally easier for plants to perform photosynthesis and accumulate biomass.

In terms of transpiration, or plant water use, an increase in atmospheric CO2 means that stomata, or the pores on leaves that exchange gases between the leaf and the atmosphere, do not need to open as much. This means that, at the scale of a leaf, less water is lost and there is lower transpiration. With increased photosynthesis and lower water loss, then the water efficiency of plants is increased.

So it seems that more carbon dioxide in the atmosphere is a good thing for plants – they grow faster and use less water. However, nothing in biology is so simple. Yes the water loss at the scale of a leaf is lower, however, there are more leaves on the plant because it is growing faster and bigger. So, at the scale of the entire plant, water use or transpiration is actually higher.

This may seem trivial but consider this: a 30 metre (100 foot) tall tree transpires about 200 litres (53 gallons) on a good day. A forest of woodland contains many thousands to millions of individual trees all transpiring about 200 litres per day. This can impact on water reserves – particularly around dams for drinking water or irrigation. With increased atmospheric carbon dioxide, all of those trees may potentially transpire more water and having a greater impact on water supplies.

Scientists measure the world's tallest trees

CO2 can also influence biodiversity

Not all species of plants are created equally. Increasing carbon dioxide does improve growth but some species perform better than others. In a woodland or rainforest, some species may grow better and out-compete their neighbouring plants. That is, some species may become more common and others scarce.

This, in turn, can have cascading effects all the way up the food chain. For example, some birds or insects specifically feed on one or a few groups of species. If these become more or less common then those animals will also become more or less common. The entire process is too complex to predict or even estimate what could happen.

Increasing atmospheric carbon dioxide can benefit plants – something greenhouse and glasshouse growers use to their advantage. However, the process and outcomes are complex. In the real world increasing carbon dioxide does not benefit all plants.

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