Could planting 20 million trees increase drought?

By Derek Eamus 10 February 2010
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The Liberal Party has argued that planting 20 million trees will reduce Australia’s carbon emissions, but Derek Eamus wonders if it’s a good idea.

The release of the Liberal Party climate change policy has suggested that planting 20 million trees in Australia will impact on our continental CO2 emissions. Tree planting has also been suggested in other parts of the world as a means of combating climate change.

But is this desirable for Australia? And are there any other implications from planting this number of trees here, in the world’s driest inhabited continent?

Adding things up

First we need to look at how much carbon will be sequestered (locked away for a significant period of time) in 20 million trees. From my simple calculation, if we think about 20 million trees each of 50 cm diameter (certainly a large tree for urban scales) this corresponds to about 341 kg of above ground biomass for each tree.

Half of this is carbon, so this equates to about 170 kg of carbon or about 622 kg of carbon dioxide per tree. Multiply by 20 million trees and you get about 12 million tonnes (Mt) of CO2 absorbed to grow those trees to that size.

Unfortunately it takes many years to get to that size, but if we assume a 10 year growth period and assume a constant average rate that means about 1.2 Mt of CO2 absorbed per year by 20 million trees.

This sounds a lot, but remember that at the moment Australia emits about 400 Mt of CO2 per year. So 20 million trees will end-up containing only about 0.3 per cent of our annual emissions, per year. Still, I guess it’s a start.

Negative impact on water

But what about the relationship between tree growth, carbon uptake and water use by trees? Two immutable facts are often forgotten in this debate. First, for every molecule of carbon dioxide absorbed by a leaf, several hundred (up to a thousand) molecules of water are lost through evaporation into the atmosphere.

Second, Australia is an arid continent where fresh water resources are being placed under increasing stress through increased demand by irrigators, human consumption and environmental needs.

These two facts suggest that planting trees may have significant impacts on water resources. Is this a problem? In order to answer this question we need to consider how plants actually grow.

In order for leaves to absorb CO2 pores in the leaf surface (stomata) must open. This opening is stimulated by sunlight, which drives the photosynthetic process of carbon fixation, which drives plant growth.

However, as soon as those stomata open (and there are many thousands of these stomata per square centimetre of leaf) the energy in sunlight causes liquid water to evaporate from inside the leaf and then diffuse out of the leaf into the atmosphere.

Leaf loss

You can expect about 150 or so molecules of water to be lost (as transpiration) for every molecule of CO2 fixed during photosynthesis. This ratio (1:150) gets much worse (by a factor of two) when we remember that about half of the carbon fixed this week in a leaf will be lost next week as respiration – the process that burns fuel in all living cells and in doing so releases CO2 back into the atmosphere.

This means that for every molecule of CO2 fixed in the long-term (that is, not respired away in the short-term) there are about 300 molecules of water lost from the soil.

To put this into a landscape perspective, if a young plantation is transpiring 1 mm of water per day per square metre of ground, this is equivalent to 365 million litres of water per year per 100 hectares of plantation. For a small area plantation of 100 square kilometres this is equivalent to the water in more than 14000 Olympic-sized swimming pools.

The follow up question we must ask is: what might be the impact of planting large acreages of plantations on landscape water balances?

Water budgets

To answer this we need to think about the water budget of a landscape. Rain that falls can recharge the soil with water, which is then used by plants in transpiration; or run off into streams and rivers to keep them flowing, or percolate past the roots and recharge our groundwater reserves.

If the land has pasture or crops, then the total amount of water lost as transpiration in each year is much smaller than if the land is covered in trees. This is because trees transpire water all year and have deep roots and so capture more of the rainfall. Crops don’t use water all year and have shallow roots.

Thus, the proportion of rainfall that is lost as transpiration each year is larger when the landscape is covered by trees compared to when the landscape is covered by crops or pasture. This means less water available for river flow and less water available for groundwater recharge. We need to ask if this is a desirable outcome.

Am I suggesting we don’t plant trees? No, I am not. Am I suggesting we cut down more of our trees? No, I am not. What I am suggesting is that we need to think carefully about where we plant and what we plant. But that is a discussion for another day.

Derek Eamus is a plant physiologist and ecophysiologist and the Chair of Environmental Sciences at the University of Technology, Sydney. We republish this opinion piece courtesy of the Australian Science Media Centre