Carbon Uptake and Water Use of Vegetation Under Climate Change

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Accumulation and storage of carbon in trees is one method of sequestration which may help offset increasing atmospheric CO2 concentrations. However, for every molecule of CO2 absorbed by a leaf, up to a thousand molecules of water are released as transpiration, water that has moved out of the soil into the atmosphere. Therefore, simply planting more trees to absorb more CO2 is not as risk-free as may originally be thought, especially in the dry continent that is Australia.

The location for planting trees is also an important factor when considering their impact on water supplies. Trees use more water than grasses and shrubs and the productivity of trees is strongly influenced by water availability. Sites with more rainfall have faster rates of carbon accumulation and store more carbon. Whether trees should be planted in groundwater discharge zones or recharge zones is an important consideration when planting trees.

Water use of a stand of trees changes as it ages. The period of greatest water use coincides with the period of most rapid carbon accumulation.Water is a scarce resource in Australia so any climate mitigation schemes involving reforestation must also consider the environmental, social and economic cost of water used by the new plantation.