South Africa is located in a predominantly semi-arid part of the world. The climate varies from desert and semi-desert in the west to sub-humid along the eastern seaboard. Average rainfall, around 450 millimetres (mm) per year, is well below the global annual average of about 860 mm. Evaporation is comparatively high. South Africa’s water resources are, from a global perspective, extremely limited.
Yes. The National Water Act (Act No 34 of 1998) declares timber plantations (afforestation) as stream flow reduction activities (SRAs). As SRAs, all existing commercial plantations must be registered as water users and will, in time, be licensed. Only lawful plantations will be granted a licence. The rules of legality for plantations are quite complex, but simply put, are as follows:
Commercial forestry plantations in South Africa account for a little less than 3% of total water use. Plantations are not irrigated; they only intercept rainfall, which reduces runoff into rivers and streams. Agriculture and crop irrigation are the dominant users of water in South Africa.
We avoid unnecessary water use by not planting trees too close to riparian zones and wetlands and ensuring that these are kept free of commercially planted trees.
Water quality is managed by regulating drainage and minimising erosion, thereby preventing runoff into streams and rivers.
Pesticide use conforms to the strict Forest Stewardship Council™ 4 (FSC™ N003159) standards, which aim to minimise the use and impact of these on the environment. The amounts of fertiliser used in plantation forestry are minimal and there is no likelihood of leakage into streams and rivers.
Consequently, rivers, streams and lakes are not polluted by forestry operations, although there is some sedimentation. We have a number of procedures to ensure that runoff after harvesting is kept to a minimum. These include leaving harvesting residue on the ground and ensuring that the correct harvesting and extraction methods are used.
Statements are often made that Eucalyptus trees use more water than pine trees. Recent investigation by members of the South African Water Research Commission (WRC) have pointed out a lack of scientific evidence supporting quoted differences in water use between Eucalyptus and pine species. Recent research also showed that water use by trees could be more affected by soil characteristics (soil depth), environment (slope and landscape position) and climate than the genus planted.
Research results from various studies comparing water use of pines to that of Eucalyptus varies greatly and does not consistently show that one genus uses more water than the other. There are a number of factors that could play a role here. A common mistake made when assessing the differences in water use between Eucalyptus and pines occurs when trees are measured at a common age and not at a common crown development (leaf area) stage.
The pattern of water use over the stand-development period is similar to that of nutrient use and follows the growth-curve pattern. It increases from planting, until it reaches a peak at the time of canopy closure and thereafter decreases over time. It is not accurate to compare the water use over just the first five years of the rotations.
Thus, water use over two Eucalyptus rotations of 10 years should be compared to that of a 20-year pine rotation. If the comparisons are made over a longer time-span, differences in water use between Pinus and Eucalyptus are not statistically significant. It has even been demonstrated in South Africa, that the water use of very old plantations returns to levels of water use in the natural vegetation prior to afforestation. Many of the water use figures reported in South Africa originate from sapflow measurements.
These have been done on a single tree or a very small number of trees on a stand only. It has been shown in other studies that single tree water use cannot be upscaled to reflect the stand-level water use, as it introduces significant error. Due to the cost of equipment required to measure tree water usage, the replication of treatments was also very limited in past (and current) studies. Measurement of water use efficiency (WUE) is normally linked to productivity or biomass measures such as stem volume production per unit of water used.
Expression of WUE in this manner - as annual stem volume increment per unit volume of water transpired for Eucalyptus species across age classes and site types in South Africa - ranges from 0.0008 to 0.0123 cubic metres (m3) stem wood produced per m3 water consumed2.
There is, furthermore, variation in WUE between varieties (of the same age and on the same site), with the tendency for the fastest-growing genotypes, to consume the least water per mass or volume of wood produced, therefore being more water efficient. Furthermore, from available measurements, on economic WUE (levelised net present value per cubic metre of water), eucalypts tend to be the most water use efficient of all species (in comparison to pine and various indigenous plantations) on a per mass or volume of wood produced basis.