I would like to respond to a comment by Ricardo Carrere regarding water use
by fast-growing tree species. He states that "timber plantations consume
huge volumes of water." I cannot tell from Carrere's statement if this is
more or less consumption than other crops (such as sugarcane). Also, it is
not clear if Carrere's statement relates to water use on a per hectare or
per tree basis.
Lets examine some data from South Africa where information on water use by
pine stands has been estimated. For a region with 1,500 mm of annual
rainfall, scrub-fynbos (burnt on a 7-year cycle) will produce about 2.5
tons/ha/yr while a 30 year-old pine stand will produce about 15
tons/ha/year. It has been estimated that for the scrub-fynbos area, 53% of
the rainfall would end up in streams (and/or groundwater) while only 27% of
the rainfall would end up in streams/groundwater from the pine plantation.
In comparison, one should expect about 66% of the rainfall on a denuded area
to end up in streams/groundwater. Although fast-growing exotic pines affect
streamflow more than fynbos, the pines use only half as much to produce a
ton of wood. These estimates suggest that indigenous trees require about
780 tons of water to produce a ton of wood, while pines require only 390
tons (see data below).
Denuded area Fynbos Pine monoculture
Rainfall 1,500 mm 1,500 mm 1,500 mm
runoff+
leached 990 mm 795 mm 405 mm
Amount of rainfall
used by trees 195 mm 585 mm
MAI tons/ha/yr 2.5 15
Water use/ton 195/2.5= 78mm/ton 585/15 = 39mm/ton
Therefore, water management decisions can be made either on an area basis
(which species uses more water per ha) or on a weight basis (which species
uses more water to produce a ton of biomass). Management decisions can vary
greatly depending on which basis is used. Although the Carrere states that
"fast-growing species of trees commonly used in timber plantations consume
huge volumes of water," this is not true when comparing water use
efficiency (g water used/g biomass produced). By considering water use on a
per hectare basis, Carrere gives us the incorrect impression that pine trees
are inefficient in their use of water. In fact, when wood production is the
major objective of the landowner, fast-growing tree species may use less
water per ton of wood produced than slow-growing native species.
I would agree that any crop (monoculture or not) that produces 15 tons/ha/yr
will use more water/ha than a crop that only produces 2.5 tons/ha/yr.
David South
School of Forestry
Auburn University, AL 36849-5418
dsouth@forestry.auburn.edu
205-844-1022
205-844-1084 (FAX)
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