PART 1: THE INCREASED WATER USE OF INVASIVE ALIEN TREES: SOME REASONS
It has become general knowledge that invasive alien plants often have much increased water usage compared with native vegetation and especially in water scarce landscapes. However, Calder and Dye (2001) pointed out that the reasons for this increased water use are perhaps less understood. Yet they are of importance especially when there is need to efficiently channel resources for alien plant clearing. Understanding some of these reasons can facilitate prioritization of areas for clearing IAPs.
Here are some reasons:
1. Difference in height
Trees usually evaporate more water than shorter and annual vegetation crops such Fynbos and or grasslands. Interception of rain is higher from trees than from shorter crops and that facilitates higher rates of evaporation from tree canopies. This is similar to the clothes line effect as clothes pegged on a washing line will dry faster than those laid out flat on the ground. A stronger pull is required to evaporate water from a lower surface than a higher one.
2. Difference in rooting depth
In drier climates trees usually adapt to these conditions by developing deeper rooting systems. This enables them to access more soil water during the dry periods, a characteristic not generally exhibited by shorter or annual vegetation. This also leads to higher evaporation rates overall.
3. Difference in senescence
Most crops and grassland have a characteristic pattern of development where green leaf area increases rapidly after germination or winter dormancy, and peaks towards maturity before degrading. This senescence of short vegetation, particularly montane grassland in South Africa is often a major cause of reduced evaporation when compared to evergreen trees, of which most IAPs are.