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Black wattles can take the heat

Invasive alien plants (IAPs) have successfully invaded many riparian zones in South Africa. Australian Acacia species are particularly widespread as invaders of riverbanks in the Western Cape [for further details on Australian acacias as invaders in South Africa, click here]. The region, most of which currently experiences a typical Mediterranean-type climate, is predicted to experience more intense droughts in the future under most scenarios of climate change. IAPs aggravate this problem due to their profligate water use. One such thirsty invasive alien tree, Acacia mearnsii, commonly known as black wattle, competes aggressively with native tree species in riparian areas. However, it remains unclear what physiological advantage (pertaining to water relations) this species has over co-occurring native species under predicted reduced streamflow scenarios.

In a recent paper in the journal; Biological Invasions, C·I·B student, Casper Crous, and co-authors set out to determine whether two key native fynbos riparian woody tree species show differences in drought-tolerance compared to A. mearnsii in three prominent fynbos riparian zones that differ in streamflow quantity. The objective of the study was to gain a mechanistic understanding of how woody species, especially the invasive alien species, adapt their hydraulic strategy across this proxy for water availability.

Acacia mearnsii tree

A young Acacia mearnsii tree. The inset shows the bi-pinnate leaf structure.

To obtain this information, vulnerability to drought-induced cavitation was measured for each species. This method measures species susceptibility towards air emboli entering xylem due to insufficient water supply. A certain percentage xylem blocked will consequently constrict water flow throughout the plant (impaired hydraulic conductivity), leading to either branch die-back, or more likely, plant death.

Findings showed that the xylem of A. mearnsii was 50% blocked under a reduced potential to access groundwater (lower water potential), whereas the xylem of the co-occurring species reached that same level of cavitation at a higher water potential, suggesting a physiological (and anatomical) advantage in tolerating water-stress for the invasive. Acacia mearnsii is likely to persist under future drier conditions in these fynbos riparian zones and therefore remains a top priority for control.

On a more positive note, the native tree Brabejum stellatifolium (Wild Almond) showed good potential for being a valuable species for restoration of riparian zones in the Western Cape, as it compared well to black wattle in terms of drought-tolerance at two sites [for further discussion of options for restoration in riparian zones, click here]. Streamflow per se was an inaccurate predictor of on-site drought-tolerance, i.e. trees were not more tolerant at lower streamflow conditions.

This study gave the researchers a glimpse into the physiological functioning of an IAP (regarding water relations) and has important implications for improving our understanding of the potential trajectories of fragile riparian communities in the face of rapid global change.

Read the paper

Crous C.J.; Jacobs S.M.; Esler K.J. 2011. Drought-tolerance of an invasive alien tree, Acacia mearnsii and two native competitors in fynbos riparian ecotones. Biological Invasions.

Contact the author

For further details contact Casper Crous, e-mail casperc@sun.ac.za