Ecosystem level impacts of invasive Acacia saligna in the South African fynbos

Published source details Yelenik S.G., Stock W.D. & Richardson D.M. (2004) Ecosystem level impacts of invasive Acacia saligna in the South African fynbos. Restoration Ecology, 12, 44-51.
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Summary

The nitrogen-fixing, golden wreath wattle Acacia saligna, (native to Australia) was introduced to South Africa where it is now considered invasive in some areas. Since at least 1968, it has been invading the undisturbed, floristically diverse fynbos within the Riverlands Nature Reserve. Originally colonising and dispersing from trees planted on neighbouring land, by 1995 A.saligna covered 300 ha of the 1,300 ha reserve. Tall, dense stands dominated large areas, thereby threatening the native flora (including 41 of Riverlands' fynbos plant species considered critically endangered) and the ecosystem as a whole.

In 1998, A. saligna became one of the woody invasives to be targeted by the Working for Water Programme and areas were clear-cut as a control measure. This allowed the opportunity to investigate changes in nitrogen cycling regimes in the fynbos and the effect of the clear-cutting on soil microclimate and other consequences resulting from its removal.

Golden wreath wattle removal: Golden wreath wattle was cleared from plots using chain saws and axes in December 1998. A 1-year study (March 1999 to February 2000) was undertaken to determine whether this clearance amplified the rate of nitrogen (N) mineralization as a result of changing soil micronutrients.

In addition to plots cleared of acacia, were two other treatments or controls. One was intact Acacia thicket and the other undisturbed pristine fynbos. All three treatments were on the same soil type comprising well-drained, aeolian acidic sands.

Each treatment had three plots containing four 3 x 3 m sub-plots. Leaf litter was not removed from the plots following Acacia clearance.

Litter and soil: Litterfall, litter quality, soil nutrients and available soil nitrogen were measured in each of the three plots.

To measure nitrogen return to the soil, leaf litter from the Acacia and fynbos plots was collected every three months from four litter traps (measuring 20 x 16 cm). The litter was dried and weighed, and total nitrogen content was determined. Inorganic nitrogen availability was calculated.

A glasshouse bioassay experiment was also conducted to see whether the changes in soil nutrient availablity would enhance the success of Erhata calycina, a native fynbos grass species.

Weather: Rainfall was 91 mm lower than the 40 year average, with rainfall peaking later in the season than usual.

Both cleared (immeadiately after clearance) and uncleared Acacia stands had four times the amount of litterfall as pristine fynbos plots, with the acacia litter containing double the nitrogen concentration of fynbos litter. Levels of nitrogen returned to the soil from above ground biomass were 10 times greater in acacia plots (cleared and uncleared) with more of this nitrogen available in acacia soils than in fynbos soils.

This demonstrates that removal of acacia stands does not exacerbate the increased levels of available nitrogen. It had been feared that clearing could enhance nitrogen mineralization by causing changes in soil moisture and temperature. Although changes in soil moisture and temperature were recorded, no evidence for enhanced nitrogen mineralization was apparent.

However, the opportunity for at least one nitrophilic species to gain a competitive advantage over other slower-growing fynbos species seedlings was demonstrated. This was the weedy grass Erharta calycina, a native to the fynbos but never usually dominant. E.calycina was shown to have an increased growth rate and greater biomass when grown in nitrogen-rich acacia soils compared with the native fynbos soils.

This raises the possibility of secondary invasion after initial clearance of Acacia. To combat this, it is recommended that after Acacia clearance, managers use controlled burns, as well as the addition of mulch with a high carbon:nitrogen ratio, to reduce the amount of available nitrogen in the soil.

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