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Ecosystem Impacts

1. Bitou bush alters the ground level microclimate

We have found that bitou bush increases the canopy cover on the fore dune (Fig 1) which causes changes to the ground level microclimate so that it is moister, darker (Fig 1) and cooler, which is very different to the usual ground level conditions created by the hot and dry sand.

 

Fig 1: Ground level light in the bitou bush invaded and non-invaded habitats

Fig 2: The mean daily minimum and maximum temperature on the ground below bitou bush canopies (open squares) and native canopies (closed triangles) in January and February 2007

2. Bitou bush alters the decomposition rates and increases the abundance of invertebrate decomposers

Using litterbags placed in different habitats and containing either native or bitou litter, we found that the succulent bitou leaves decomposed at a significantly higher rate than the sclerophyllous native leaf mix. Time to 99% leaf loss was estimated to be 0.9–1.3 years for bitou and 3.1–4.4 years for the native species. This reflects the physical properties of the leaves and the lower leaf mass area of bitou. Bitou leaves decayed faster in coarse mesh bags compared to fine mesh, indicating leaf litter invertebrates positively influenced their decomposition. Bitou leaves generally decomposed faster within the weed infestations, partly due to invasion creating a protected environment with an altered microclimate. Replacement of native species with C. monilifera will alter nutrient cycling through changes in litter quality and decomposition rates which has implications for ecosystem resilience and stability.

 

3. Bitou bush alters the nitrogen cycle of sand dune vegetation

Field samples have identified that there is a greater input of litter to native areas, which is highly seasonal, compared with the bitouhabitat. Together with a slower decomposition rate, this results in a greater  mass of litter on the ground in the native areas. The litterfall is estimated to contain more than twice the amount of nitrogen and phosphorous within the native areas, because of the high leaf fall rate rather than a higher nutrient content. However, the total soil nitrogen was two to five times higher in the invaded areas  than within the native areas. This was accompanied by an increase in ammonia content. The bulk density was lower in invaded areas, but there was no clear trend in the change in soil pH. The higher soil nitrogen beneath the bitou infestations could increase the competitive superiority of C. monilifera directly by increasing growth rate, or indirectly by impairing the establishment of native species that are adapted to growing in nutrient-poor soils.

 

Further reading:

Lindsay E.A. and French K. (2005)   Litterfall and nitrogen cycling following invasion by Chrysanthemoides monilifera spp. rotundata' J. Appl. Ecol. 42, 556-566.

Lindsay E.A and French K. (2004) Chrysanthemoides monilifera ssp. rotundata invasion alters decomposition rates in coastal areas of New South Wales, Australia. Forest Ecology and Management  198, 387-99.

 

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