Can fire reverse the invasion effects of Cytisus scoparius in a montane grassland ecosystem?

Background/Question/Methods

Invasion of grasslands by woody plants severely alters the native plant community composition, structure and ecosystem function. Native plants are directly displaced, or indirectly impacted by novel ecosystem properties created by invasive species. Feedbacks between invasion and disturbance regimes push recipient ecosystems to new states. The montane grasslands of the Nilgiris are an important part of the Western Ghats (of India), a global biodiversity hotspot. The grasslands are maintained by climatic and edaphic factors. The extent of these grasslands has been greatly reduced over the past two centuries by large-scale commercial plantations, agriculture and hydel projects, and more recently invasion by exotic woody plants. Historically, the indigenous people burned the grasslands periodically, but now the local authorities have followed fire suppression for the past few decades. My research examines the impact of invasion by Cytisus scoparius (a nitrogen-fixer) on the native plant community and ecosystem properties, and the role of fire in altering these relationships. Three surveys, in wet and dry seasons, were carried out in four natural treatments: interactions of two levels (presence/ absence) each of fire and invasion. Percent cover of herbaceous species, above ground biomass, available and mineralizable soil nitrogen were sampled in the surveyed plots.

Results/Conclusions

Invasion resulted in significant decreases in species richness regardless of burning. Invaded sites had significantly lower diversity and higher dominance. The species composition of burned-invaded plots was similar to burned- and unburned-non-invaded plots. Invasion increased the cover of a common C3 grass and decreased that of C4 grasses. The cover of one common native forb was higher in burned sites while that of another was higher in invaded sites. There was significant decrease in available ammonium in burned sites in the wet season while the pattern was reversed in the dry season. Invasion failed to significantly influence the levels of soil available nitrogen or the rates of mineralization. Patterns in plant community properties under the different natural treatments suggest that fire can promote the recovery of native communities in invaded sites. The effects of invasion appear to be stronger on plant community structure than on ecosystem function. High diversity and endemicity are special features of the Nilgiris grasslands and over 150 plant species can potentially be displaced by C. scoparius. This study has highlighted the deleterious consequences of invasion on native habitats, and suggested that fire may be a useful tool in reversing invasion effects.