Alien grass invasion of Renosterveld : influence of soil variable gradients

[摘要] This thesis examines the role of agricultural activity in the process of invasion of westcoast renosterveld fragments by annual alien grass species. This highly endangeredvegetation type has less than 5% remaining, it is vital to understand the mechanismsallowing invasion of annual alien grasses in order to effectively prevent the loss of themany rare and endemic species found in west coast renosterveld. This study wasdivided into three major components.Firstly the distribution of indigenous and alien plant species in relation to fence lines,separating active agricultural fields from untransformed vegetation, was described.Regression analysis was used to test for relationships between distances fromagricultural fields and soil physical and chemical characteristics in natural vegetation.Cover by annual alien invasive grasses in untransformed vegetation decreasedsignificantly with distance away from agricultural land.Secondly alien and indigenous grass seed banks were sampled along the transects, atthe same sites, in order to establish whether the seed banks correlated with aboveground cover. Results varied among sites and seed banks were correlated with thevegetation cover at only one site. It appears that there are a multitude of factorsdetermining the distribution of annual alien grass cover.Thirdly a greenhouse experiment established the role that nitrogen plays in the successof the alien grass Avena fatua. This species was grown in competition with threeindigenous species, an annual forb (Dimorphotheca pluvialis), a geophyte (Oxalispurpurea) and an indigenous perennial grass (Tribolium uniolae) at three levels of soilnitrogen. The geophyte was largely unaffected, while growth of the annual andindigenous perennial grasses was negatively affected by competition with A. fatua.Nitrogen did not seem to affect competitive interactions. Management of these renosterveld patches, in order to conserve them effectively, willrequire a multi-faceted approach, including prevention of further invasion and removalof invasive grasses already present.

[发布机构] Stellenbosch University