Soil seed bank under variegated thistle does not explain thistle dominance

Variegated thistle (Silybum marianum) is a large, spiny annual that often forms dense monospecific communities on dry ridges and sunny hillslopes. The owner of a typical Poverty Bay hill-country farm with persistent variegated-thistle infestations reported that winter applications of herbicide were ineffectual in the long term as more variegated thistles simply recolonised the sprayed sites. An absence of preferred species, particularly perennial ryegrass (Lolium perenne) and legumes (Lotus and Trifolium spp.), in the soil seed bank under dense thistle populations may explain the persistence of these monospecific populations. To test this hypothesis, soil samples were collected from a dense and sparse variegated-thistle population in each of seven paddocks and incubated in a glasshouse. Emerged seedlings were identified and counted. The incubation was repeated three times. Total soil seed numbers were similar under both the dense and sparse populations with similar numbers of preferred legumes under both. However, there were significantly more perennial ryegrass seeds under the dense variegated-thistle populations compared with the sparse ones. Domination of thistles in densely infested patches was not due to lack of preferred species, or indeed other weed species, in the soil seed bank.