Germination and growth responses of co-occurring grass species to soil from under invasive Thymus vulgaris

Identifying the mechanisms underlying invasion success is important for our understanding of why some exotic plants negatively impact the biodiversity and functioning of only certain ecosystems. Allelopathy is one potential mechanism of invasion in which invasive plants produce secondary compounds (allelochemicals) that inhibit the growth of nearby organisms. Thymus vulgaris L.is an aromatic perennial, endemic to the western Mediterranean, which in its native range affects other species via allelopathy, but overall appears to facilitate native diversity. Thymus vulgaris has invaded thousands of hectares of Central Otago, southern New Zealand where it grows at high densities in relatively monocultural communities in which native species are less common than exotic species. We examined the effects of soil collected from under thyme and from away from thyme, from both north- and south-facing slopes, on the germination and seedling biomass of three common exotic (Bromus diandrus Roth, Dactylis glomerata L. and Vulpia myuros (L.) C.C.Gmel. var. megalura (Nutt.) Auquier) and two common native (Anthosachne aprica (Á.Löve et Connor) C.Yen et J.L.Yang, Poa colensoi Hook.f.) thyme-associated grass species. We detected small quantities of the allelochemicals thymol and carvacrol in soil under thyme. Soil from under thyme had no effect on germination rates or seedling growth for either native or exotic grasses. Native grasses had inherently lower germination rates and seedling biomass than exotic grasses, and these differences between natives and exotics were the most striking in our results.