Germination, growth, reproduction, and population structure of three subspecies of lepidium sisymbrioides (brassicaceae) with respect to taxon rarity

Abstract

Lepidium sisymbrioides comprises three subspecies endemic to South Island, New Zealand. The taxa have very different abundances: L. s. ssp. sisymbrioides is geographically widespread, L. s. ssp. kawarau has a few populations in two localities, and L. s. ssp. matau has only one main population. This study sought ecological reasons for these differences in abundance. Population structures, flowering, and seed production were recorded in two populations of Lepidium sisymbrioides ssp. sisymbrioides (Patearoa, Pisa Flats), two populations of L. s. ssp. kawarau (Slapjack Creek, Falls Dam), and the only known population of L. s. ssp. matau (Galloway). Seed germination for each population was tested in vitro. Germination, growth, flowering, and rosette production were recorded in reciprocal sowings of seed from the five populations in soils from each of the sites. Taxonomic characters were compared between plants of these five and a third population of L. s. ssp. kawarau also grown in these soils. In vitro seed germination varied between populations and between years. L. s. ssp. kawarau seeds from Falls Dam failed to germinate in vitro, and their germination rates in soil were lower than those of the other populations. Seedling growth rates were similar between populations. Falls Dam L. s. ssp. kawarau plants grown from seed flowered one (male) or two (female) years later than those of the same sexes from other populations. Thereafter, flowering rate and frequency, and rosette production, did not differ markedly between populations in cultivation, as was also the case in the wild. Soil bioassay (Lolium perenne dry weights) indicated that soils occupied by L. sisymbrioides plants were more fertile than adjacent soils, but there were no differences in fertility between the soils occupied by different populations of L. sisymbrioides. There was a gradient of taxonomic characters between two populations of L. s. ssp. sisymbrioides and L. s. ssp. matau, between three populations of L. s. ssp. kawarau, and between these two main groupings. Differences in germination, growth, reproduction, and population structure between taxa and populations did not explain their relative abundance in the wild. © 1998 Taylor & Francis Group, LLC.