The environmental determinants of Ascarina lucida distribution in south Westland, New Zealand

Abstract

Possible environmental determinants of the South Island distribution of Ascarina lucida (Chloranthaceae) Hook. f. were studied at three spatial scales: within 40x20m permanent plots in Saltwater Forest, across Saltwater Forest, and across south Westland. Ascarina is a widespread, sparse, endemic plant species. Palynological records show the density and distribution of Ascarina has declined substantially since c. 5000 years BP from its maximum distribution in the late post glacial (c. 5000 - 10000 years BP). In the North Island Ascarina is rare, and is known only in small, localised populations. However, in the Westland region of the South Island, Ascarina is more abundant. Environmental variables (canopy cover, establishment site, ground cover and plot characteristics such as slope), were measured in the Timberlands West Coast Ltd permanent plots at Saltwater Forest, to establish determinants of Ascarina distribution at individual and forest scales. Environmental determinants of Ascarina distribution at a regional scale and species associations with Ascarina were studied through existing records held in the South Westland Management Evaluation Programme (SWMEP) database at Landcare Research. The effect of increased light and disturbance on the density, age, and size of Ascarina individuals was also studied in Saltwater Forest through a comparison of Ascarina on cut line tracks and in the intact forest. Ascarina was found to establish on raised microsites, under decreased canopy cover, and in sloped plots, located on moranic landforms at low altitudes. It was not found in areas with little or no canopy. Strong species associations were found in the SWMEP data Ascarina was found with a suite of species known to occur on well drained, unpodsolised soils, including Hedycarya arborea, Myrsine australis, Freycinetia baueriana and Ripogonum scandens. Despite occurring under decreased canopy cover in the permanent plots, populations of Ascarina on the cut line showed no differences in density, size or age to intact forest populations; most likely an artefact of study design. The results suggest that three environmental variables determine the distribution of Ascarina at all scales studied; Ascarina requires high light, free-draining soils, and high humidity sites free from severe frost. Sites with these characteristics are limited in New Zealand, and these somewhat contrary environmental conditions for establishment and growth, which are most likely due to the tropical origins of this species, limit the distribution of Ascarina.