Untangling spatial distribution patterns of the invasive herb Hieracium lepidulum Stenstr. (Asteraceae) in a New Zealand mountain landscape

Abstract

The invasive herb Hieracium lepidulum Stenstr. (Asteraceae) has been observed increasing in a range of indigenous ecosystems in New Zealand in recent years. The ability of this species to occupy and potentially to dominate a wide range of indigenous ecosystems at the expense of native biota triggered alarm among conservation managers, and set the stage for this research. The overall aim of this thesis was to identify and explain patterns of H. lepidulum invasion on a national and landscape scale, and to provide the data necessary to model the future spread of H. lepidulum throughout the landscape.

A reconstruction of spatiotemporal spread patterns of H. lepidulum in New Zealand showed that spread has been focussed around three centres in the eastern South Island ranges where H. lepidulum was recorded relatively early. More recently, H. lepidulum has spread to West Coast and Fiordland ranges, areas of high rainfall that are potentially favourable for H. lepidulum, and which potentially provide a new 'frontier' for spread.

A survey of the spatial distribution of H. lepidulum among habitats in an upland mid-Canterbury landscape showed that high population abundances of H. lepidulum in forest creek habitats appear to act as sources for spread of H. lepidulum into adjacent beech forest and alpine habitats. These results served as the framework with which to generate hypotheses about how habitat variation in habitat resistance to invasion, propagule supply, plant performance in established populations and dispersal patterns might have generated the observed spatial distribution patterns.

A seed-sowing experiment measured among habitat resistance to invasion by quantifying how the interaction between habitat resistance to establishment and seed supply determined seedling establishment. Tussock grassland and alpine creek habitat were least resistant to H. lepidulum establishment. Forest creek and canopy gap habitat were less resistant than forest interior habitat. Variation in habitat resistant to establishment thus appeared to at least partially explain the variable pattern of distribution of H. lepidulum among forest habitats, while a lack of seed reaching alpine habitats appeared to explain the low abundance of H. lepidulum in the alpine.

Monitoring of plant performance in natural populations supported this, with plant size and reproductive output highest in tussock grassland and alpine creek habitats, and higher in forest creek and canopy gap populations than in forest interior populations.

Among habitat variation in dispersal patterns was examined by measuring dispersal curves in five habitats under different wind conditions. Wind conditions best explained variation in the number of seed dispersing, and the rate of decline of seeds dispersing with distance from source. However, there was a trend for greater seed dispersal (in terms of distance dispersed and number of seeds) in forest creek habitat relative to forest interior and canopy gap habitat, suggesting that facilitation of dispersal in forest creeks may contribute to high abundances in forest creek populations relative to the forest interior.

The uneven spatial distribution of H. lepidulum among habitats in the mid-Canterbury landscape is predominately a function of variation in habitat resistance to invasion, subsequent population growth due to variation in plant performance among habitats, and variation in propagule supply rates.