weeds

The naturalised European blackbird (Turdus merula) is the most widely distributed avian seed disperser in New Zealand. Together with the native silvereye (Zosterops lateralis) they are the major seed dispersers over large areas of New Zealand. I review the international literature on aspects of the ecology and behaviour of blackbirds relevant to their potential for dispersing weeds in New Zealand. Blackbirds eat a wide range of native and exotic fruit including many naturalised species.

Wild ginger (Hedychium gardnerianum) is a tall rhizomatous herb that invades forests and shrubland fragments in northern New Zealand. In order to determine the impacts of this invasive weed on forest processes, comparisons of conifer-broadleaved forest patches with different densities of ginger were made at Opononi and Whangarei in Northland. Soil properties, vegetation structure, floristics, and seed rain were recorded. Annual litter fall biomass and chemical composition were measured.

The relationship between fleshy-fruited indigenous species and adventive weeds in the diet of 500 mist-netted birds was studied in forest remnants of differing size and degree of modification. Fruit abundance Peaked in March and April, and most fruit was either red/orange or purple/black. The physical parameters of adventive and indigenous fruits were not significantly different. Six of the 15 passerine species netted are frugivores, and of those netted 77% had eaten fruit.

Recent concern that honey bees may threaten natural areas by increasing weed abundances through increased pollination was investigated by reviewing the literature to determine which weed taxa surveyed from New Zealand Protected Natural Areas (PNAs) are visited by honey bees. The contribution made by honey bees to weed reproduction was assessed by checking reproductive strategies and pollination mechanisms of a subset of problem weeds. A substantial proportion of surveyed weeds in PNAs are probably visited by honey bees (43%) including half of the problem weeds.

New Zealand's protected natural areas are being increasingly threatened by weeds as the natural landscape is fragmented and surrounding land use intensifies. To assist in designing management to reduce the threat, we attempted to determine the most important reserve characteristics influencing the presence of problem weeds in forest and scrub reserves. Data on 15 reserve characteristics were derived from surveys of 234 reserves.

Replacement patterns under buddleia (Buddleja davidii) groves aged between 2 and 17 years were studied in streambeds in the western Ikawhenua Range and in the upper Waioeka catchment, Te Urewera National Park. Height and basal diameter growth followed an exponential pattern, with rapid early growth (0.5 m/year and 1 cm/year respectively), levelling off after 15 years or more. Intense self-thinning occurred in younger stands. Typical forest floor vegetation was developing within 15 years of colonisation by buddleia.

Seedlings of fifteen species of shrubs and small trees, commonly found in open sites and early stages of secondary succession, were grown in a glasshouse under light intensities of 16% and 66% full daylight, and their growth parameters (height, number of leaves, dry weight, mean relative growth rate) recorded. Three species from open habitats, Coprosma robusta and Dodonaea viscosa, and an adventive shrub, Crataegus monogyna, had the highest mean relative growth rates in 66% daylight and 16% daylight, but their ranking for other parameters (e.g. height) was variable.

Hawthorn (Crataegus monogyna) is a naturalised tree that is classified as a noxious plant in several counties in the eastern South Island. It is locally abundant in lowland forest remnants, and with the indigenous spiny shrub matagouri (Discaria toumatou), on grazing land in Canterbury. Two scrub sites near Porters Pass, where the original hawthorn trees still existed, and a forest site near Kowai Bush were sampled by measuring stem diameters and counting growth rings, to determine the age structure and dynamics of hawthorn.

Tiritiri Matangi Island (‘Tiri’) in the Hauraki Gulf of the northern North Island of New Zealand was deforested, pastorally farmed, and then farming was abandoned in 1972. This history is typical of many northern New Zealand islands. The island’s modern history is less typical; since 1984 it has been the focus of a major restoration project involving thousands of volunteers. No original forest remains, but grazed secondary forest in a few valley bottoms covered about 20% of the island when farming was abandoned. Tiri’s wild vascular flora was recorded in the 1900s and again in the 1970s.