Forest processes in the presence of wild ginger (Hedychium gardnerianum)

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. There were no significant differences between soils under forest with sparse (< 5% cover) and dense (> 50% cover) ginger. A sparse subcanopy layer and fewer saplings were common factors in ginger plots. Forest canopy litter fall (c. 6.8 t ha-1 yr-1) and total N (45.1-56.2 kg ha-1 yr-1) were similar in plots with sparse or dense ginger. Species richness of adult trees and seed fall richness varied little. In contrast, seedling density and richness were lower in dense ginger stands and seedling composition was dominated by species with large (> 200 mg) seeds, i.e. Corynocarpus laevigatus, Dysoxylum spectabile and Rhopalostylis sapida. These seedlings may produce a different forest canopy from that currently found in areas where ginger is sparse.