Population dynamics of the emergent coniferAgathis australis(D. Don) Lindl. (Kauri) in New Zealand II. Seedling population sizes and gap-phase regeneration

Abstract

The view that kauri (Agathis australis) is a successional species which does not regenerate in mature forest is entrenched in the New Zealand literature. However, seedling and sapling populations ranging from c. 200 to >2000 stems ha^-1 were recorded in 25 mature kauri stands throughout the species' range in the North Island. Higher densities were recorded in gap-phase and successional communities. Gaps created by the fall of mature kauri trees at Trounson Kauri Park averaged 0.04 ha, and a few contained abundant kauri seedling and sapling populations. Denser seedling aggregates were sometimes associated with rotting stumps or logs on the forest floor. Trays placed in gaps showed kauri germination and survivorship rates greater than those placed beneath adjacent canopies. Our data support a “cohort regeneration model” in which dense regeneration occurs in successional communities following large-scale disturbance. This leads to self-thinning “ricker” stands in which seedling recruitment is rare, producing a localised “regeneration gap”. Continued mortality, increasing as the initial survivors begin to senesce, creates a higher frequency of canopy gaps, allowing a second less synchronous wave of recruitment to occur. However, many gaps may be lost to hardwood species so that succeeding cohorts are less dense. Despite a low efficiency of gap capture, the great longevity of kauri ( ≥ 600 years) implies that the species will survive on any site for 1500 to 2000 years, long enough for large scale stochastic disturbance by landslip, storm, or fire to reinitiate the process.