Distinctive vegetation communities are associated with the long-lived coniferAgathis australis(New Zealand kauri, Araucariaceae) in New Zealand rainforests

Abstract

The conifer Agathis australis (New Zealand kauri; Araucariaceae) has a significant influence on soil processes beneath its canopies, reducing soil pH, stalling nitrogen cycling processes, and sometimes forming podzols. Distinctive plant species assemblages have been anecdotally observed to occur in association with A. australis stands; however, the authenticity of these proposed associations has not been formally assessed. Owing to the effects of A. australis on its soil environment and the recorded vegetation patterns, we hypothesized that this species may act as a foundation species, playing a significant role in structuring plant community composition in its vicinity. To test this, we investigated the influence of proximity to A. australis on plant community composition at stand and individual tree scales. We also investigated compositional variation with distance from the conifer Dacrydium cupressinum (rimu, Podocarpaceae) within the same forests to directly compare A. australis effects to those of another large conifer. We examined changes in stand composition relative to the abundance of each of these conifers at two forests, and measured changes in environmental conditions and plant composition with increasing distance from mature individuals at one of the same and one other study site. The organic soil formed beneath A. australis individuals was highly acidic, with high levels of NH4-N, carbon and total nitrogen, but low levels of NO3-N. We recorded a difference in species composition in the vicinity of A. australis compared to forest without this species in the same environment, describing three groups of species: stress-tolerant species dependent on the presence of A. australis within mature forest; those dependent on areas with A. australis absent; and those with distributions unaffected by A. australis presence. Such effects on the abiotic and biotic environments were not recorded in the vicinity of individuals of D. cupressinum. These results highlight the substantial effect that A. australis has in enhancing landscape-scale habitat heterogeneity and influencing overall forest diversity.