The regeneration dynamics of Araucaria laubenfelsii in maquis and forest, Mont Do, New Caledonia

Many of the southern hemisphere conifers occur as emergent trees in high diversity angiosperm forests. Despite it's small size, New Caledonia has a flora which includes 43 endemic species of conifer. This study examines the regeneration ecology of the New Caledonian conifer, Araucaria laubenfelsii Corbasson, a species which occurs on ultramafic soils as an emergent tree in rain forest and in an unusual structural association with maquis vegetation. The hypotheses focus on the processes which create and maintain the structure of the maquis-conifer community and the stand dynamics of these conifers within closed forest. The establishment, growth, survival and reproduction of approximately 2,500 individuals was followed in permanent plots, within the Mont Do Botanical Reserve. This study utilized accepted demographic techniques to examine the regeneration status of Araucaria laubenfelsii in both maquis and forest. Community vegetation was also sampled and analysed and several environmental variables were explored including light environment and soil properties. The ability of Araucaria laubenfelsii to regenerate continuously in both maquis and some forest plots, given a lack of disturbance, is clear from the reverse-J shape of the size class distributions. Seedling and sapling densities in maquis plots suggest that Araucaria laubenfelsii is able to establish within the maquis environment, while densities in forest suggest more limited establishment success. Forest stands represent a successional gradient from immature sites dominated by Araucaria laubenfelsii, to more mature sites where Nothofagus codonandra or mixed species dominate. Within forest light may be the limiting factor, while in maquis cloud-combing of moisture may contribute to the successes of germination and establishment. Araucaria laubenfelsii seems able to tolerate poor soil conditions showing no signs that soil limits establishment success. Forest/maquis boundaries are maintained by disturbance and topography rather than by edaphic factors, with data showing evidence of forest expansion into maquis. Growth rates suggest Araucaria laubenfelsii is a slow growing species, showing evidence of suppression in the sapling size classes in forest environments. Growth rates in the maquis are generally faster than in the forest, for all size classes. Seedfall was minimal during the period of study and a mast year was not recorded, therefore estimates of fecundity from in-field germination probably underestimate actual fecundity for the species. Araucaria laubenfelsii on Mont Do can be considered a longlived pioneer, able to thrive under high light conditions, reflected by estimated X values in maquis consistently > 1. Araucaria laubenfelsii on Mont Do is able to survive fire once it reaches a critical size leaving adults to act as a seed source for the establishment of a new cohort. Most of the existing seedlings and saplings are killed by fire. Cyclonic disturbances are infrequent and may result in blow-down of large individuals within both maquis and forest. All of the individuals which were blown-down during the study were previously fire scarred. Around fallen stumps, seedling densities can be high, reflecting both the low dispersability of Araucaria seeds and enhanced moisture from the shading of the adult, when it was alive. Disturbance by fire and wind, seed dispersal, and slow growth rates play a role in the regeneration dynamics of Araucaria laubenfelsii on Mont Do, New Caledonia