Abstract:
In New Caledonia, dense rainforest on ultramafic substrate is the richest, one of the most endemic and most threatened ecosystems. Since human settlement about 3,500 years ago, repeated fire events led to the significant reduction of these forests, and their replacement by maquis shrubs. Intensive exploitation of nickel on ultramafic environments since the late 1800s, and forest prospecting efforts for logging since colonial times, further contributed to the high degradatoin of this ecosystem. Dense rainforests on ultramafic soils are now dramatically fragmented and occupy a mere 7 percent of the land surface of Grande Terre, the main island of New Caledonia. In order to limit the threat to this rich ecosystem, the ecological restoration of endemic corridors between residual patches of a fragmented forest was planned, with the long-term objective of rehabilitating a self-sustaining endemic rainforest. However, the austere composition of these ultramafic soils, the competition with persistent species that have replaced the forest, and the complex biology of endemic species that are adapted to ultramafic substrates, require a detailed study of candidate species for this revegetation project. The aim of this study was therefore to identify suitable endemic species for this purpose, and to determine some natural conditions that might influence their successful establishment. The identification of suitable species was implemented after a careful pre-selection of 26 candidate species, based on consultations with experts, review of the botanical literature, and collection of field data. Field work was carried out to determine the critical environmental parameters that exert an influence on the presence and recruitment of candidate species. The suitability of these species was assessed with regards to their ecological behaviours, which were compared to the theoretical model of long-lived pioneers. Indeed, such species are recognized to colonize forest gaps, where they rapidly set up a canopy that recreates a favourable microenvironment for shade-dependant and understorey forest species to thrive. Multivariate statistical analyses of data led to the development of a final list of the most suitable species, and the key environmental variables that influence their establishment. Results of this research allowed for the proposition of a first draft of ecological corridors design and recommendations for complementary management measures, which should be applied to optimize the ecological relevance and improve the likelihood of success of the project.