The anomaly of monodominant tropical rainforests: some preliminary observations in theNothofagus-dominated rainforests of New Caledonia

ABSTRACTNothofagusspecies dominate small patches of rainforest on ultramafic soils in New Caledonia, forming an almost monospecific upper canopy on some sites. These stands are commonly bordered by rainforests of greater floristic and structural complexity, also on ultramafic soils. In two lowland stands, the population size structures ofNothofagus aequilateralisstems greater than 50 cm high had an approximately skewed bell-shaped distribution with a modal size class of 15–25 cm stem diameter, and with no evidence of old trees. This indicates that the establishment ofNothofagushas occurred over a limited time span, without sufficient recent regeneration to allow the future replacement of the existing canopy trees. The factors initiating the major period of regeneration are not certain, but large-scale disturbance is implicated and evidence of past wild-fire on both sites is consistent with this hypothesis. It is unlikely that significant regeneration ofNothofaguswill occur as the stands mature and canopy gaps are created by the death of old trees, since saplings of other species already occur at high densities in the understorey and will probably pre-empt the resources made available by canopy gaps. Two other study sites were situated above 900 m asl. At Dzumac, in an apparently older stand dominated byN. codonandra, there is some evidence of recent regeneration, predominantly confined to a single large canopy gap. The population size structure ofN. baumanniaeon the fourth site, on the summit of Mt Mou, shows a closer approximation to a reverse-J curve. Seedlings, however, are relatively uncommon on this site, and the numerous saplings may be suppressed rather than young.Analyses of topsoil provided no consistent evidence to suggest that mineral content is controlling the distribution ofNothofagusacross vegetation boundaries.The environmental factors promoting the formation of these monodominant canopies are uncertain, but appear to involve large-scale disturbance by windstorm or fire. The population size structures suggest that the canopies of the two lowland stands, at least, are likely to become more species-rich, with a progressive decline in the frequency ofNothofagus. Therefore, these lowland monodominant forests cannot be regarded as equilibrium communities, irrespective of whether more detailed studies reveal environmental differences across the community boundaries. The status of the two high altitude sites is less certain.