Monodominance in Nothofagus (Nothofagaceae) forests from contrasting latitudes: Insights from permanent plots into mechanisms of persistence in species-rich forests

Background/Question/Methods

Dominance of forest canopies by 1-2 species is common at temperate latitudes, but is more unusual in species-rich forests in tropical-subtropical climates. Nothofagus species dominate forest stands across a wide geographic range, from temperate to tropical latitudes (0.5-55.5˚S). Here we investigate the mechanisms allowing canopy dominance in a species-rich system, using permanent plots to compare regeneration dynamics of Nothofagus forests in tropical New Caledonia against predictions from population size structures. Trends in dominance and regeneration dynamics in Nothofagus forests are then compared along gradients in latitude and species richness.

Results/Conclusions

Population size structures in New Caledonia measured in 1990 suggest that stand-level establishment of Nothofagus populations has occurred following large-scale disturbance – lowland stands comprise many trees of similar size, with little recent recruitment past the seedling stage. These data suggest that the Nothofagus canopy will be replaced over time by a more species-rich canopy comprising species with greater shade-tolerance, but with stand-level replacement likely if a severe cyclone or wildfire occurs during the reproductive lifespan of the Nothofagus trees. However, in two study sites, multi-tree canopy gaps have been created by cyclone damage plus endogenous tree death over the last 20 years, and Nothofagus saplings are now abundant in these canopy gaps. We suggest that the unimodal population structure of many of the lowland Nothofagus-dominated forests in New Caledonia may result from an unusual regime of disturbance in the past (natural plus human causes), with insufficient time for multi-generation populations to develop. The recruitment seen in these two study sites suggests that dominance by Nothofagus might persist beyond the first generation, although it is uncertain to what extent it would persist in terms of abundance and the number of generations without ‘re-setting’ by a large-scale disturbance. It is also uncertain whether such long periods without major forest damage are likely in this cyclone-prone environment. Similar trends are evident in nearby forests dominated by the monocarpic canopy tree, Cerberiopsis candelabra (Apocynaceae). For both these taxa, mast seeding and rapid growth rates contribute to high seedling densities and recruitment in multi-tree canopy gaps. Hence, for these forests, data from permanent plots suggest a more complex pattern of forest dynamics than was originally predicted from population size structures. Despite differences in climate and regional species richness, the regeneration dynamics seen in tropical Nothofagus forests is similar to that observed in many temperate Nothofagus forests.