A third layer of shade-dwelling trees is sometimes identified beneath the main canopy, with a sparse ground vegetation under that. The emergent layer is dominated by members of the Dipterocarpaceae family, of which there are ten genera and around 270 species in Borneo alone, and there may be over 2,000 species of trees with stems at least 10. cm in diameter in the primary forests of Sarawak and Brunei (ibid.). The forest structure is spatially extremely heterogeneous, with topographic influences limiting the occurrence of large trees on the steepest slopes, and smaller-scale disturbances such as tree falls and occasionally landslides creating gaps which are available for recolonization by pioneer species. The overall