The rainforest rises above 1500 m, each level associated with a particular type of vegetation. This central mountainous zone is interspersed with wide alluvial plains, below 500 m, covered in rich sago palm swamp forests. It is an area of high relief with deep valleys in between the mountain peaks. The major Highland valleys have turned into grassland areas as a result of intense agricultural activity.
Population densities display great disparity in New Guinea.
In the Highlands, sweet potato monocropping and a healthy climate allow for high population densities, over 100 inhabitants per km 2 in some places. On the other hand, forest populations have lower densities, inferior to 10 and often closer to 1 or 2 inhabitants per km 2 . The groups living in lowland grassland areas have population densities of 15 to 30 inhabitants per km 2 . New Guinea's forests are on very rugged terrain and there are therefore hardly any roads, but a few of the larger rivers are navigable. As a consequence, all ethnic groups who do not live along these routes are scattered throughout the mountains and live off the forest.