The Trus Madi Association occurs above about 1,200 m on the Crocker Range and extensively on the Trus Madi Range reaching 2,580 m at the summit of Gunung Trus Madi.
The range is composed of sandstone, shale and siltstone, the sandstone being coarse-grained, often quartzitic and with many characteristics of greywacke. These rocks outcrop on many of the steep mountain slopes and also on ridge crests but over large areas soils are formed on mixed colluvium derived from these rocks.
Dipterocarp forest covers the lower slopes of the association below about 1,350 m and also extends upwards beyond this height in sheltered valleys. It gives way an oak/conifer forest dominated by Fagaceae and Lauraceae species. Above 1,950 m, low trees and shrubs are covered by leafy hepatics and mosses in what can be described as a moss forest. Species include Ericaceae, Podocarpaceae, Fagaceae, Theaceae, Lycopodiaceae, Burmaniaceae, Myrsinaceae, Guttiferae and Myrtaceae.