The Harenna Forest covers almost half of the park in area, at the southern park boundary and is a magnificent example of moist tropical forest, about which very little is known. It extends over a wide range of altitude, from 1,500 to 3,500 m, bridging the gap between Combretum/Terminalia dry, wooded grasslands and Afro alpine moor lands in just 60 km, probably the only place in Africa where this still occurs.
The upper part of this forest area is on steep slopes and differs little from the northern woodlands. A notable absence is Juniperus. The tree cover is very high and dense stands of Mountain Bamboo (Arundinaria alpine) cover large areas.

Below about 2,200 m, the slope is much gentler, and larger tree species appear, Podocarpus gracilior in particular. Trees over 30 m in height are common, there is good shrub undergrowth, a secondary canopy, lianes and dense epiphytic growths on all larger trees.
The steeper slopes of the forest on the Harenna Escarpment are swathed in mist for much of the year, often all day. There are no rainfall records for the area, but it is evident from the many small streams, dense epiphytes, and the dense forest and undergrowth, that this area receives, retains, and slowly releases, much water (Hillman, 1986).