About thirty million years ago, the area was covered by extensive
rainforest. During a cooler and drier period some ten million years ago, the lowland forests were converted to
savanna, leaving the mountain ranges as "islands" where the tropical forests continued to flourish. The long-term persistence of a humid climate and the isolation of each mountain range has led to a great deal of
endemism, and a very diverse flora and fauna. The Ukaguru and other Eastern Arc mountains have extremely high biodiversity with numerous endemic species (more than 25 percent of the vertebrate species). The Ukaguru Mountains are covered with
miombo woodland on the eastern and southern slopes,
Acacia-Commiphora woodland and savanna on the northern and western slopes, and montane rainforest, dry montane forest, montane grassland at higher elevations. Forests extend from 1500 to 2250 meters elevation. An analysis of satellite images taken between 1999 and 2003 found 172 km2 of the mountains were still covered in evergreen forest. Tree species in the montane evergreen forests include
Balthasaria schliebenii, Ocotea usambarensis, Podocarpus milanjianus, and
Polyscias stuhlmannii. The forests are less diverse, and have a lower canopy, than montane forest in the other Eastern Arc mountains. Large areas of degraded montane grassland lie west of the main ridgeline. The toads
Nectophrynoides laticeps and
N. paulae are endemic to the Ukaguru mountains. The plants
Peddiea thulinii and
Lobelia sancta are endemic to the Ukagurus. ==Protected areas and conservation==