Tropical montane forests are fragile ecosystems that provide a wide range of ecosystem services such as hydrological services, protection of biodiversity, and a contribution to climate change mitigation, yet they face degradation as well as losses due to deforestation. Deforestation poses a major threat yet whether these tropical montane forests recover from these changes is not well understood, especially for African montane forests. This study assessed rates of deforestation, and recovery using remote sensing of two important tropical montane forests of East Africa: the Mau Forest complex and the Mount Elgon forest. An in-depth study of aboveground biomass, species diversity and richness, and soil carbon and nitrogen stocks were conducted for the Mau forest complex. To conduct the detailed study, 47 forest plots were established to collect data subsequently used to calculate the rate of recovery of the aboveground biomass (AGB) and species recovery in 3 blocks of the Mau forest complex. From the same plots, soil samples were collected to assess the response of soil carbon (C) and nitrogen (N) stocks to 60 cm of soil depth from the different recovery stages.
This study found that 21.9% (88,493 ha) of the 404,660 ha of the Mau forest Complex was lost at an annual rate of -0.82% yr-1 over the period between 1986-2017. However, 18.6% (75,438 ha) of the forest cover that was cleared during the same period and is currently undergoing recovery. In the Mt Elgon forest, 12.5% (27,201 ha) of 217,268 ha of the forest cover was lost to deforestation at an annual rate of -1.03 % yr-1 for the period between 1984 - 2017 and 27.2% (59,047 ha) of the forest cover that was lost is undergoing recovery. The analysis further revealed that for the Mau forest complex, agriculture (both smallholder and commercial) was the main driver of forest cover loss accounting for 81.5% (70,612 ha) of the deforestation, of which 13.2% was due to large scale and 68.3% was related to the smallholder farming. For the Mt Elgon forest, agriculture was also the main driver of forest loss accounting for 63.2% (24,077 ha) of deforestation followed by the expansion of human settlements that contributed to 14.7% (5,597 ha) of forest loss. For the aboveground biomass (AGB), it was found that AGB recovered rapidly in the first 20 years at an annual rate of 6.42 Mg ha-1, but the rate of recovery slowed to 4.67 Mg ha-1 at 25 years and 4.46 Mg ha-1, at 30 years of age. At 25 years, the mean AGB (198.32 ± 78.11 Mg ha-1) was statistically indistinguishable from the mean AGB in the old growth secondary forest (282.86 ± 71.64 Mg ha-1). Stem density, species diversity, and richness (i.e., Evenness index, Shannon’s index, and Simpson’s index) did not show any significant changes with the recovery stages of the secondary forest, although there existed a significant variation between the young secondary forests of age below 15 years from the old growth secondary forests. The study further found that, unlike the AGB and aboveground carbon (AGC), the soil C and N stocks were not significantly different across the recovery periods with mean soil C in the youngest forest 184.1 ± 41.0 Mg C ha-1 and old growth secondary forest as 217.9 ± 51.8 Mg C ha-1, the N stocks in the youngest forest was 16.4 ± 4.8 Mg N ha-1 and 20.1 ± 3.9 Mg N ha-1 for the old growth secondary forest.
The findings of the study indicate that these tropical montane forests of East Africa are under threat resulting from forest clearance and deforestation. The forest AGB recovers after 25 years while the tree species richness and diversity, soil C and N stocks do not change significantly with the recovery stages. The effects of disturbances i.e., forest fire, charcoal burning, grazing (livestock), elephant damage, and fuelwood collection on the soil C and N stocks within the different recovery stages were not significantly different between old growth secondary forests and the other recovery stages. These findings contribute to the knowledge on the response of the tropical montane forest of East African to pressures of forest clearance and deforestation.