Silvicultural contributions towards sustainable management and conservation of forest resources in the highlands of Ethiopia
Zusammenfassung der Projektergebnisse
The Project 'Silvicultural contributions towards sustainable management and conservation of natural forest resources in the highlands of Ethiopia' was undertaken to design sustainable forest management strategy for the utilization of forest resources in the highlands of Ethiopia. Two different field experiments were conducted in the Munessa-Shashemene-Forest: One in plantation forests of exotic tree species and the other in the natural forest. While the natural forest is heavily degraded, plantation forests are not appropriately managed. In the natural forest, enrichment plantings were undertaken as a measure to restore degraded natural forests. Four ecologically and economically important indigenous tree species (Junipurus procera, Cordia africana, Prunus africana, and Podocarpus falcatus) were raised in a local nursery and planted in the gaps of the degraded natural forest. The success/failure of the enrichment planting was examined on 20 gaps selected from five blocks. In addition, five gaps with no planting were used as a control variant. Results of the enrichment plantings in the natural forest indicated that survival was different among species. Two years after planting, only 23% of the C. africana and P. africana seedlings had survived, while J. procera and P. falcatus showed higher survival rate of 76% and 47%, respectively. The development of height and diameter over the two-year observation period was reasonable, except for P. africana, which was strongly affected by herbivores. C. africana showed also weakly height development, as it suffered from drought. Highest photosynthetic and transpiration rates were recorded for this species confirming its highly intolerant to drought. In contrast, P. falcatus exhibited lower photosynthesis and lower transpiration rates, which were associated with higher water use efficiency in comparison to the other three species. The result demonstrated the potential of enrichment plantings with native tree species to restore the degraded natural forest in the Ethiopian highlands. However, the success of species susceptible to herbivory, as P. africana, can only by achieved by protection measures. The success of species intolerant to drought as C. Africana can not be achieved in the event of scarcity of rainfall particularly at the early stage after planting. In the plantation forests, different management options involving three silviculture treatments (control, intense promotion and conversion) and two different variants of protection (fenced and unfenced plots) were conducted in four different age classes of the most dominant plantation forests (Pinus patula, Cupressus lusitanica and Eucalyptus saligna) in the study area. The silviculture treatments are characterised by three intensity levels of thinning: non-intervention (control), promotion of potential crop trees (PCTs) by removing the competitors of the PCT (intense promotion), and the removal of all trees except the PCTs to promote the natural regeneration of the native tree species (conversion). In total 46 experimental plots, representing nine different combinations of forest types and age classes, were installed. The mature stand, ground vegetation and regeneration were in-depth investigated. Results of the plant sociological records according to Braun-Blanquet conducted on the 46 plots, showed the high diversity of the understory vegetation. In total 382 different species were counted, which cover in average 57% of the forest floor. The understory vegetation was composed of 90 woody species (timber species, shrubs, and woody climbers and lianas), and 292 non-woody species (herbs, climbers, ferns and grass species). Vegetation parameter as number of species, coverage, and species diversity were dependent of the type and age class of the forest plantations. The ethno-botanical results revealed that a total of 133 understory species were subjected to local use. Mainly herb species is used by the local residents, followed by trees and shrubs. Herbaceous plants are mainly used for pasture, but also for alimentation and for traditional medicine. Woody plants are mainly used as fire-wood and construction material. Within a total survey area of 6,762 m2 (46 plots x 49 circular plots x 3m2), 14,357 woody regeneration plants were counted under the canopy of the 46 plots. Although the study area is characterised by extensive grazing, there was unexpected sufficient number of naturally regenerated saplings of native species under the canopies ofthe forest plantation stands. However, the height of the regeneration varied between 10 cm and 450 cm under the different stands with lower average height and root collar diameter in the middle-aged plantations with very dense crown canopy. Most of the regeneration plants are damaged or used by the local residents. However, the most frequent cause of damage is herbivory. The silvicultural treatments, already one year after intervention, had surprisingly a significant impact on the diameter growth of the selected potential crop trees. Even the old-aged stands exhibited a considerable reaction on the silvicultural treatments. Results clearly indicated that the more intense the promotion, the higher the radial increment, irrespective of stand age. The preliminary results of the short-term observation period were promising, but a longer term observation-period is required for gaining sufficient information about the growth of the regeneration and mature stands in order to provide recommendations on a scientific basis for the sustainable management of the plantation as well as the natural forest in the highlands of Ethiopia.
