Yirdaw, E. 2002. Restoration of the native woody-species diversity, using plantation species as foster trees, in the degraded highlands of Ethiopia. Doctoral thesis. Univ. Helsinki Tropical Forestry Rep. 24.
The potential of fast-growing forest plantation species to enhance the recruitment, establishment and succession of native woody species in the degraded Ethiopian highlands was studied. The naturally-regenerated woody species diversity and ground layer vegetation cover were studied in plantations of Eucalyptus globulus, Pinus patula, Cupressus lusitanica, Grevillea robusta, and Juniperus procera, and in surrounding natural forests in Wondo Genet, Menagesha and Chancho, Ethiopia. Furthermore, the canopy photosynthetic photon flux density transmittance of these five forest plantation species and the growth of native Podocarpus falcatus seedlings in canopy gaps of plantations were investigated. In addition, to understand the germination ecology and consequently the recruitment of native tree species, the germination response of Cordia africana, Juniperus procera, Acacia abyssinica, and Faidherbia albida seeds to red to far-red ratios and temperature were studied.
At Wondo Genet, a total of 53 naturally regenerated tree and shrub species belonging to 31 families were recorded in the understory of the plantations; important indigenous timber species were also represented. Trees accounted for 72% of all naturally-regenerated woody plant species. In eucalypt plantations at Menagesha and Chancho, a total of 22 and 20 woody species belonging to 18 and 17 families were found and, out of these, trees accounted for 68 and 55%, respectively. About 77 and 83% of the woody species found in the adjacent natural forest were also represented in the understory of plantations at Wondo Genet and the eucalypt plantation at Menagesha, respectively. However, the relative abundance of species in the plantations and the adjacent natural forest varied considerably. The understory woody plant density in plantations was up to 8,325 stems/hectare. There was no significant variation in understory woody species richness among plantations. The herbaceous ground cover percentage in G. robusta and P. patula stands was considerably higher than that observed in C. lusitanica and J. procera stands.
Woody species richness and abundance at Menagesha were on the average 2.4 times and 5.7 times higher, respectively, than the corresponding values at Chancho, and these differences were significant. This result demonstrated the crucial role of the remnant small patches of natural forests, as a source of diaspores for the restoration of the woody species diversity in degraded areas of the Ethiopian highlands.
Canopies of E. globulus, P. patula and G. robusta transmitted about three times as much photosynthetic photon flux density as J. procera or C. lusitanica plantations. In contrast to J. procera and C. lusitanica, E. globulus and G. robusta had relatively open crowns, higher crown-bases and lower leaf area indices, and, as a result, their canopies had a higher photosynthetic photon flux density transmittance percentage as well as higher below-canopy red/far-red ratio and temperature.
The mean height and root-collar diameter of P. falcatus seedlings decreased steadily from gap centre towards gap edge and further to the plantation understory. As the gap size decreased from 668 m2 to 449 m2, the height and root-collar diameter of P. falcatus seedlings decreased by 27% and 19%, respectively. In general, opening of gaps in plantations of heavy-shading tree species seems to increase the herbaceous layer ground cover, enhance the colonisation and growth of native woody species and, consequently, may also increase the floral diversity of mono-specific plantations.
Germination of Cordia africana seeds was strongly inhibited at low R/FR and increased as the temperature increased from 15 to 30oC. It appears that C. africana has evolved a light quality sensing mechanism that prevents seed germination beneath leaf canopies. R/FR, temperature, and their interaction significantly affected germination of J. procera. The effect of R/FR on germination of J. procera was most pronounced, and the highest germination probability was recorded at 20oC. Seeds of this species displayed a complex dormancy mechanism and germinated only within a narrow range of temperatures and R/FR. The germination probability of F. albida seeds increased with rising temperature (from 15 to 30oC), but there was no significant effect of light. Neither R/FR, temperature, nor their interaction had a significant effect on the germination of A. abyssinica seeds.
The density of naturally-regenerated woody plants in plantations was over three times the usual planting density in Ethiopia, indicating a high potential of forest plantations for restoring the natural forest ecosystems on degraded lands at a comparatively low cost. In order to fully re-establish the diverse and economically valuable natural forest, complementary measures such as enrichment planting of missing primary forest species may be required. The small isolated remnant natural forests are the only native woody species refuges left in many parts of the highlands, and they are also the only source of diaspores. Therefore, the linkage between plantations and natural forests should be realised and hence the conservation of these natural stands should be given high priority. Although there is a lack of quantifiable practical standards for biodiversity evaluation, natural forest stands near a restoration site can initially provide baseline data for the evaluation of the extent and rate of woody plant recruitment and establishment in plantations.
Key words: Afromontane forests, degraded lands, plantations, natural regeneration, natural succession, restoration, woody species diversity, Ethiopia
Author's address: Eshetu Yirdaw, Viikki Tropical Resources Institute (VITRI), P.O.Box 28 (Koetilantie 3), FIN-00014 University of Helsinki, Finland