Pattern and dynamics of remnant dry afromontane forests : a case study in Northwestern Ethiopia

Thesis (PhDFor)--Stellenbosch University, 2019.

ENGLISH ABSTRACT: Often forest managers get confronted with the management of a new forest area for which no information is available. The concept of this dissertation is to use a combination of common analytical tools to develop a first approximation of the ecological status of Afromontane forests in northwest Ethiopia. This included analyses of the floristic-structural composition and species associations of these forests, their plant-plant and plant-site relationships, and their response to different disturbance factors. These aspects form the basis for developing a sustainable resource use management system. The floral composition, plant-plant and plant-environment interactions, spatial scales of disturbance affecting regeneration of species differently, and population status of species, were investigated in three Afromontane forests of northwest Ethiopia, to develop a first approximation of the ecological status of forests. A systematic sampling design in homogenous stands was employed to collect vegetation and environmental data. Data were collected from 150 nested circular plots, with 50 plots sampled in each of three Afromontane forests, namely Alem Saga, Gelawudiwos and Tara Gedam. Each plot consisted of a main plot of 100 m2 and a sub-plot of 1 m2 , respectively for the woody and herbaceous species. Soil and litter were collected from the subplot. Topographical variables, including altitude, aspect and slope, were collected from each plot. Species were grouped into four propagule types, based on the type and size of the part of the fruit or seed or combination of the two, that gets dispersed by dispersal agents: fleshy large, fleshy small, dry large and dry small. Propagules considered as small when <5 mm diameter, and large with ≥5 mm diameter. Diversity and diversity profiles were analysed, using Shannon index and Renyi index, respectively. The floral similarity with other Afromontane forests in Ethiopia and Africa, and woodlands from Ethiopia, were analysed, using Sørensen similarity index. TWINSPAN and DCA analyses (indirect gradient analysis) were used separately for woody and herbaceous species to identify plant communities. Direct gradient analysis (CCA) was used to investigate the relationship between the identified communities and various environmental variables. Spatial ordination analysis, using DCA, was used as an analytical approach to investigate the scales of disturbance (gap sizes), affecting the regeneration of canopy tree species. The population structure of selected tree species was analysed using stem diameter class distributions across different communities. A total of 209 vascular plant species, including 109 woody and 100 herbaceous species, were recorded. Seven of these species were known to be endemic to Ethiopia. The species presented 58% with dry and 42% with fleshy propagules, with 40% small, dry propagules. In herbaceous species, dry and small propagules were predominated with 64%. The area showed a Shannon-Weiner diversity and evenness value of 4.0 and 0.26, respectively; indicating that the study area has high floristic diversity despite uneven distribution of the individuals among the encountered species. The three most diverse Afromontane forests, with their shared number of species and Sørensen similarity percentages between brackets, within the present study area, were Wondo Genet with 80 (35.6 %), Gendo with 70 (37.1%) and Denkoro with 68 (35.5%), but 23 species (6.6%) were shared with the Southern Cape in South Africa at the southern end of Africa. The Metema woodlands contributed a 20.6% similarity for herbaceous species. Such floral similarity of Afromontane forests maybe attributed to fragmentation of their historical landmass connectivity (vicariance) and dispersal between more local forests. Four main woody plant communities along with sixteen sub-communities and two main herbaceous plant communities along with eight subcommunities were identified. Different communities showed different affinities towards different gradients, despite some overlap occurring among communities. Radiation index (calculated from slope and aspect) and altitude were found to be the common highly significant environmental variables to explain the occurrence and composition of both herbaceous and woody communities. Various scales of disturbance affected regeneration of canopy tree species across forest communities, ranging from fine to coarse spatial scales. Some species, including Prunus africana, Olea capensis subsp. macrocarpa, Olea europaea subsp. cuspidata and Schefflera abyssinica showed critically poor regeneration, which need conservation attention. Plant communities were also characterized by different gradients and spatial scales of disturbance affecting their regeneration, indicating the need for different management interventions. The information obtained in this study, about floristicstructural composition, species associations, their plant-plant and plant-site relationships, and their response to different disturbance factors, provide the essential basis for guiding better resource use management of these forests.

AFRIKAANSE OPSOMMING: Dikwels word woudbestuurders gekonfronteer met die bestuur van ‘n nuwe woud waarvoor geen inligting beskikbaar is nie. Die konsep van hierdie tesis is om algemene analitiese tegnieke in kombinasie te gebruik om ‘n eerste benadering te ontwikkel van die floristies-strukturele samestelling, species-assosiasies, hulle plant-plant and plant-groeiplek verhoudings, en hul reaksie op verskillende versteuringsfaktore, as basis vir die ontwikkeling van volhoubare hulpbrongebruiksbestuursisteme. Inligting oor die flora van ‘n woud, die plantgemeenskappe en hul onderliggende omgewingsveranderlikes, en die versteuring-herstelprosesse, is basiese insette vir volhoubare bestuur van ‘n woudekosisteem. Die aanwesige plantsoorte en hul samestelling in plantgemeenskappe, die plant-plant en plant-omgewing interaksies, ruimtelike skale van versteuring wat verjonging van kroondakboomsoorte verskillend beinvloed, en die populasie-status van boomsoorte, is in drie Afromontane-woude van noordwestelike Ethiopië ondersoek. ‘n Sistematiese opname in homogene boomsamestellings is gedoen om plantegroei- en omgewingsdata te versamel. Data is op 150 genestelde sirkelpersele versamel, met 50 persele in elk van drie Afromontane-woude, naamlik Alem Saga, Gelawudiwos and Tara Gedam. Elke perseel het bestaan uit ‘n hoofperseel van 100 m2 vir houtagtige en ‘n sub-perseel van 1 m2 vir kruidagtige soorte. Grond- en blaarvalmonsters is binne die sub-perseel versamel. Hoogte bo seevlak, aspek en helling, is vir elke perseel aangeteken. Plantsoort-diversiteit en diversiteitsprofiele is onderskeidelik met behulp van Shannon- en Renyiindekse ontleed. Die plantsoort-ooreenkomste met ander Afromontane-woude in Ethiopië en in Afrika, en met boomveld in Ethiopië, is ontleed met die Sørensen ooreenkomsindeks. Afsonderlike ontledings vir houtagtige en kruidagtige soorte, met TWINSPAN- en DCA (indirekte gradientanalise), is gebruik om plantgemeenskappe te identifiseer. Die verwantskap tussen geïdentifiseerde gemeenskappe en verskeie omgewingsveranderlikes is met CCA (direkte gradientanalise) ontleed. ’n Ruimtelike-ordeningsontleding met DCA is gebruik om die invloed van die ruimtelike skaal van versteuring (kroondakopening-grootte) op die verjonging van kroondakboomsoorte te ondersoek. Die stamdeursneeklas-verdeling van geselekteerde boomsoorte is gebruik om hul populasiestruktuur oor plantgemeeskappe te ontleed. ‘n Totaal van 209 plantsoorte bestaande uit 109 houtagtige en 100 kruidagtige soorte, is aangeteken. Sewe plantsoorte is endemies tot Ethiopië. Die Shannon-Weiner-diversiteit en gelykheidswaardes van onderskeidelik 4.0 en 0.26, wys dat die studiegebied ‘n hoë floristiese diversiteit het, ongeag die ongelyke verspreiding van plante van die versamelde soorte. Die drie mees diverse Afromontane-woude in die studiegebied, met hul onderskeie gedeelde aantal plantsoorte, en hul Sørensen ooreenkomsindeks tussen hakies, is Wondo Genet met 80 (35.6 %), Gendo met 70 (37.1%) en Denkoro met 68 (35.5%), maar 22 species (6.6%) is gedeel met die Suid-Kaap in die suide van Afrika. Die Metema boomveld het 20.6% bygedra tot die ooreenkomste met kruidagtige soorte. Sulke ooreenkomste in plantegroei van die Afromontanewoude kan toegeskryf word aan fragmentasie van hul historiese landmassa-konneksies (vikariansieteorie) en saadverspreding tussen meer naby-gelee woude. Die houtagtige samestelling het in vier hoof- en sestien sub-plantgemeenskappe verdeel, en die kruidagtige samestelling in twee hoof- en agt sub-plantgemeenskappe. Die gemeenskappe verskil in terme van verskille in omgewingsgradiënte, afgesien van oorvleueling in samestelling tussen gemeenskappe. Radiasie-indeks (gebaseer op helling en aspek) en hoogte bo seevlak was die algemene en hoogsbeduidende omgewingsveranderlikes wat die voorkoms en samestelling van beide houtagtige en kruidagtige gemeenskappe bepaal. Verskeie skale van versteuring (fyn tot growwe grein) beinvloed verjonging van kroondakboomsoorte oor verskillende gemeeskappe. Sekere boomsoorte, soos Prunus africana, Olea capensis subsp. macrocarpa, Olea europaea subsp. cuspidata en Schefflera abyssinica, het kritiese swak verjonging, wat bewaringsaandag benodig. Plantgemeenskappe is gekenmerk deur verskillende skale van versteuring in terme van hul verjonging, wat aandui dat verskillende tipes van bestuursingryping nodig is. Die inligting wat met hierdie studie verkry is, oor die floristies-strukturele samestelling, plantspecies assosiasies, hulle plant-plant en plant-groeiplek verhoudinge, en hul reaksie op verskillende versteuringsfaktore, sal leiding voorsien tot beter hulpbronbestuur.

Doctoral