Ecology of Regeneration and Phenology of Seven Indigenous Tree Species in A Dry Tropical Afromontane Forest, Southern Ethiopia
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2008-04
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Addis Ababa University
Abstract
The Munessa-Shashemene Forest, a typical dry afromontane forest located on the eastern escarpment of the Rift Valley at about 250 km from Addis Ababa, is under severe pressure due to logging, cattle grazing and conversion of the forest to arable land. Ecology of regeneration and phenology of seven indigenous tree species known to have considerable socio-economic and ecological importance were investigated. Seedling growth experiment within the forest was carried out in three light environments for 136 days and their survival followed for 12 months in the permanent plots. About 1200 understorey seedlings located systematically at different sites within the forest were marked and monitored for their mortality and herbivory rates for a period of 2 years. Seedling densities and their spatial distribution along gradients of altitude, disturbance and canopy light were investigated in the forest using quadrats having sizes of 10 m x 5 m along line transects. Tree and sapling densities, their spatial distribution along altitudes, and size-class distribution were investigated using quadrats of 20 m X 20 m along line transects. About 210 focal mature trees have been monitored for their leaf, flower and fruit every month for a period of two years. Various statistical tools and ecological software were employed for the data analyses.
Seedling morphological and physiological traits showed significant variation among species and light levels. Seedling growths such as total plant biomass, height, total leaf area and root length were highly responsive to light levels while, specific leaf area, leaf area ratio, leaf, root and stem mass ratios were more of species-specific. Relative growth and net assimilation rates were highly influenced by the plant biomass. Understorey seedling mortality and herbivory rates year-1, respectively, ranged 11% (Podocarpus) – 67 % (Syzygium), and 0 (both Croton and Podocarpus) - 56 % (Celtis). They were significantly different between dry and wet seasons of the year. Mean densities of seedlings varied among species from 0 (Polyscias) to 5,334 (Prunus) individuals ha-1, while trees and saplings varied from 3 (Polyscias) to 102 (Podocarpus) plants ha-1. Size-class distributions of the species have shown three major groups:- (1) species with inverse J-shape curve, and thus having healthy regeneration status, (2) species with higher proportion of individuals in the lowest size-class, but missing individuals in the subsequent middle size-classes and thus hampered regeneration and; (3) species with no individuals in the lowest and middle height classes, but represented by large sized individuals only and thus at risk of local extermination. Better reproductive performance evidenced by high seedling densities (e.g. Celtis and Prunus) or adaptive defenses to herbivory (e.g. Croton and Podocarpus) were common attributes of healthy regenerating species.
In deciduous species, the period of leaflessness/shedding lasted from January to March. Leaf flushing peaked in April coinciding with start of small rains. In most species, flowering pattern was strongly seasonal and annual. Unlike flowering, fruiting was non-seasonal and extended over several months of a year. The peak fruiting period was occured around mid to late of the long dry season. Rainfall and temperature triggered the phenological cycles in the tree species. In general, understorey light environment, drought (soil moisture stress), seedling herbivory damage, reproduction, and human disturbances were among the major factors affecting regeneration of tree species in the forest. Species of high conservation priority in the immediate future are Polyscias and Pouteria; while Celtis, Croton and Prunus are encouraged for sustainable utilization options. The implications of the results of seedling growth, survival, herbivory damage, plant population structure and phenology of the species are discussed in detail.
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Biology