LITTERFALL AND NUTRIENT RELEASE BY DECOMPOSITION IN 3 PLANTATIONS COMPARED WITH A NATURAL FOREST IN THE ETHIOPIAN HIGHLAND

The pattern of litterfall, its nutrient element content, rate of weigh t loss and release of nutrient elements were investigated in 28-40 yea r old plantations of two exotic species, Cupressus lusitanica and Euca lyptus globulus and one indigenous species, Juniperus procera, and com pared with that of an adjacent Juniperus-Olea-Podocarpus montane fores t in the Ethiopian highland. The total annual fine litterfall was 501 g m-2 year-1, 583 g m-2 year-1, 653 g m-1 year-1 and 1087 g M-2 year-1 in the C. lusitanica, Eucalyptus globulus, J. procera plantations and the natural forest, respectively: of this litterfall, leaves constitu ted 57.6%, 77.2%, 71.9% and 65.5%. Litterfall was higher in the natura l forest than in any of the plantations in 18 of 24 months studied. It was generally high during the dry months and low during wetter months and varied between the 2 years of study. The concentration of nutrien t elements in the litterfall to some extent showed a seasonal pattern, most pronounced for K. The four sites could be arranged in this seque nce with respect to the rate of nutrient release from decomposing litt er after 12 months: J. procera= C lusitanica > Eucalyptus globulus= na tural forest. Nutrient elements and dry weight (DW) followed the relea se pattern: K > Mg > Ca > DW > N = P. After 24 months, the weight loss was less in the natural forest and C lusitanica site than in the Euca lyptus globulus and J. procera sites, with 11.8%, 10.6%, 7.2% and 4.6% remaining DW, respectively. The high N to P ratio in litter in the fi rst phase and at the end of decomposition, or during the whole period in the case of Eucalyptus globulus, indicates that P was generally lim iting for the decomposition. Owing to the lower amount of litter and i ts lower nutritional quality, the annual nutrient input by litter of t he two exotics generally was much lower than that of J. procera and, i n particular, that of the natural forest. A consequence of the less ef ficient within-stand cycling in the J. procera and natural forest site s may be that the herbs, shrubs and tree seedlings here may benefit mo re from nutrient elements released from litterfall than plants in the C lusitanica and Eucalyptus globulus sites.