Litter production and nutrient cycling in two plantations and a Podocarpus falcatus dominated natural forest ecosystems in south-eastern highlands of Ethiopia

Abstract

The production of litter plays a fundamental role in the biogeochemical cycle of organic matter and mineral nutrients in forest ecosystems. The amount of litter produced and nutrients returned, and their accumulation in the organic layers of Eucalyptus globulus and Cupressus lusitanica plantations, and an adjacent Podocarpus falcatus dominated natural forest were monitored over a one year period in Ethiopia. The objective of the study was to asses the general trends of nutrient cycling by litter fall under the two plantations relative to the natural forest. The results showed that annual litter production was significantly highest in the natural forest (4.4 t ha-1year-1) and least in Cupressus plantation (2.2 t ha-1year-1). Litter fall in the natural forest was of high quality and richer in most nutrients than the two plantations which behaved similarly. The annual fluxes of N, S, P, Ca, Mg and K were 1.3 to 2.3 times as much in the natural forest as in Eucalyptus and Cupressus. The organic layer under Cupressus had poor quality litter compared to Eucalyptus and natural forest which behaved similarly mainly due to the influence of the understorey vegetation on Eucalyptus organic layer. Although statistically not significant, dry mass accumulation in the organic layer of Cupressus was higher than in Eucalyptus and lower than in the natural forest, but the approximate residence time was in the order: Cupressus >natural forest>Eucalyptus. The organic layer in the natural forest had always the maximum quantity of nutrients compared to the two plantations. Except for Ca in which Cupressus far better, the other elements were accumulated in larger quantity in the Eucalyptus organic layer than in Cupressus. Overall, the results show that the Cupressus plantation is poor self fertilizing owing to the low quality homogenous materials which could result in a gradual impoverishment of the soil. Key words: Cupressus, Ethiopia, Eucalyptus, litter fall, nutrient cycling, organic layer.