GROWTH AND NUTRIENT ACCUMULATION BY PINUS-CARIBAEA ON 3 SAVANNA SITES IN NORTHERN NIGERIA

Growth and nutrient accumulation by a 14 year old stand of Pinus caribaea var. hondurensis (Barr. and Golf.) on three different savanna sites in northern Nigeria were determined. There were differences in the rate and magnitude of productivity and nutrient accumulation between the stands growing on the different sites. These differences were attributed to contrasts in soil characteristics, altitude and mean annual rainfall. Growth was higher and nutrient use efficiency better on sites where soil and climatic factors were more favourable. Comparison of the amounts of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) held in the tree biomass with the inventory of the corresponding soil reserves of plant 'available' nutrients showed that the amounts of N, Ca and Mg immobilised in the above-ground tree layer were small. In contrast a much greater proportion of each of P and K was held in the biomass. The fraction of plant 'available' soil P immobilised in the biomass varied between 72% and 176%; the respective K fractions were 64% and 92%. Based on these data, the estimates of nutrient loss that might accompany different intensities of biomass utilisation were calculated. The estimates indicated potential problems of P and K nutrition in future rotations. This prediction was qualified in view of the fact that precise data on some of the nutrient fluxes of the ecosystems were lacking. It was concluded that savanna sites with inherently low nutrient quality were at a risk of nutrient depletion.