Productivity, site evaluation and state of nutrition of Gmelina arborea plantations in Oluwa and Omo forest reserves, Nigeria

The high productivity associated with many tropical forest plantation species has contributed to their importance in meeting the world's growing demand for wood products. However, there is concern that their high growth rate and intensive management system might lead to site nutrients depletion. A total of 20 stands (10–28 years) of Gmelina arborea plantations in Oluwa and Omo forest reserves, Nigeria, was used to investigate the effect of Gmelina plantation development on site nutrients, aboveground productivity and nutrient accumulation. Standing biomass ranged from 81.5 to 392.1 t ha−1 in the youngest and oldest stands. Regardless of stand age, biomass partitioning was about 83.0, 13.5 and 3.5% to stem, branches and foliage, respectively. The size and age of trees did not significantly affect nutrient concentration in tree components. Nitrogen, Ca and Mg contents in tree tissue increased in the order of foliage > stem > branches while that of K and P increased in the order of foliage > branches > stem. Tree tissue nutrients concentration exhibited little change with stand development. Stand nutrient accumulation followed the same trend as standing biomass, with about 80% of each nutrient stored in the stem and 20% in branches and foliage. The accumulation of nutrients in stem, branches and foliage followed the order: Mg > N > Ca > P > K > Na. Though soil nutrients were slightly depleted between 10 and 19 years and re-built up afterwards, the overall effect of stand development on soil nutrients was not statistically significant, implying that the development of Gmelina plantations did not adversely affect the soil nutrient status. Consequently, productivity during the next rotation will most likely be affected by harvesting methods of current stands and management practices of the next rotation. The 20% accumulation of aboveground nutrients in branches and foliage implies that apart from the already replenished site nutrients, there will be an additional 20% nutrient input into the soil if the branches and foliage are left on the site after harvest. For long-term site quality and sustainability of production, successive plantations should be managed on 25 years rotation as lower rotation will most likely lead to steady depletion of site nutrients.