Carbon X nutrient interactions-the limitations to productivity

The interaction of nutrient cycles with the production, accumulation and death of organic matter is considered, in particular the relation to changes with stand age. It is shown that, because of efficient recycling within the tree and through the litter, the demands made by forests on the soil nutrient capital fall markedly after canopy closure. Because this effect is primarily a result of the ability of the tree to conserve and reuse nutrients in the foliage the significance of the evergreen habit is examined. Although there are many reports of a net photosynthetic contribution from foliage older than one year the relationships examined suggest that whereas there is virtually no correlation between growth rate and total foliage carried, there is quite a good relationship with weight of current-year foliage irrespective of whether trees are deciduous or the age at which evergreen foliage is shed. It is suggested that the evergreen habit would be a nutritional disadvantage prior to canopy closure but thereafter a considerable advantage.