Do nutrient-poor soils inhibit development of forests? A nutrient stock analysis

Abstract

Nutrient-poor soils often support low-stature grasslands, savannas and shrublands where the climate is warm enough and wet enough for closed forests. Though this pattern has long been recognised, the causes are debated and poorly explored. I tested the hypothesis that forest fails to develop where the total nutrient pool is too small to construct both the foliage and the wood. I estimated potential woody biomass from the difference between soil nutrient stocks and forest foliage stocks. Nutrient stocks required for foliage were estimated from leaf tissue concentrations and foliage biomass typical of Amazon forests. Potential wood biomass was estimated from wood nutrient concentrations typical of Amazon forests. Data on soil nutrient stocks were assembled from studies from South American and African forests and savannas and from south-western Australian and south-west African heathlands. According to these calculations, estimated nutrient stocks (kg ha−1) to build a forest would need to be > = P: 20–30, K 200–350, Ca 300–600 and Mg 55–65. Many surface soil horizons from both savanna and heathland sites were below these thresholds. However when deeper soil layers were included, most soils had adequate nutrient stocks. The nutrients in shortest supply were Ca and K and not P. This study suggests that nutrient stocks are usually adequate for constructing the wood needed to build a forest, except where soils are highly leached and very shallow. The implication is that, at steady state, low nutrient stocks seldom constrain forest development. The apparent failure of low nutrient stocks to explain the missing forests on nutrient-poor soils emphasises the need for new ideas on how nutrients, alone or in combination with other factors such as fire, influence vegetation structure.