Are forests restricted by nutrient poor soils? : an analysis of soil nutrient stocks and associated vegetation in the fynbos biome, South Africa
The realized woody biomass of the Fynbos Biome in the Western Cape, South Africa falls below the climatic potential to support woody biomass. The lack of substantial tracts of woody forests has long puzzled ecologists, although patchy nutrient distribution and fire disturbance are thought to play a role. The issue has been confounded in the recent past by the invasion of non-indigenous woody plants into formerly low stature fynbos dominated areas. Despite low wood nutrient concentrations, a substantial proportion of nutrients are locked up in the wood of forests due to the large volume of wood. Nevertheless, nutrient stock analysis indicated that plant available nutrients in the poorest global soils (including fynbos soils) are sufficient to support forests (indigenous and alien). I hypothesized that soil nutrient stocks of the Fynbos Biome are sufficient in quantity to support closed canopy indigenous forests with a woody biomass greater than 225 000 kg ha⁻¹ and that alien Pinus spp. and Eucalyptus spp. have lower nutrient stocks than indigenous fynbos species. The study was conducted in the Orange Kloof Forest Reserve and Jonkershoek Nature Reserve within the Western Cape (South Africa). Soil, wood and leaf samples of representative species were collected from indigenous forest, fynbos, E. globulus and P. halepensis and assessed for nutrient contents. Estimates of potential woody biomass on four different soils indicated that indigenous forests would be limited by fynbos soil stocks to below the 225 000 kg ha⁻¹. Pine forest had lower wood nutrient concentrations (mg kg⁻¹, n=6) for N (2466), K (2433), Ca (383), and Fe (34) than indigenous forest species (n=11) N (3427), K (4254), Ca (1636) and Fe (140). The low nutrient stocks in pine wood may allow them to grow tall and be competitive in the shrubby fynbos biome where indigenous forests are limited by expensive wood costs.
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