A preliminary test of catabolic nutrients in explanation of the puzzling treelessness of grassland in mesic Australia
Résumé
The puzzling exclusion of native trees from grassland under mesic climates can hypothetically be explained by nutritional regimes that make grasses competitively superior to trees. One hypothesis holds that where nutrient concentrations in soils allow catabolism to be as rapid as anabolism at the scale of the whole plant, photosynthates are respired instead of being allocated to stems, resulting in dominance of herbaceous over woody growth-forms. This matching of catabolic and anabolic rates would depend on the ratios of catabolic to anabolic nutrient elements in soils at a particular site. We consequently investigated elemental concentrations of soils near boundaries between treeless grassland and eucalypt woodland in New South Wales, Australia. Soils were analysed for Be, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Cd, Ba and Pb. Based on our preliminary classification of nutrients as mainly catabolic (e.g. Cu) or mainly anabolic (e.g. Mn), the results showed that, relative to adjacent woodland, the soils under treeless grassland were naturally enriched in anabolic nutrients, but even more so in catabolic nutrients. Furthermore, the greater concentration of Mo - which can induce deficiency of the catabolic nutrient Cu - in soils under woodland than in soils under treeless grassland suggests that catabolic dystrophy allows eucalypt seedlings to prevail in competition with grasses. The results support the theory that where ratios of catabolic to anabolic nutrients are sufficient grasses will achieve competitive superiority over woody seedlings. Our study indicates that soil nutrient concentrations may partly explain the incidence of natural patches of treeless grassland in Australian landscapes otherwise dominated by woody plants.