| Abstract: | Populations of subterranean clover were used to examine the effect of lowering nutrient supply on crowding in even-aged monospecific populations. Two hypotheses were being tested. Under one, termed ‘altered-speed’, reducing the nutrient supply to populations merely slows down the crowding process. Under the other, called ‘altered-form’, reducing the nutrient supply intensifies the crowding process (Morris & Myerscough 1984). The populations were grown at eight densities of sowing and three levels of nutrient supply. Non-thinning yield-density curves and self-thinning lines were fitted to the data. For the thinning populations, reducing the nutrient supply led to altered-form crowding, with thinning lines of reduced slope and intercept for total and shoot weight being observed in populations grown at lower levels of nutrient supply. For the non-thinning yield-density curves, lowering the nutrient supply did alter crowding effects, but not to the extent necessary to give full altered-form crowding. Rather, crowding effects intermediate between altered-speed and altered-form were observed. No altered-speed crowding was observed in this experiment. Comparison was made between the altered-speed crowding observed in other data (Hozumi & Ueno 1954: White & Harper 1970) with the altered-form crowding observed here and elsewhere (Hozumi & Ueno 1954). In these data, altered-form crowding was consistently associated with an increase in the proportion of root in the plants' growth as nutrient level was reduced, while in altered-speed crowding there was a complete lack of any such nutrient effect on root-shoot proportion. This was taken as evidence that under altered-speed crowding, variations in the level of nutrient supply did not lead to major change in the way that plants interfered with each other. For altered-form crowding, major change in the way plants interfered with each other's growth did occur as nutrient level fell. |
