Effects of nutrient level on thinning and non‐thinning crowding in even‐aged populations of subterranean clover

Abstract

AbstractPopulations 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.