The effects of spatial pattern of nutrient supply on yield, structure and mortality in plant populations

1 Early in growth, plant populations subjected to heterogeneous conditions can achieve significantly higher yields than populations grown in homogeneous conditions that provide the same amount of nutrients. We hypothesized that this yield enhancement would be ephemeral, and that yields of populations grown under different patterns of nutrient supply will converge as time passes. We also predicted that plant size frequency distributions will continue to differ between populations in heterogeneous and homogeneous conditions, that spatial patterns of mortality will differ, and that greater mortality will occur in populations under heterogeneous conditions. An experiment was carried out on Cardamine hirsuta to test these hypotheses. 2 The experiment was harvested after 60 days, when flowering had begun in all populations. At this time, populations grown with the same nutrient supply but different patterns of supply did not differ in yield. Populations grown at higher nutrient levels had greater yields but suffered more mortality. 3 In heterogeneous conditions, plants in nutrient-rich locations were larger than plants in nutrient-poor locations, and larger than plants in identical positions in populations growing under homogeneous conditions. Plants in nutrient-rich patches in heterogeneous treatments suffered more mortality than plants in nutrient-poor patches. Contrary to prediction, total mortality at all levels of nutrient supply was significantly higher in populations in homogeneous conditions. 4 Coefficient of variation in shoot sizes was higher in populations at higher nutrient levels, but unaffected by pattern of nutrient supply. CV of root mass per unit of substrate was greater under heterogeneous than homogeneous conditions. 5 Frequency distributions of shoot size, and root mass per unit of substrate, were strikingly different between populations grown under different patterns of nutrient supply. The differences are interpreted as showing that under homogeneous conditions all parts of the habitat are heavily exploited, giving rise to uniformly high levels of interplant competition. In contrast, nutrient-poor patches in the heterogeneous treatments contain low root masses. Plants in these patches remain small, but have a high probability of survival. We propose that these differences in intensity of habitat occupation, and presumably in competition, explain the lower mortality in populations growing under heterogeneous conditions.