Variation in Competition Along an Environmental Gradient: Hieracium Floribundum in an Abandoned Pasture

The results support the proposal that competition should intensify as soil fertility and plant standing crop increase, even when moisture availability also varies, because the intensity of competition is affected by variation in soil moisture availability.

Abstract

(1) To judge if plant survival, recruitment and population growth were affected by competition equally at opposite ends of a complex environmental gradient (i.e. low nutrient, high light, low plant standing crop to higher nutrient, lower light, higher plant standing crop), the response of Hieraciumfloribundum to the removal of interspecific and some intraspecific neighbours was recorded in an abandoned pasture. To determine if soil moisture availability affected the intensity of competition, precipitation was excluded for the first part of the experiment and plants were watered either frequently or infrequently during this period. (2) Rosette survival, recruitment and population growth all increased when neighbours were removed, but only at one end of the gradient (i.e. higher nutrient, higher standing crop, lower light). Reducing soil moisture at the start of the experiment did not affect rosette survival but it increased rosette recruitment where nutrient and standing crop were higher on the gradient. However, recruitment increased whether neighbours were present or absent, so the intensity of competition, measured as the difference in recruitment with neighbours present and absent, was not affected by variation in soil moisture availability at either end of the gradient. (3) These results support the proposal that competition should intensify as soil fertility and plant standing crop increase, even when moisture availability also varies.