Plant Populations and Global Environmental Change: The Effects of Different Temperature, Carbon Dioxide and Nutrient Regimes on Density Dependence in Populations of Vulpia ciliata

1. Monocultures of Vulpia ciliata spp. ambigua were subjected to a range of temperatures, CO2, nutrient and density regimes in a factorial design housed within solar-domes. Temperature treatments were imposed at ambient and +3 ⚬C levels, CO2 at ambient and +340 ppm, and there were three levels of nutrients and eight levels of densities ranging from 156 to 31 250 seeds m-2. The abiotic treatments were imposed after emergence. 2. There was little mortality and this was unrelated to the treatments. Plants grew more quickly at the high temperature, high nutrient and low density regimes, and flowering was earlier at the high temperature regime. 3. At seed set, biomass per plant and seed production per plant were analysed by analysis of variance and by fitting mean yield-density models expanded to account for different environmental conditions. Biomass and fecundity were greatest at high temperature, high nutrient and low density regimes. Allocation of biomass to shoots was greater at the high temperatures, as were seed number/shoot biomass ratios. Any effects of CO2 were negligible. The parameter b describing the nature of the relationship between seed production per plant and density was always less than unity but was greater at the higher temperature regime. The response to density was therefore under-compensating in all conditions, implying that populations would display monotonic damping to equilibrium densities. 4. Under proposed future environmental regimes, V. ciliata has the capacity for more rapid population growth from low levels and for a northwards range shift. However, if open ground is not maintained, its habitat may become dominated by species that are more competitive or that have a higher rate of increase.