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 ran ge of temperatures, CO2, nutrient and density regimes in a factorial d esign housed within solar-domes. Temperature treatments were imposed a t ambient and +3 degrees C levels, CO2 at ambient and +340 ppm, and th ere were three levels of nutrients and eight levels of densities rangi ng from 156 to 31250 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, h igh nutrient and low density regimes, and flowering was earlier at the high temperature regime. 3. At seed set, biomass per plant and seed p roduction per plant were analysed by analysis of variance and by fitti ng mean yield-density models expanded to account for different environ mental conditions. Biomass and fecundity were greatest at high tempera ture, 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 d escribing the nature of the relationship between seed production per p lant and density was always less than unity but was greater at the hig her temperature regime. The response to density was therefore undercom pensating in all conditions, implying that populations would display m onotonic damping to equilibrium densities. 4. Under proposed future en vironmental regimes, V. ciliata has the capacity for more rapid popula tion 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 incre ase.