Yield-density Relationships: the Influence of Resource Availability on Growth and Self-thinning in Populations of Vulpia fasciculata

Abstract

Monocultures of Vulpia fasciculata were grown over a wide range of densities to investigate the influence of crowding and nutrient supply on growth and self-thinning. For a given time and density series the relationship between mean yield per plant (w) and the density of survivors (N) could be described by the equation w= w_m (1+aN)^−b. where w_m is the yield of an isolated plant, a is the area required to achieve a yield of w_m and b describes the effectiveness with which resources are taken up from the area. All three parameters increased with time. Adding nutrients changed not only the rate at which the effects of crowding occurred but also the intensity of crowding since w_m = C(a^b)^D. where C and D are constants. The addition of nutrients resulted in an increase in the value of C. Such an increase means that a larger weight can be supported by a given area because the resources within that area are greater. During the early phases of growth, populations of V. fasciculata conformed to the −3/2 power law, w = cN^−3/2, but only at very high densities with a plentiful supply of nutrients. However, once the maximum standing crop had been reached the trajectory of the thinning line switched to a slope of just less than −1 when weight was ploted against density on logarithmic scales. The intercept of the −3/2 thinning line was considerably higher (log c = 5.74) than those for most trees and forbs but was similar to those of a number of other grasses.