The Dynamics of a Grassland Ecosystem: Botanical Equilibrium in the Park Grass Experiment

Abstract

The published results of the Park Grass Experiment (PGE), begun in 1856 [at Rothamsted, UK], provide up to 30 yr of annual data which may be used to determine whether the botanical composition of these grasslands was at equilibrium. Data covering a period exceeding 80 yr are available to test for relationships between hay yield (biomass), species diversity, species number and time. Species diversity and species number show negative relationships with plot biomass and with pH. These relationships were constant over time. The effects of biomass and pH on species number and species diversity were additive. Analysis of the flora of 9 plots, each divided into grasses, legumes and a miscellaneous component, showed that these components were at equilibrium. The effect of various endogenous factors on this botanical equilibrium was examined. No regular or irregular cycles of component biomass operating between years were detected and it is inferred that populations were regulated by processes operating within individual years. The biomasses of all 3 components were positively correlated within an unfertilized plot, but the floristic components of plots receiving a fertilizer treatment showed few within-plot correlations. By contrast, between-plot correlations of components were common for all plots with the exception of those receiving N fertilizer. The mechanisms of population regulation which maintained the Park Grass ecosystem at equilibrium are discussed and tests for these are proposed.