STABILITY IN THE PLANT-COMMUNITIES OF THE PARK-GRASS-EXPERIMENT - THERELATIONSHIPS BETWEEN SPECIES RICHNESS, SOIL-PH AND BIOMASS VARIABILITY

The Park Grass Experiment (PGE), begun at Rothamsted Experimental Stat ion in 1856 and still running, affords a unique opportunity to test fo r the influence of species number and soil reaction on biomass variabi lity in a suite of comparable plant communities. Biomass variability w as measured by calculating the coefficient of variation (CV) over time of annual hay yield in an eleven-year moving window. CV and species n umber were both strongly negatively correlated with biomass; both rela tions were affected by time and pH. Multiple regression of CV on speci es number and mean biomass for non-acidified plots in 42 years between 1862 and 1991 showed a relationship between biomass and CV which was negative in most years and significantly so in nearly three quarters o f them (30/42). We are unable to tell how much of this effect is intri nsic to the statistical relation between the mean and CV of biomass. S pecies number was negatively correlated with CV in 29/42 years, but th is was statistically significant on only three occasions. Because this relation was highly significant in the year (1991) for which we have the largest sample size (34 plots), we tentatively conclude that bioma ss variability may be lower in more species-rich communities, although the effect is possibly a weak one. We suggest that physiological stre sses imposed by low pH may explain the greater variability of plots wi th acidified soil. An increase in the variability of biomass that occu rred across plots with time may be due in part to acidification across the whole experiment. Three hypotheses are proposed to explain the re lationship between species richness and biomass variability: (i) bioma ss variability on more species-rich plots is better buffered against c limatic variation because species differ in their response to climatic conditions: (ii) there are fewer species on plots with greater biomas s variability because species have been lost by competitive exclusion in years when biomass reaches high values; (iii) species richness and variability are both correlated with a third variable, for example soi l moisture deficit within a plot. All three hypotheses are susceptible to testing within the PGE.