Changes in allozyme frequencies in Festuca ovina populations after a 9-year nutrient/water experiment

Abstract

1. The grass Festuca ovina is an important constituent of the species-rich 'alvar' grasslands on the Baltic island of Ȯland. Levels of allozyme polymorphism are high and variation is known to be correlated with habitat variation (soil moisture, pH and depth). 2. A 9-year field experiment on species diversity provided replicate plots (in three sites) that had been subjected to six different experimental treatments (control; N + P + K; P + K; N + K; K; water). 3. Samples of F. ovina were collected and analyses of deviance were used to investigate associations between allele frequencies, at each of four polymorphic loci, and the nutrient/water treatments. We also used the models to estimate predicted values for the alleles in different nutrient/water treatments and in interactions involving the nutrient/water treatments and additional explanatory variables (vegetation height and clipping). 4. There were significant allozyme frequency differences between samples of F. ovina from the six different nutrient/water treatments in the grassland experiment. Frequencies in the fertilized or watered plots had diverged from those in the control plots. There were also significant allele-habitat associations (after the removal of site effects), especially at the Pg1-2 locus. 5. Soil moisture was the only variable that was common to this study and an earlier study of variation in F. ovina in natural habitats. In natural populations, the Pgi-2-2 allele was significantly associated with soil moisture and was more common in dry habitats. Our findings that the frequency of the Pgi-2-2 allele was significantly affected by the nutrient/water treatments, and that it was rarest in the treatment that involved the addition of extra water, were therefore as predicted. 6. The study supports the conclusion, from an earlier study of populations in unmanipulated grassland habitats, that selection is contributing to the fine-scale patterning of genetic variation in the alvar populations of F. ovina.