THE DYNAMICS OF AN EXPERIMENTAL POPULATION OF SALVIA LYRATA: THE POPULATION CAGE APPROACH APPLIED TO PLANTS

We report a four-year study of the dynamics of a population of Salvia lyrata L., growing in a greenhouse. The study exemplifies the population cage approach, which has been the basis of many long-term studies of dynamics of animal populations for over 50 years, but which has been overlooked by plant population biologists. The cohort of plants that founded the population had a very high probability of survival and, as adults, produced the greatest numbers of seeds and seedlings. Thereafter, the numbers of individuals in each stage class declined. This decline appeared to be a consequence of density-dependent processes. At the conclusion of the study, the population density was far lower than at the onset. The behaviour of the population differed from that of the population of S. lyrata studied in a nearby mown field in several respects, including timing of seedling emergence, average size of individuals and final density. Likely causes of these differences are discussed. We recommend the use of population cages in further studies of plant populations. Because it facilitates long-term observation, this approach can be of great use in elucidating the dynamic consequences of demography.

The measurement of small-scale environmental heterogeneity using clonal transplants of Anthoxanthum odoratum and Danthonia spicata

To assess the scale of micro-environmental heterogeneity perceived by two co-occurring grass species, Anthoxanthum odoratum and Danthonia spicata, cloned tillers of each species were planted into the natural habitat at a range of spacings (from 2 cm to more than 2 m apart) and measured for survival and fecundity over three years. A. odoratum responded to heterogeneity at a scale of 4-8 cm and at a scale of 2-8 m but not to intermediate scales. D. spicata did not respond significantly to heterogeneity. However one genotype infected with the systemic fungus Atkinsonella hypoxylon showed a large response to heterogeneity at both small and large spatial scales. The results showed that the scale and level of environmental heterogeneity as measured by its fitness impact depends on the species and genotype involved. The results indicate that small scale environmental heterogeneity could play a role in the maintenance of sexual reproduction in A. odoratum.

Paternal and maternal effects on propagule size in Anthoxanthum odoratum

Propagules (caryopsis plus glumes) from a diallel cross among eight genotypes of Anthoxanthum odoratum were weighed to asses the relative contribution of nuclear, reciprocal, and environmental effects to propagule size. Maternal reciprocal effects were large whereas zygotic nuclear effects were small, although consitently present. There were no interactions between maternal reciprocal and nuclear effects. Environmental effects were large, including environmentally induced maternal effects and within individual variance. These results suggest that response to selection for seed size as a result of sib-competition will be weak, and that seed size differences are relatively unimportant for the maintenance of sexual reproduction in natural populations.