Variations in the Demography of Natural Populations of the Wild Strawberries Fragaria Vesca and F. Virginiana

Abstract

(1) This study examines the variability in demography within and between two closely related, clonal, perennial plants, Fragaria vesca and F. virginiana. Populations were studied at three sites for each species. (2) About 100 ramets were marked at each site and followed for 2.5 to 3 5 years. Survival, size, sexual reproduction and vegetative propagation were recorded for each ramet at approximately 6-week intervals. (3) Ramet densities varied greatly among the sites, from 1.8 m-2 to 340 m-2. (4) Mean size of ramets differed significantly among the sites, as did the relationship between size and age; in the F. vesca populations, older ramets were larger; in the F. virginiana populations they were not. (5) The patterns of age-specific mortality fell into two groups: sites where newly-produced ramets were subject to the same risk as established adults; and sites where newly-produced ramets had 5-10 times greater risk. Survivorship within cohorts was consistently logarithmic with time. (6) Cyclic annual variation in mortality rate was marked (and synchronous across cohorts) in some sites, but absent in others. (7) The probabilities of flowering and producing stolons differed significantly among sites and between years. The number of flowers, fruits, stolons or rooted nodes produced per reproductive individual did not differ, however. (8) The two species cannot easily be characterized by the demographic parameters measured. The recent history of each site seemed to influence local demography more than taxonomic identity.