Spatio-temporal dynamics of sex structure in trioecious and dioecious populations of Salix myrsinifolia

Abstract

Most Salix species are characterized by a complex sex structure with populations that contain different ratios of male, female (dioecious populations) and even hermaphroditic individuals (trioecious populations). However, our knowledge of the temporal and spatial variability of such populations is rudimentary. I studied how sexual structure in trioecious populations of Salix myrsinifolia changes over time (from 1995 to 2012) and space in 32 populations across NE Poland and Lithuania. Additionally, the sex of marked individuals was monitored in a population over eight seasons. A number of land cover variables, latitude and population size were investigated to determine what possible mechanisms/factors affect the occurrence of hermaphroditism in this species. Sex structure was found to differ between the northern and southern parts of the study area (primary and secondary ranges), with trioecious populations occurring significantly more often in the secondary part of the range. Females dominated in northern dioecious populations, while sex ratios were balanced in the southern part of the range and in trioecious populations. The mean frequency of hermaphrodites reached 14 % and was decreasing toward the north. The sex structure remained stable over the study period, with an increasing frequency of hermaphrodites (by nearly 6 %). A detailed study of marked individuals revealed that sex is labile in this species. Over 12 % of individuals changed sex from one to three times. Most changes (67 %) were to hermaphroditism, which frequently occurred (34 %) just before death or when entering the post-reproductive period.