Differential grazing of female and male plants of prostrate juniper (Juniperus communis L.)

Summary Females and males of dioecious species may not allocate the same proportions of assimilate to plant protection and so may be differentially grazed. The distribution range of the dioecious, coniferous shrub, juniper (Juniperus communis) is declining in the UK and populations are becoming fragmented. The main aims of this study were to (i) assess the sex structure of populations of prostrate juniper at four sites in northern Scotland and (ii) assess the extent of grazing on individual plants. A total of 518 prostrate juniper plants were inspected in the four populations. The sex of 30–85 % of them could not be determined. There were differences between the sites in grazing intensity, plant density, plant size, and the proportion of females. Between June and September, there was almost no grazing of current growth and no difference between males and females in the amount of grazing. Most grazing took place over winter. This was supported by the observation that dung counts over summer did not correlate with grazing of current or old shoots. Female, male and unsexable plants had similar size ranges. Female and unsexable plants had similar numbers and proportions of old shoots grazed but male plants had significantly less. This suggests that many of the unsexable plants were non-reproducing females and that the greater grazing on unsexable plants might be responsible for the cessation of reproduction. The impact of differential winter grazing of prostrate female and male juniper plants is discussed in relation to the conservation of this species in Scotland and the UK.