Sex-related spatial segregation along environmental gradients in the dioecious conifer, Taxus baccata

Sex-related differences in dioecious woody plants commonly result in spatial segregation of the sexes and a male-biased sex ratio, leading to strongly structured populations. We report results from more than 1000 precisely georeferenced individuals of a threatened European dioecious tree species, Taxus baccata in central Italy. We hypothesized differences between males and females in both spatial distribution and growth pattern. We used point pattern analysis (PPA) to evaluate the spatial distribution of trees as influenced by dioecy and tree size, and dendrochronological methods to study growth pattern differences between males and females. We found a dominance of female trees giving a SR of 0.79, which increased along a gradient of elevation and slope. At the microsite level, female Taxus trees were clumped to a scale of 30 m while male trees were randomly distributed. We observed a significant effect of both sex and decade in the growth pattern of Taxus trees, where annual radial growth of females was 8.3% greater than that of males. Female trees are more commonly found in wetter sites and males are widely but sparsely distributed, causing the absence of a clear spatial segregation of the sexes. Observed differences in spatial and growth patterns of males and females could possibly influence the climate change response of T. baccata, given that range expansion depends upon the availability of seed-bearing, female trees that tend to be clumped in climatically favorable sites.