Sex-specific plant responses to light intensity and canopy openness: implications for spatial segregation of the sexes.

In seed plants, the proximate causes of spatial segregation of the sexes (SSS) and its association with environmental variation are thought to be linked to sex-specific morphological and physiological variation. To address the general question of linkage among SSS, plant traits and environmental gradients, Marchantia inflexa was used, for which male plants are found under more open tree canopy than females. We hypothesized that males are adapted to higher light intensity and are better able to tolerate water stress than females, as is the case with seed plants. We tested for sex-specific habitat and trait relationships by quantifying plant traits (morphological and physiological) and estimates of the light conditions (percent canopy openness and light intensity) in the field. Using path analysis, we found that edge pore density in both sexes was negatively correlated with canopy openness, while in males, edge pore density had a weak but positive relationship to light intensity. These responses suggest that canopy openness and light intensity have opposing effects on edge pore density in males and that males might be more responsive to water stress than females. Additionally, the greater importance of female support tissue, which functions as storage, in explaining and being explained by other variables in the path analysis, relative to male support tissue, may reflect sex-specific allocation differences related to resources needed for female function.