Seedling performance in a dioecious tree species is similar near female and male conspecific adults despite differences in colonization by arbuscular mycorrhizal fungi

Abstract

Patterns of seedling growth and survival near conspecific plants have important consequences for species diversity in plant communities, but the factors causing intraspecific variation in seedling performance are unclear. Greater seed production or size of adult plants could both drive the local accumulation of specialized antagonist species in the environment and affect conspecific seedling performance. Experiments with dioecious species, in which only female individuals produce seeds, decouple these factors in areas of high conspecific density. To assess whether conspecific seedling performance is reduced in the environments associated with seed-producing female trees relative to male or heterospecific trees, we conducted shadehouse and field experiments with a dioecious tropical tree species, Virola surinamensis (Myristicaceae), on Barro Colorado Island, Panama. The shadehouse experiment isolated the effect of soil microbial communities on seedling performance and allowed us to quantify colonization by mutualistic arbuscular mycorrhizal (AM) fungi, while the field experiment allowed us to assess seedling performance in a context that included the effects of nearby conspecific trees. Seedling biomass at the end of the 8-mo shadehouse experiment was similar between experimental plants exposed to soil microbial communities from underneath female conspecific, male conspecific, and heterospecific adult trees. However, seedling colonization by AM fungi was higher in male soil microbial communities. Similarly, at the end of the 7-mo field experiment, survival and biomass did not differ among experimental seedlings grown in female, male, and heterospecific environments (but biomass was reduced near larger conspecific adults). Together, our experiments did not support the hypothesis that conspecific seedling performance is reduced in female environments relative to male environments, despite reductions in colonization by mutualistic AM fungi in female soil microbial communities. Thus, intraspecific variation in the biotic interactions between conspecific seedlings, established adults, and mutualist species may not translate directly to patterns of tropical tree seedling survival.