Negative correlation between dispersal investment and canopy openness among populations of the ant-dispersed sedge, Carex lanceolata

Flowering plants exhibit a wide variation in the resources they invest in dispersal structures (dispersal investment), but the environmental correlates still remain unclear in many cases. Canopy openness is predicted to be negatively correlated with dispersal investment, because selective pressures on increased dispersal investment, including host-specific natural enemies and the paucity and/or ephemerality of safe sites, will be more prevalent in shady sites. Here, we tested this prediction by firstly examining the correlation between dispersal investment and canopy openness as well as abundance of the representative natural enemy (rusts, Puccinia spp.) through seven populations of an ant-dispersed sedge, Carex lanceolata (Cyperaceae). Secondly we conducted a cafeteria experiment to verify the effect of intraspecific variation in dispersal investment on diaspore preferences of seed dispersing ants. Lastly, a seedling transplant experiment was performed to clarify whether seed dispersal distances by ants are sufficient to reduce infection by the rusts. We found a negative correlation between dispersal investment and canopy openness, thus supporting the prediction. Moreover, there were more signs of rust infection caused by Puccinia spp. on adult plant leaves in more shady sites. The cafeteria experiment showed that a large ant species (Formica japonica) with relatively long seed dispersal distances tended to prefer diaspores with greater dispersal investments, while smaller ant species with shorter dispersal distances preferred diaspores with lower dispersal investments. The seedling transplant experiment revealed that rust severity in sedge offspring was considerably reduced by the typical seed dispersal distance (ca. 4 m) afforded by the large ant species, F. japonica. The increased rust severity at shady sites, combined with the narrow dispersal ranges of rusts, can partially explain the negative correlation of dispersal investment with canopy openness. These results support the importance of canopy openness as a factor underlying the variations in dispersal investment seen among flowering plants.