Mycorrhizal fungal colonization of plant roots is typically thought to benefit plants through enhancing nutrient acquisition, but this symbiosis also incurs a photosynthate cost to the plant. Mycorrhizal colonization therefore represents an ecological and evolutionary tradeoff depending on the particular species and environment involved. Species with consistently high colonization rates are thought to be more dependent on mycorrhizal fungi for nutrient acquisition, whereas species with low colonization rates use alternative strategies. However, consistent levels of colonization in a species could also reflect the fertility of the habitats and microsites where they are typically found. Plants are known to induce increased mycorrhizal colonization when soil nutrients are low, and reduce colonization (and photosynthate loss) when soil nutrients are high. Interactions between these sources of variation in mycorrhizal colonization are poorly understood, with data from natural populations particularly lacking. To further understand drivers of species differences in mycorrhizal colonization, we surveyed colonization in field populations of 21 closely related species from the genus Lobelia at 70 sites across the eastern United States. We were also able to examine four pairs of species that co-occurred at the same site.
Results/Conclusions
We observed strong variation in mycorrhizal colonization among Lobelia species in their native sites across the United States. Average colonization ranged from a low of 14% in narrowly distributed Lobelia brevifolia to a high of 49% in widespread Lobelia inflata. In the four pairs of co-occurring species, differences in colonization between the species at co-occurring sites reflected differences observed across the species ranges. This indicates that differences in colonization among species are likely due to variation in mycorrhizal dependence rather than soil fertility, although analysis of soil nutrients at these sites is ongoing. In one pair (L. brevifolia and Lobelia nuttallii), the difference in colonization was exaggerated at sites where the species co-occurred compared to sites where they did not co-occur, indicating enhanced niche partitioning when together. These results highlight the role that mycorrhizal dependency may play in niche partitioning and the co-existence of closely related species.