Local adaptation to mycorrhizal fungi in geographically close Lobelia siphilitica populations.

Mutualism between plants and arbuscular mycorrhizal (AM) fungi is common, and plant populations are expected to have adapted to the AM fungal communities occupying their roots. Tests of this hypothesis have frequently been done with plant populations that are tens to hundreds of kilometers apart. However, because AM fungal community composition differs at scales < 1 km, local adaptation of plant populations to AM fungi may occur at small spatial scales, but this prediction has not been tested. Furthermore, prior experiments do not often experimentally identify whether adaptation is related to specific mycorrhizal functions. To test for plant adaptation to AM fungal communities at small spatial scales, and whether adaptation is associated with the nutritional benefits that AM fungi provide to plants, we grew Lobelia siphilitica plants from two geographically close populations (1.4 km apart) in a greenhouse reciprocal transplant experiment with soil biota that either included (whole soil) or excluded AM fungi (microbial wash) at both low and high soil phosphorus availability. Though both plant populations responded positively to the presence of AM fungi in the whole soil biota treatment relative to the microbial wash treatment, the average growth response of plant populations to mycorrhizal fungi was highest when local populations were grown with local AM fungi. In addition, local adaptation was only observed in the presence of AM fungi at low phosphorus levels. Thus, local adaptation of plant populations to AM fungi is present at spatial scales that are much smaller than previously demonstrated and occurred primarily to enhance phosphorus acquisition.