Fungus among us: Biotic and abiotic factors influencing the distribution of alpine mycorrhizae

Background/Question/Methods: Habitat heterogeneity can influence species distributions across landscapes. At our study site in the Colorado Rocky Mountains the distribution of mycorrhizal associations, symbiotic relationships between root-colonizing fungi and plants, varies across alpine habitats.  In particular, arbuscular mycorrhizae (AMF) are more abundant in open meadows while ectomycorrhizae (ECM) are more abundant in willow understory habitats. In this study we identified factors underlying these distributional patterns. We hope to gain insight into the relative importance of biotic and abiotic conditions in determining AMF and ECM distributions.

We surveyed three different sites to explore correlations between the abundance of mycorrhizae, collembola (springtails), and leaf litter in open meadow and willow understory habitats. In each habitat we measured relative temperature, relative humidity, leaf litter biomass, collembola abundance and mycorrhizal root colonization rates. To further investigate leaf litter effects on mycorrhizal associations, we conducted a leaf litter addition/removal experiment. We removed leaf litter from willow understory plots and added it to corresponding open meadow plots. After two months we assessed mycorrhizal colonization proportions in plots with and without leaf litter in each habitat.

Results/Conclusions: We found that the proportion of root length colonized by AMF was negatively correlated with leaf litter biomass in the willow understory. ECM were positively correlated with collembola abundance in the open meadow, while AMF were positively correlated with collembola abundance in the willow understory. Results show that abiotic factors also differ across this habitat gradient. Temperature and humidity were lowest in the willow understory and highest in the open meadow, and may effect mycorrhizal colonization. Overall, this survey suggests that both biotic and abiotic factors likely influence the distribution of ECM and AMF.  We found that over this short time span leaf litter addition and removal had little effect on ECM colonization in either habitat. However, results indicate that removing leaf litter from the willow understory influenced AMF abundance.

This study demonstrates that biotic and abiotic factors vary across habitats, correlating with spatial distribution of mycorrhizal associations. It suggests that AMF and ECM respond distinctly to this variation.  Using these concepts, we created an educational lesson plan about mutualisms, habitat variation and species distribution. This lesson integrates the experiment and inquiry to teach secondary students about the role of habitat variation in creating complex ecological communities. The diverse factors studied that underlie habitat variation each play an important yet coordinated role in determining the composition of the surrounding community.