Changing climactic conditions will shift the position along the continuum from parasite to mutualist of many plant symbionts. Arbuscular mycorrhizae fungi (AMF) provide nutrients in exchange for photosynthate, but they can also increase drought tolerance. Epichloë produce alkaloid compounds, which can protect hosts from herbivory. Climate change models predict warming and increased seasonality of precipitation in the Pacific Northwest (PNW), which may exaggerate plant drought stress in the region. Thus, the interaction between Epichloë endophytes and AMF has implications for plants’ ability to adapt to these climactic changes. We hypothesized that with warming and drought stress, grasses that host both symbionts will favor AMF at the expense of Epichloë.
We tested this hypothesis in a manipulative climate change study embedded within a 520-km natural climate gradient in three PNW grasslands. The climate treatments are a full factorial crossing of heat (+3.0˚C) and precipitation (+20%) at each site. We collected the grasses Bromus hordeaceus and Agrostis capillaris from all plots in which they were available. The grasses were tested for Epichloë infection using an immunoblot test, and AMF colonization was quantified along with dark septate root endophytes (DSE), a poorly understood, polyphyletic group of root-colonizing ascomycetes.
Results/Conclusions
In B. hordeaceus, heating increased AMF colonization (p=0.071, n=30), while Epichloë infection decreased AMF colonization (p=0.047, n=30). There was also a non-significant trend for Epichloë infection to decrease with heating. There was no interaction between the DSE and AMF. However, Epichloë infection decreased colonization by DSE (p=0.003, n=30), in the same way that it decreased colonization by AMF.
One major effect of heating is faster drying of the soil, and plants experienced drought stress in heated treatments. There is evidence for apparent competition between AMF and Epichloë in Bromus hordeaceus. The fact that Epichloë infection decreased both DSE and AMF colonization, but the two did not seem to affect each other, indicates inhibition of other symbionts by Epichloë-produced alkaloid compounds, rather than competition for photosynthate. Since AMF colonization is higher in the heated plots, while Epichloë infection is more rare, it is possible that the host plants encourage AMF over Epichloë thereby increasing their drought tolerance.