A dual perspective: Plant and fungal traits that underlie shifts in mycorrhizal growth responses to rising [CO2]

Background/Question/Methods

Mycorrhizal associations play a critical role in terrestrial ecosystems, and changes in atmospheric [CO₂] have likely impacted mycorrhizal functioning by altering resource availability to plants and fungi. Previous mycorrhizal-CO₂ studies have mainly focused on the effects of modern vs. future conditions. However, assessing mycorrhizal responses to a wider [CO₂] gradient is critical for (1) establishing a baseline for how these symbioses functioned prior to anthropogenic forcing, (2) assessing potential nonlinear responses to global change, and (3) characterizing mechanisms that underlie shifts in mycorrhizal functioning. Here, we examined mycorrhizal associations in two Taraxacum hosts across a broad [CO₂] gradient reflecting glacial through future conditions in a controlled growth chamber experiment. Taraxacum ceratophorum and T. officinale (Asteraceae) were grown with and without mycorrhizal fungi under 180, 270, 390, 700, and 1000 ppm [CO₂] (constant nutrient availability). After 30 d, we assessed [CO₂] effects on the mycorrhizal growth response of each host (R). To better understand mechanisms that underlie shifts in R, we further assessed [CO₂] effects on both plant and fungal traits. We hypothesized that R would increase with rising [CO₂] due to shifts in the relative degree of carbon vs. nutrient limitation within hosts.

Results/Conclusions

We found that (1) changes in [CO₂] had a larger effect on plant traits compared to fungal traits, (2) rising [CO₂] differentially affected the mycorrhizal growth response of T. officinale and T. ceratophorum, and (3) traits underlying shifts in R differed for the Taraxacum hosts. For T. officinale, R increased with rising [CO₂], and 700 ppm [CO₂] represented a “tipping point” above which R shifted from negative to positive.  Shifts in R were strongly correlated with mycorrhizal effects on relative leaf C, N, and P content. For T. ceratophorum, R increased slightly from 180 to 390 ppm [CO₂], which further supported our hypothesis that mycorrhizal benefits increase with rising [CO₂]. However, 390 ppm [CO₂] represented a "tipping point" after which further increases in [CO₂] caused R to decrease and eventually shift from positive to negative. For this host, fungal growth increased with rising [CO₂], and shifts in R were positively correlated with mycorrhizal effects on photosynthetic rates. Overall, our results suggest that rising [CO₂] impacts the functioning of mycorrhizal associations largely through effects on plant traits. Furthermore, rising [CO₂] generally increases mycorrhizal growth responses; however, this trend may not hold for all species.