Mycorrhizal functioning across the glacial-interglacial transition: Evidence from stable isotopes

Background/Question/Methods

Mycorrhizal associations are symbiotic plant-fungal interactions that function along a continuum from mutualism to parasitism (M-P continuum) depending on the balance between plant carbohydrate production and soil nutrient availability. Dramatic changes in atmospheric [CO₂] over geologic and contemporary time scales have altered the cost of carbohydrate production within plants, and these changes likely influenced mycorrhizal functioning. To date, little is known about whether mycorrhizal associations shifted along the M-P continuum over geologic time. In this study, we applied stable isotope techniques to plant fossils from ancient packrat middens (35,000–4,000 years old), herbarium specimens (30–70 years old) and modern specimens to assess whether mycorrhizal functioning shifted across the glacial-interglacial transition when atmospheric [CO₂] rose from 180 ppm to 390 ppm. Specifically, we used leaf carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to assess plant C and N dynamics. All plant specimens were collected from forested habitats in the Snake Range (NV, USA). Ectomycorrhizal fungi (EMF) colonize the dominant pine tree species in these habitats. Co-occurring juniper trees and understory species, which are colonized by arbuscular mycorrhizal fungi, were used as “controls” in comparisons with EM pines.

Results/Conclusions

Leaf δ¹⁵N suggest that pine trees from the Snake Range were consistently colonized by EMF over the glacial-interglacial transition. However, leaf δ¹³C suggest that low [CO₂] during glacial periods reduced the amount of C available for photosynthesis, which may have restricted the ability of pine trees to support EMF. Furthermore, leaf N content indicates that N availability has decreased with rising [CO₂], which may have altered the relative benefit of EMF during glacial and interglacial periods. Taken together these results suggest that changing atmospheric [CO₂] impacted mycorrhizal functioning in one of two ways. First, EMF may have been more likely to function as parasites under low [CO₂] due to shifts in the balance between C and N availability. Second, EMF may have functioned as mutualists under low [CO₂] if enhanced N demands for photosynthesis outweighed the carbohydrate costs of these fungi during glacial periods. In conclusion, this study provides the first quantitative evidence that mycorrhizal functioning may have shifted with changing atmospheric [CO₂] over geologic time. Previous estimates of C availability to plants during glacial periods did not account for mycorrhizal fungi as a C sink. Thus, by incorporating mycorrhizal fungi into plant C and N dynamics, this study provides novel insights into the importance of shifting resource availability for both plants and their fungal symbionts.