Using fungal traits of ectomycorrhizas to understand carbon and nutrient dynamics of boreal forests

Background/Question/Methods

Mycorrhizal fungi are often viewed as functional extensions of roots. This perspective interprets fungal function in the light of host demands. However, this view ignores fungal agency. When viewed from the fungal perspective, what resources a host can give may be more relevant to fungal function than what resources a host demands. In two separate boreal forests, we examined shifts in exploration type, a trait of ectomycorrhizal fungi representing foraging strategy, in response to changes in soil resources. We evaluated the evidence for two competing hypotheses: 1) ‘host-demand’, and 2) ‘host-supply’. When viewed as functional extensions of roots, we predicted that ectomycorrhizal fungi with abundant emanating hyphae, namely distance-exploration types, should increase in abundance when soil resources are limited, representing the ‘host-demand’ hypothesis. Here, emanating hyphae facilitate resource uptake. When viewed from the fungal perspective, we predicted that distance-exploration types should increase under conditions of host carbon surplus, representing the ‘host-supply’ hypothesis. Here, emanating hyphae increase in response to carbon. To test our predictions, we surveyed exploration types in Pinus banksiana forests along a gradient of stand age (2–76 years) where inorganic nitrogen typically declines, and in Populus tremuloides forests where inorganic nitrogen had been added for 13 years.

Results/Conclusions

In Pinus banksiana forests, both fine root and leaf area increased in stands up to 30–36 years old, and then plateaued. As expected in older forests, decreases in inorganic nitrogen likely coincided with stand decline. The abundance of distance-exploration types paralleled changes to leaf area, contrary to predictions of the host-demand hypothesis. That the abundance of distance-exploration types tracked stand leaf area suggests carbon may limit the production of emanating hyphae as stands grow old. In Populus tremuloides forests with added inorganic nitrogen (30 kg ha−1 year−1), stem radial growth initially increased, but it was not sustained at the time we characterized ectomycorrhizas of the trees. After 13 years, the abundance of distance-exploration types doubled, counter to predictions of the ‘host-demand’ hypothesis. This latter outcome may be the result of a host carbon surplus brought on by a secondary resource limiting tree growth, which increased the production of emanating hyphae. Taken together, results from these field studies demonstrate that ectomycorrhizal fungi are not mere functional extensions of roots, i.e., foraging for resources to meet their host demands. To understand the function of ectomycorrhizal fungi requires a broader recognition of their agency.