Functional complementarity of Douglas‐fir ectomycorrhizas for extracellular enzyme activity after wildfire or clearcut logging

Summary 1. Clearcut logging results in major changes in ectomycorrhizal fungal communities, but whether this results in the loss of key functional traits, such as those associated with nutrient acquisition from soil organic matter, is unknown. Furthermore, little is known about the importance of resource partitioning in structuring ectomycorrhizal fungal communities following disturbance because most research on these communities has focussed on life history strategies. By studying functional traits, such as activities of enzymes involved in the catabolism of organic macromolecules in soil, we can determine whether a physiological potential for resource partitioning exists in pioneer ectomycorrhizal communities and whether severe disturbance affects these important ecosystem services. 2. We used activities of key hydrolytic enzymes in the ectomycorrhizospheres of Douglas‐fir seedlings regenerating at clearcut sites as a functional trait to test whether these differed from those at recent wildfire sites or control forests. We sampled the most abundant types of ectomycorrhizas from 16‐month‐old seedlings from sites exposed to (i) low or (ii) high severity wildfire, (iii) sites that had been clearcut logged in the same year as the fire and (iv) sites that contained control stands of mature Douglas‐fir. We expected differences in activities among ectomycorrhizas sampled from different disturbance treatments and among those formed by different fungal species. 3. In spite of large differences in soil chemistry, activities of acid phosphomonoesterase, N‐acetylglucosaminidase and β‐glucosidase, when averaged among the ectomycorrhizas sampled per site, were not affected by disturbance agent. However, activities varied up to sixfold among mycorrhizospheres of different fungal species on the same seedling. Multivariate analysis also indicated some consistent differences in enzyme profiles among ectomycorrhizas formed by specific fungal species, independent of treatment. 4. The finding that ectomycorrhizal fungal communities exposed to different disturbance agents are functionally similar with respect to the activities of three mycorrhizosphere enzymes supports the conclusion that complementarity exists among ectomycorrhizal fungi. The substantial physiological diversity among ectomycorrhizal fungi at the scale of an individual seedling’s root tips, especially at control mature forests, indicates the potential for resource partitioning within the ectomycorrhizal community and access to a wider range of nutrient sources by each seedling. 5. Functional similarity among ectomycorrhizal fungal communities across a disturbance severity gradient suggests that dry interior Douglas‐fir forests are resilient to severe disturbances such as high severity wildfire and clearcutting with forest floor removal. Moreover, our results suggest that current harvesting practices emulate natural disturbances with respect to site‐level mycorrhizosphere enzyme activity. The large variation in activity among fungal species, however, indicates that a substantial simplification of the fungal community through other perturbations, as expected with climate change, has the potential to affect ecosystem function.

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