A comparison of the physiologies of ectomycorrhizal fungal communities from clearcuts and mature forests

Background/Question/Methods

Ectomycorrhizal fungi (EMF) form associations with plant roots and play a significant role in acquiring nutrients for the plant in exchange for carbon. After clearcutting, a shift in EMF community structure has been repeatedly documented in clearcuts compared to adjacent forests. It is unknown why this shift occurs or if the EMF communities differ physiologically. There are two theories to explain this species shift:  (i) it is due to a reduction of ectomycorrhizal fungal inoculum from living mycorrhizas in the clearcut thereby allowing fungi that propagate from spores to dominate, (ii) EMF that are more effective at acquiring nutrients in each soil environment (clearcut vs. forest) are the most successful at colonizing roots in that environment. If the latter is true then one would expect clearcut and forest EMF communities to differ functionally with respect to taking up nutrients. It would then be expected that plants colonized by clearcut fungi would acquire more nutrients in clearcuts than if colonized by forest fungi, and vice versa. To address this, naturally regenerated 1-yr-old subalpine fir (Abies lasiocarpa(Hook.) Nutt.) seedlings were reciprocally transplanted between clearcut and forest environments at snowmelt and harvested at the end of the growing season. Apical buds produced during the growing season were analyzed for nitrogen content and biomass. Percent colonization was also determined. 

Results/Conclusions

Soil analysis showed that nitrogen forms differed between clearcut and forest soils. Preliminary results demonstrated that seedlings originating from the clearcut accumulated significantly more nitrogen and produced larger apical buds than seedlings from the forest when grown in either clearcut or forest environments. Seedlings originating from the clearcut had a lower percent colonization rate than seedlings originating from the forest and percent colonization rates did not change when seedlings were grown for three months in the other environment. Initial morphological observations (to later be confirmed with molecular identification) suggest that root systems were initially colonized by different fungi and that these did not change after transplantation.   

These results indicate that seedlings originating from clearcuts acquired more nitrogen after transplantation into either environment than seedlings originating from the forest environment. This suggests that ability to supply nitrogen to a seedling in a particular environment is not a major factor determining successful colonization by EMF. It also implies that any loss of forest fungi from the community after clearcutting does not have an immediate negative effect on seedling establishment.