Mycorrhizas and secondary succession in aspen-conifer forests: Light limitation differentially affects a dominant early and late successional species

Background/Question/Methods

Plant succession and mycorrhizal fungi both play crucial roles in shaping the development of forest ecosystems.  Despite the strong potential for interactions between them, few studies have examined how patterns of forest succession affect mycorrhizal associations.  The majority of tree species in subalpine forests form ectomycorrhizal (EM) associations and are reliant on the increased resource acquisition to be competitive in these nutrient restricted environments.  Fire suppression in subalpine forests over the last century has changed successional patterns in ways that may affect mycorrhizal associations with host trees.  To better understand these relationships we conducted field and greenhouse studies in which we examined mycorrhizal infection along gradients of light intensity and soil nutrient availability that develop as aspen become seral to conifers under longer fire return intervals.   We examined whether ectomycorrhizal associations of quaking aspen (Populus tremuloides), a shade intolerant, early succession species were more sensitive to light and soil resource limitations than subalpine fir (Abies lasiocarpa), a shade tolerant, late succession species.

Results/Conclusions

In the field study, ectomycorrhizal infection of aspen roots was reduced by 50% in conifer dominated stands relative to aspen stands.   In contrast, subalpine fir maintained its EM associations regardless of the successional status of the stand.  The greenhouse results were consistent with field results and indicated that light limitation was the driving force behind reductions in EM infection of aspen roots in later stages of succession.  These results suggest nutrient limitations that constrain early successional species may be aggravated by losses in EM associations via light limitations created by late successional species.  This is a potential mechanism by which climax forest species create a competitive advantage over early successional species and our data suggest that this succession is exacerbated by longer fire return intervals.