Root-associated ectomycorrhizal fungi shared by various boreal forest seedlings naturally regenerating after a fire in interior alaska and correlation of different fungi with host growth responses

Abstract

The role of common mycorrhizal networks (CMNs) in postfire boreal forest successional trajectories is unknown. We investigated this issue by sampling a 50-m by 40-m area of naturally regenerating black spruce (Picea mariana), trembling aspen (Populus tremuloides), and paper birch (Betula papyrifera) seedlings at various distances from alder (Alnus viridis subsp. crispa), a nitrogen-fixing shrub, 5 years after wildfire in an Alaskan interior boreal forest. Shoot biomasses and stem diameters of 4-year-old seedlings were recorded, and the fungal community associated with ectomycorrhizal (ECM) root tips from each seedling was profiled using molecular techniques. We found distinct assemblages of fungi associated with alder compared with those associated with the other tree species, making the formation of CMNs between them unlikely. However, among the spruce, aspen, and birch seedlings, there were many shared fungi (including members of the Pezoloma ericae [Hymenoscyphus ericae] species aggregate, Thelephora terrestris, and Russula spp.), raising the possibility that these regenerating seedlings may form interspecies CMNs. Distance between samples did not influence how similar ECM root tip-associated fungal communities were, and of the fungal groups identified, only one of them was more likely to be shared between seedlings that were closer together, suggesting that the majority of fungi surveyed did not have a clumped distribution across the small scale of this study. The presence of some fungal ribotypes was associated with larger or smaller seedlings, suggesting that these fungi may play a role in the promotion or inhibition of seedling growth. The fungal ribotypes associated with larger seedlings were different between spruce, aspen, and birch, suggesting differential impacts of some host-fungus combinations. One may speculate that wildfire-induced shifts in a given soil fungal community could result in variation in the growth response of different plant species after fire and a shift in regenerating vegetation.

Fig. 1.

Diagram of sampling scheme used to harvest plants in the vicinity of each alder (A, center). Quadrants were established on the basis of compass points (northeast, northwest, southeast, and southwest) and areas of various distances from alder (0 to 3 m and 4 to 7 m) identified. From each of these eight areas, one spruce, one aspen, and one birch seedling were located and harvested as described in the text. Samples depicted indicate locations of seedlings harvested relative to alder 1 (not to scale).

Fig. 2.

Proportions of each plant species' total observed ARISA peaks per fungal ribotype for alder (a), aspen (b), birch (c), and spruce (d) seedling roots. Minor ribotypes are defined as those containing less than 1% of the total peaks observed for all species. The letter A or B on pie slices corresponding to 705 to 708 bp indicates different OTUs (see Table 1 and text for details). Numbers on minor ribotype pie slices indicate how many ribotypes are contributing to the size of the slice.

Fig. 3.

Result of an NMS ordination where fungal root tip community data from seedlings of the same species are pooled for each of the three sampling areas around each focal alder, as described in the text. Different plant species are represented by different symbols: ▾, aspen; ▴, birch; ▪, spruce, and •, alder. Note that the alder communities are distinct from those of the other three plant species.