Limited overall impacts of ectomycorrhizal inoculation on recruitment of boreal trees into Arctic tundra following wildfire belie species-specific responses

Abstract

We tested whether post-fire seedling establishment of common boreal tree and expanding shrub species at treeline and in Arctic tundra is facilitated by co-migration of boreal forest mycorrhizal fungi. Wildfires are anticipated to facilitate biome shifts at the forest-tundra ecotone by improving seedbed conditions for recruiting boreal species; at the same time fire alters the composition and availability of mycorrhizal fungi critical to seedling performance. To determine the role of root-associated fungi (RAF) in post-fire seedling recruitment and future biome shifts, we outplanted four dominant boreal tree and shrub species inoculated with one of three treatments at treeline and in tundra: burned boreal forest, unburned boreal forest, or a control treatment of sterilized inoculum. We compared survivorship, growth, and physiological performance of the seedlings in relation to mycorrhizal inoculum treatment and among host species, characterized the RAF communities based on ITS-rDNA sequencing of individual root tips sampled from surviving seedlings, and tested for correlations between RAF composition and the inoculation treatments, host species, and duration of the experiment. We explored correlations between RAF composition and seedling metrics. Both live and sterile autoclaved inoculation treatments had similar effects on seedling survivorship and growth for all species. RAF composition did not vary by treatment, suggesting that most colonization was due to local fungi. However, seedling traits and growth were correlated with RAF species composition, colonization, and the relative abundance of specific RAF taxa. Picea sp. performance in particular showed strong co-variation with RAF metrics. Our results suggest that mycorrhizal co-migration is not a primary limiting factor to boreal seedling recruitment because the experimental provision of inoculum did not affect seedling recruitment; yet, RAF did influence seedling performance, particularly resident RAF at treeline and in tundra, suggesting that mycorrhizal fungi are important to vegetation processes at the treeline-tundra ecotone.

Fig 1. Proportion of fine root length colonized by mycorrhizal fungi for each host plant species and mycorrhizal inoculation treatment.

Seedlings were harvested two years after outplanting in Arctic tundra. The lower and upper bounds of the boxplot show the first and third quartiles (the 25^th and 75^th percentiles), the middle line shows the median, whiskers above and below the boxplot indicate 1.5* inter-quartile range, and points beyond the whiskers indicate outlying points.

Fig 2

Shifts in isotope signatures of foliar a. nitrogen and b. carbon for seedlings treated with mycorrhizal inoculum and harvested two years after outplanting in Arctic tundra. Δ¹⁵N and Δ ¹³C were calculated by subtracting the average foliar δ¹⁵N and δ¹³C for each seedling species at the time inoculation from the foliar signature of each seedling at the time of harvest two years after outplanting. See S6 Table for specific contrasts. The lower and upper bounds of the boxplot show the first and third quartiles (the 25^th and 75^th percentiles), the middle line shows the median, whiskers above and below the boxplot indicate 1.5* inter-quartile range, and points beyond the whiskers indicate outlying points.

Fig 3

The proportion of root length colonized by root-associated fungi and foliar a. δ¹⁵N and b. δ¹³C of Picea glauca and Picea mariana. Panel a regression line fit for Picea glauca with S.E. shaded. Panel b regression line fit for Picea glauca (dotted) and Picea mariana (dot-dashed) with S.E. shaded.

Fig 4. Venn diagram of fungal taxa observed in the live inoculum and on seedling roots.

Fig 5. NMDS bi-plot of root-associated fungi (RAF) associated with seedlings outplanted for one or two years in Arctic tundra.

Vectors show direction and magnitude of significant correlations between RAF composition and RAF taxa.

Fig 6. NMDS bi-plot of root-associated fungi (RAF) associated with seedlings outplanted for two years in Arctic tundra.

Vectors show direction and magnitude of significant correlations between RAF composition and seedling metrics.

Fig 7. Spearman correlations between the traits of seedlings outplanted in tundra and harvest after two years and the relative abundance of root-associated fungi.

Green colors indicate positive and gray indicates negative Spearman Rho correlation coefficients. All relationships are significant at p<0.05. Solid colors indicate correlations with Picea mariana and the dotted pattern indicates correlation with Betula neo-alaskana.