Deciduous shrub encroachment is occurring across the arctic tundra. Increases in shrub cover could alter belowground carbon and nutrient dynamics as a consequence of shifts in root function. However, root dynamics associated with shrub expansion remain one of the least understood aspects of plant function in the Arctic. The consideration of tundra root-trait data of both encroaching shrubs and endemic tundra species across an environmental gradient may enhance our understanding of belowground functioning in the Arctic. We sampled roots of common deciduous shrub genera (Betula, Alnus, and Salix) and tussock grass/sedge vegetation (dominantly Eriophorum spp.) in five moist acidic tundra sites along a latitudinal gradient in Northern Alaska (67.0 °N – 69.3 °N). Root morphology, chemistry and mycorrhizal colonization of the non-woody absorptive roots were measured for both shrubs and tussock vegetation. Vertical distributions of absorptive root biomass within a shrub or tussock patch (1-m diameter) were determined from soil coring to the bottom of the active layer.
Results/Conclusions
We found that nutrient strategies of shrub roots are acquisitive whereas tussock roots are conservative. The absorptive roots of shrubs were relatively thin, frequently branched, and exhibited high nitrogen concentration ([N]) whereas tussock roots were thick, rarely branched and low in [N]. We also found frequent ectomycorrhizal colonization for all three types of shrub vegetation, which allow shrubs to mine organic nutrients from the arctic soils. In contrast, inorganic nutrients derived from microbial decomposition of plant litters are the obligate N source for the non-mycorrhizal tussock species. The different forms of nutrient sources were further confirmed by contrasting values of natural abundance of d15N in shrub vs. tussock roots. Finally, we found that shrub root can explore deeper soil in some areas, indicating that accumulated nutrients in the deep layer of the arctic tundra could have likely been immobilized by the deep roots induced by the recently established shrubs. Furthermore, there were also pronounced differences in root traits among shrub genera. Salix had absorptive roots that were generally thinner and less dependent on ectomycorrhizal fungi than Betula and Alnus. The accumulative root biomass in the active layer was higher under Alnus patches than Betula patches at the southernmost site. In contrast, Betula patches had the most abundant roots at the two northern sites, especially in the deeper layers. These climate-dependent variations indicate that deciduous shrub genera employ unique root strategies despite their similar growth form.