Plant-soil-microbial interactions mediate vegetation succession in retreating glacial forefields.

Global warming is accelerating glacial retreat and leaving open areas for vegetation succession on young developing soils. Soil microbial communities interact with plants affecting vegetation succession, but the specific microbial groups controlling these interactions are unclear. We tested whether plant-soil-microbial interactions explain plant primary succession in the Gongga Mountain glacial retreat chronosequence. The direction and intensity of plant-soil-microbial interactions were quantified by comparing the biomass of one early-, two mid- and two late-succession plant species under sterilized vs. live, and inter- vs. intra-specific competition. The performance of most plant species was negatively affected by soil biota from early habitats (5-10 yr), but positively by soil biota from mid- (30-40) and late-succession (80-100) habitats. Two species of Salicaceae from middle habitats, which are strong competitors, developed well on the soils of all successional stages and limited the establishment of later serial plant species. The strongest microbial drivers of plant-microbial interactions changed from i) saprophytic fungal specialists during the early stage, to ii) generalists bacteria and arbuscular mycorrhizal fungi in the middle stage, and finally to iii) ectomycorrhizal fungal specialists in the late stage. Microbial turnover intensified plant-soil-microbial interactions and accelerated primary succession in the young soils of the glacial retreat area.

Post-fire vegetation succession in the Siberian subarctic tundra over 45 years.

Wildfires are relatively rare in subarctic tundra ecosystems, but they can strongly change ecosystem properties. Short-term fire effects on subarctic tundra vegetation are well documented, but long-term vegetation recovery has been studied less. The frequency of tundra fires will increase with climate warming. Understanding the long-term effects of fire is necessary to predict future ecosystem changes. We used a space-for-time approach to assess vegetation recovery after fire over more than four decades. We studied soil and vegetation patterns on three large fire scars (>44, 28 and 12 years old) in dry, lichen-dominated forest tundra in Western Siberia. On 60 plots, we determined soil temperature and permafrost thaw depth, sampled vegetation and measured plant functional traits. We assessed trends in Normalized Difference Vegetation Index (NDVI) to support the field-based results on vegetation recovery. Soil temperature, permafrost thaw depth and total vegetation cover had recovered to pre-fire levels after >44 years, as well as total vegetation cover. In contrast, after >44 years, functional groups had not recovered to the pre-fire state. Burnt areas had lower lichen and higher bryophyte and shrub cover. The dominating shrub species, Betula nana, exhibited a higher vitality (higher specific leaf area and plant height) on burnt compared with control plots, suggesting a fire legacy effect in shrub growth. Our results confirm patterns of shrub encroachment after fire that were detected before in other parts of the Arctic and Subarctic. In the so far poorly studied Western Siberian forest tundra we demonstrate for the first time, long-term fire-legacies on the functional composition of relatively dry shrub- and lichen-dominated vegetation.