Fine Root System Acclimations before Dieback with Artificial and Geothermal Soil Warming

Global warming is predicted to impact high-latitude and high-altitude forests strongly, threatening their functioning and carbon storage, both of which depend on the warming response of tree fine root systems. This presentation reports the effects of soil warming on the biomass, morphology, and colonizing ectomycorrhizal (EcM) community of spruce fine and absorptive roots. We compare the results from a man-made long-term soil warming (+4°C) experiment to the responses of spruce roots growing at a geothermal soil temperature gradient (+1 to +14°C) extending to the forest die-off edge, to shed light on the generalisability of the findings and reveal any thresholds in acclimation ability. The artificial warming site (a Picea abies stand) was located in central Europe, Austria, the geothermal gradient (a Picea sitchensis stand) was situated in northern Europe, Iceland.

Soil warming caused simultaneous changes in all studied organizational levels of the tree fine root systems. Trees in warmer soils formed longer and less branched absorptive roots with higher specific root length and area, and lower root tissue density in both spruce stands, irrespective of location and warming method. Soil warming at the central European site also supported a more varied EcM community and an increase in the abundance of Tomentella spp., implying a shift in nutrient foraging. Fine and absorptive root biomass decreased towards warmer soil, with a rapid decline occurring between +4°C and +6°C from the ambient and leading to the collapse of the fine root system at the northern European site. At the central European site, the applied +4°C warming had no effect on fine and absorptive root biomass. The similar fine root responses at the two warming sites suggest that the observations possibly reflect general acclimation patterns in spruce forests to global warming.