Belowground growth responses of mature Picea sitchensis forest stand at different levels of soil warming

Climate warming is predicted to be more pronounced in higher latitudes. As climate warms, it results in higher surface temperatures, subsequently raising the soil temperature, thus affecting soil processes, such as root growth and belowground C input. To improve our predictions of the response of boreal forests to climate change, it is important to better understand the effects the warming has on fine root production and mortality and how belowground NPP changes as compared to aboveground NPP. The ForHot research site in Iceland is a natural soil warming experiment, created in May 2008 by a major earthquake, after which the geothermal bedrock channels became warm in previously cold areas. In this study we use an experimental site with a stand of 50 year old Sitka spruce (Picea sitchensis), growing in soils, with a warming gradient from 0 to +6°C between 2008 and 2018.

For this study we used soil coring to assess fine root biomass (FRB), and minirhizotron technique to assess the fine root longevity. By combining these data, we were able to estimate the absolute values of fine root turnover, and thus the belowground litter input as well as belowground NPP along the soil warming gradient. By assessing the aboveground litter input with litter traps, we were as well able to estimate the ratio of above- and belowground litter inputs into the soil along the warming gradient. Our results showed a decrease in the fine root biomass with higher soil temperature, higher fine root turnover rate, and a higher ratio of above/belowground litter input.