Geothermal ecosystems as natural climate change experiments: The ForHot research site in Iceland as a case study
Sigurdsson, B.D., N.I.W. Leblans, S. Dauwe, E. Guðmundsdóttir, P. Gundersen, G.E. Gunnarsdóttir, M. Holmstrup, K. Ilieva-Makulec, T. Kätterer, B. Marteinsdóttir, M. Maljanen, E.S. Oddsdóttir, I. Ostonen, J. Peñuelas, C. Poeplau, A. Richter, P. Sigurðsson, P.v. Bodegom, H. Wallander, J. Weedon and I. Janssens. 2016.
Abstract
This article describes how natural geothermal soil temperature gradients in Iceland have been used to study
terrestrial ecosystem responses to soil warming. The experimental approach was evaluated at three study sites
in southern Iceland; one grassland site that has been warm for at least 50 years (GO), and another comparable
grassland site (GN) and a Sitka spruce plantation (FN) site that have both been warmed since an earthquake
took place in 2008. Within each site type, five ca. 50 m long transects, with six permanent study plots each, were
established across the soil warming gradients, spanning from unwarmed control conditions to gradually warmer
soils. It was attempted to select the plots so the annual warming levels would be ca. +1, +3, +5, +10 and +20
°C within each transect. Results of continuous measurements of soil temperature (Ts) from 2013-2015 revealed
that the soil warming was relatively constant and followed the seasonal Ts cycle of the unwarmed control plots.
Volumetric water content in the top 5 cm of soil was repeatedly surveyed during 2013-2016. The grassland soils
were wetter than the FN soils, but they had sometimes some significant warming-induced drying in the surface
layer of the warmest plots, in contrast to FN. Soil chemistry did not show any indications that geothermal water
had reached the root zone, but soil pH did increase somewhat with warming, which was probably linked to
vegetation changes. As expected, the potential decomposition rate of organic matter increased significantly with
warming. It was concluded that the natural geothermal gradients at the ForHot sites in Iceland offered realistic
conditions for studying terrestrial ecosystem responses to warming with minimal artefacts.
Key Words
geothermal soil warming; subarctic grasslands; climate change; spruce forest; decomposition
