Long and medium-term interannual assessment of sub-Arctic grassland aboveground biomass at different soil warming levels

Climate change affects ecosystems considerably worldwide, but as warming is happening at an accelerated pace at higher latitudes, it is essential to study how warming affects ecosystem structure and function in Arctic and sub-Arctic ecosystems.

In this research, we looked at changes in the aboveground biomass (AGB) of two Icelandic sub-Arctic grasslands located at the ForHot site. The ForHot site is an exceptional studying site where the soils are naturally warmed. Thus, making it an important natural laboratory to assess and learn more about the long-term effect of global warming. At the research site, one grassland ecosystem has soils that have been warmed for over 60 years (long-term warming; LTW) and the other since 2008 (medium-term warming; MTW), when an earthquake disrupted geothermal channels in the underlying bedrock. Fifty permanent survey plots were established in the autumn of 2012 along the two grassland soil temperature gradients (ranging from 0 to +18°C).

We assessed how vegetation structure (non-vascular; AGB_non-vasc and vascular plants; AGB_vasc) and the ecosystem's maximum aboveground total biomass(AGB_tot) were affected by different levels of soil warming over multiple studied years (i.e. 2013, 2016, 2018, 2020, 2021 and 2022).

Our preliminary results showed unexpectedly relatively small changes in AGB_tot with warming. We hypothesise that changes in AGB_vasc would typically induce opposite changes in AGB_non-vasc, probably because of light competition. When looking separately at the vegetation structures, for AGB_vasc, the duration of soil warming induced contrasted responses between MTW and LTW grasslands. That is, in the MTW grassland, there were no changes for most years (p > .05) and strong negative responses (p < .05) with warming in seasonally maximum AGB_vasc for other years. Whereas in the LTW grassland, warming generally increased AGB for most years (p < .05), and also a strong negative response as observed in the MTW for the respective years despite statistically not significant. This strong negative response could be because of untypically high AGB production in control (unwarmed) plots during those years and less positive reactions with different levels of soil warming. We will show some potential drivers (environmental variables) for those unexpected temporal variations in the warming response. AGB_non-vasc, such as lichens and mosses, have an unclear pattern across the soil warming gradient in both grassland ecosystems.