PhD defense: Qiaoyan Li

Qiaoyan Li defends her thesis,

Effects of climate change on the carbon exchange and balance in shrubland ecosystems

Zoom link

Supervisors:
Associate Professor Klaus Steenberg Larsen, IGN, Denmark
Professor Per Gundersen, IGN, Denmark

Assessment committee:
Professor Bjarni Sigurdsson, Agricultural University of Iceland, Iceland
Associate Professor Andreas Ibrom, DTU, Denmark
Associate Professor Jesper Riis Christiansen (Chair), IGN, Denmark

Summary:
Europe has experienced several major summer heatwaves and drought events in the past decades, which have severely influenced terrestrial ecosystems. The response of soil carbon dynamics as well as the CO2 fluxes to global climate change remains one of the largest uncertainties for future climate projection. This dissertation investigated the effects of climate change on soil carbon dynamics and CO2 fluxes in European shrubland ecosystems.
In the CLIMAITE project, we found that soil carbon stocks increased by approximately 19% during the long-term, multi-factorial climate experiment with elevated CO2, warming, and summer drought in 2005-2013. The fate and stability of this extra carbon were studied seven years after the experiment terminated, indicating that soil carbon and nitrogen pools in this ecosystem are highly dynamic and may change rapidly in response to changes in major ecosystem drivers and causing potentially large feedback to climate change. Another study focuses on the changes in different ecosystem CO2 flux components as well as the total carbon balance over the lifecycle of Calluna vulgaris (L.). We found the ecosystem carbon balance was highly non-linear across a three-decade timescale, exhibiting a sinusoidal curvature between carbon sink/source strength and time after disturbance. This study indicated optimizing the vegetation management cycles could be beneficial from the perspective of maximizing the ecosystem carbon uptake and storage capacity in heathland ecosystems. Additionally, we also proposed a general conceptual framework, which explained the observed responses of gross primary productivity (GPP) and ecosystem respiration (Reco) to drought and warming across six European shrubland ecosystem sites based on the site-specific positioning of each site on the growing season soil moisture curves. This study demonstrated that GPP is more sensitive to drought and warming than Reco. Furthermore, a novel stepwise modeling approach was applied to a Danish heathland/grassland ecosystem, revealing asymmetric responses of GPP and Reco in response to long-term drought. This research underlines the importance of understanding different treatment effects on model parameters, which could improve our projections of ecosystem responses to future climate change.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk