PhD defence: Hanbo Yun

Hanbo Yun defends his thesis, 

Symphony of Climate Change on the Tibet Plateau Links between permafrost thawing, plant dynamics, and greenhouse gas

Supervisor:
Professor Bo Elberling

Assessment Committee:
Professor Lena Stöm, Lund University
Associate Professor Mats Björkman, University of Gothenburg
Professor Per Lennart Ambus (chair), IGN

Abstract (shortened):
The Tibetan Plateau is highly vulnerable to climate change, rise to remarkable shift in vegetation. These changes in vegetation can subsequently impact the region's carbon balance, contributing to broader climate feedback loops. Specifically, climate change accelerates surface soil organic matter decomposition and drives permafrost thawing, both of which increase soil carbon release and enhance nitrogen availability for plants, and alter plant growth, species composition.
As the climate continues to warm, two key processes influence the carbon balance on the Tibetan Plateau: rising temperatures that stimulate soil organic matter decomposition and increase ecosystem respiration, and shifts in plant biomass distribution, diversity and growth patterns. However, the status, extent, trends, and potential mechanisms driving this carbon balance under climate change remain poorly understood. Gaining insight into ecosystem carbon dioxide dynamics and how vegetation responds is crucial for identifying the underlying drivers of carbon balance on the Tibetan Plateau.
This thesis investigates the net ecosystem carbon exchange of alpine steppe (more than one third of Tibetan Plateau area), along with plant growth (maximum root depth, biomass distribution between aboveground and belowground) and community changes (species abundance and richness) in response to environmental changes, particularly nitrogen released from permafrost thawing and increased soil organic matter decomposition due to climate warming. The research is based on three field experiments conducted in alpine meadow, alpine steppe, and alpine wetland ecosystems on the Tibetan Plateau. In Chapter II, stable isotope
labeling was employed to simulate natural nitrogen release processes and to monitor the nitrogen uptake and turnover of deep–rooted and shallow–rooted plants over time.
This study demonstrates that the alpine steppe is shifting from a carbon dioxide sink to a source of carbon dioxide to the atmosphere. This shift in carbon balance is primarily driven by climate change–induced increases in ecosystem respiration including soil organic carbon decomposition. Climate change not only impacts ecosystem respiration and soil carbon decomposing but also influences plant growth, abundance, and species composition. For IX instance, changes in maximum root depth across the Tibetan Plateau, which are strongly linked to climate warming induced nitrogen released from permafrost thawing. Although plant growth, abundance, and species composition exhibit spatial and temporal variability and uncertainty in response to climate–driven changes across different vegetation types and environmental conditions, these shifts ultimately impact plant biomass allocation and the broader ecosystem carbon cycle, thereby further influencing climate change.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk before the defence. After the defence the thesis will become available from the Royal Danish Library Royal Danish Library | kb.dk