Soil carbon storage in the forest-tundra ecotone: insights from field- and remotely sensed data

Claire Céline Devos defended her PhD-thesis entitled “Soil carbon storage in the forest tundra ecotone: insights from field- and remotely sensed data” on December 15, 2023.

Foto: Claire Devos

The trial lecture was entitled “Advancing Carbon Cycle Research with Remote Sensing: Challenges and Pathways”

Summary of the thesis
Due to climate warming, forests are moving to higher elevations and latitudes at
the expense of treeless tundra. Recent studies suggest that these vegetation shifts
promote soil organic carbon (SOC) losses that exceed aboveground carbon gains,
resulting in a positive feedback to climate warming. However, confidence in this
prediction is limited due to the small number of forest-tundra ecotones that has
currently been sampled. To draw meaningful conclusions about the implications
of treeline shifts, it is crucial to move beyond individual case studies, and consider
data from a wide diversity of study sites. In addition, we urgently need to identify
remotely measurable SOC stock proxies to reduce the need for expensive soil
sample efforts and laboratory analyses. To address these needs, I sampled SOC
stocks in the surface soil horizons of 36 forest-tundra ecotones spread along a
1100 km long latitudinal gradient in Norway. My results show that SOC stocks
vary greatly within and between treeline ecotones. Interestingly, study sites with
larger SOC stocks in the tundra than in the forest are mainly situated in the
southern half of Norway, while study sites with larger SOC stocks in the forest
than in the tundra are mostly found in the northern half of the country.
Furthermore, I did not find a correlation between SOC stocks and tree biomass or
tree species. SOC stocks do increase with temperature, and vary with slope
steepness, slope aspect, and soil parent material. Unfortunately, terrain and
vegetation variables derived from UAV-borne lidar seem to be of little help in
explaining SOC stocks. From a space-for-time substitution perspective, my results
suggest that tree encroachment into treeless tundra is unlikely to have shortterm
consequences for SOC stocks. While this study provides valuable insights
into SOC stock responses to treeline shifts, it also highlights the pressing need for
further research. At present, we can only account for some SOC stock variability,
and we are not yet able to accurately map SOC stocks using remotely sensed data.

Supervisors were
Ole Martin Bollandsås (NMBU)
Mikael Ohlson (NMBU)
Erik Næsset (NMBU)
Kari Klanderud (NMBU)

The evaluation committee were
Maja Sundqvist (SLU)
Jan U. H. Eitel (University of Idaho)
Johan Asplund (NMBU)