Finromlige strålingsmålinger for heterogen boreal–alpin og subalpin vegetasjon

Eirik Næsset Ramtvedt forsvarte sin doktorgradsavhandling, “Fine-spatial radiation measurements for heterogeneous boreal–alpine and sub-alpine vegetation”, den 17 juni 2022.

Tema for prøveforelesningen var “Radiative feedback effects of vegetation changes in the Scandinavian mountains”. Vi gratulerer med graden!

Sammendrag av avhandlingen (engelsk)

Net radiation constitutes the most important component shaping the climate in the lower atmosphere and the land surface below. The flux determines the amount of energy available for exchange as sensible and latent heat, and for biochemically and physically storage in the vegetation cover and ground. Given that the incoming radiation for a heterogenous land surface has the same density over the whole surface, the net radiation will vary among the different surface patches of vegetation. This is dependent on the unique combination of the shortwave reflectance, surface temperature and the longwave emissivity. Because of global warming, rapidly northwards and elevational shifts of tree lines and altered vegetation compositions occur in the boreal–alpine and sub-alpine ecotones. The fine-spatial heterogeneity of vegetation cover and topography in these ecotones requires radiation measurements with fine-spatial resolution able to capture only one surface patch at a time and such assure homogeneity within the measurement. The main objective of this thesis was to develop knowledge of effective use of fine-spatial radiation measurements for heterogeneous boreal–alpine and sub-alpine vegetation, and as such improve the understanding of the radiative feedback effect on climate as the vegetation changes in these ecotones.

The first study presented a methodology for providing surface cover-corrected net radiation at heterogeneous eddy covariance sites. Despite significant differences in net radiation detected for the typical sub-alpine vegetation types, the surface covercorrected net radiation did not improve the energy balance. Based on the experience of the in situ use of the net radiometer tower from the eddy covariance site, the second study focused on providing summertime mountain birch albedo measurements in the boreal–alpine tree line. Fine-spatial shortwave reflected radiation was measured for single trees in sloping terrain by using an easily movable and adjustable tower structure. It was found that taller and wider tree crowns of mountain birch will reduce the summertime boral–alpine albedo. Additionally, it was revealed that the appropriateness of horizontally measured albedo of single trees in sloping terrain depends on the spatial size of the footprint of the downward-looking pyranometer relative to the size of the tree subject to observation. The height of the trees included in the second study was limited by the height of the movable radiation tower. Hence, the third and last study exploited an unmanned aerial vehicle (UAV) mounted with an upward-looking and downward-looking pyranometer to measure single-tree albedo. The study focused on Norway spruce trees in the boreal–alpine tree line during winter. The snow-masking effect had statistically significant effect on the albedo and was found stronger than the effects of the tree structure. To improve the vi spatial precision of UAV-measured albedo for small single trees, it is necessary to equip the UAV with a high-precision positioning system and the downward-looking pyranometer with a field-stop device.

This thesis reports on findings about net radiation and albedo for typical vegetation in the boreal–alpine and sub-alpine ecotones. The radiative proximal remote sensing methods demonstrated here, gain important experience which is valuable for further fine-spatial radiation measurements. This might be particularly useful for heterogeneous vegetation surfaces where typical coarse-scaled tower measurements and remote sensing products fail. Hopefully, the thesis will inspire future seeking of effective radiation measurement strategies which are necessary as the vegetation changes in the mountainous areas.

Veiledere

Hovedveileder: Professor Terje Gobakken, MINA-NMBU
Medveiledere: 
Professor Ole Martin Bollandsås, MINA-NMBU
Bedømmelseskomité

Opponenter:
Associate Professor Miina Rautiainen, Aalto University, Finland
Professor Hans Renssen, University of South-Eastern Norway, Norway
Komitéens administratior: Professor Johan Asplund NMBU/MINA

Referanse