Doctoral defence: Alisa Krasnova "Greenhouse gas fluxes in hemiboreal forest ecosystems"

On 22 June at 10:15 Alisa Krasnova will defend her doctoral thesis „Greenhouse gas fluxes in hemiboreal forest ecosystems“ for obtaining the degree of Doctor of Philosophy (in Geography).

Supervisors:
Associate professor Kaido Soosaar, University of Tartu
Professor Ülo Mander, University of Tartu
Professor Steffen M. Noe, Estonian University of Life Sciences

Opponent:
Professor Asko Noormets, Texas A&M University (USA)

Summary
Forests are essential components of global greenhouse gas (GHG) cycles. A hemiboreal transitional zone is located between boreal and temperate biomes and is represented by mixed forest stands with varying tree species ratios. The aim of the thesis was to analyse the GHG fluxes and the main environmental factors controlling them in hemiboreal forest ecosystems. The particular focus was on the CO2 uptake, the influence of the European 2018 heatwave on CO2 fluxes and the role of methane (CH4) and nitrous oxide (N2O) in total climate forcing. Mixed (spruce/birch), pine and riparian alder forest stands were net annual sinks of CO2, with coniferous forest having lower sink strength. CH4 and N2O played a minor role in the total climate forcing of a riparian forest; the mechanisms within the ecosystem require further studies. Two clear-cuts and a mixed forest (pine/spruce/birch) with clear-cuts were net sources of CO2. Air temperature was one of the main factors with soil moisture modifying its impact. The heatwave 2018 impact varied for different forest ecosystems. A mixed conifer-broadleaved forest with clear-cuts was less resistant than expected and could be pushed towards a strong carbon source. An upland coniferous forest acclimated to low soil moisture was the most resistant to the heatwave concurrent effect. A riparian alder forest could act as a booster for the net carbon uptake, albeit a short-lived one due to the usual life cycle and usage of such forest stands. A clear-cut was the most vulnerable to the weather extremes; thus, the increase in forest harvesting of this type can be detrimental to balancing the hemiboreal zone carbon footprint. In the next decades, a global increase in the frequency of disturbances and extreme weather events causing alterations in the GHG balance of terrestrial ecosystems is anticipated; thus, a comprehensive understanding of possible consequences for the hemiboreal zone is essential.