Energy balance closure, water balance, carbon exchange, and water use efficiency : observed and modeled outcomes for a managed, hemiboreal forest in Southern Sweden

Abstract: The claims for the potential of boreal and sub-boreal forests to combat negative effects of climate change have been substantial with over half of Earth’s primary forests found within boreal and temperate regions of the Northern Hemisphere. Given the likely occurrence of more extreme climate events in the future, ascertaining a better understanding of how climate effects, such as profound fluctuations in precipitation, decreases in yearly snow, faster snow melt rates, variability in evapotranspiration due to environmental stress, and related changes in the energy, hydrological, and carbon cycles will change these northern, boreal landscapes is crucial. Energy, water, and ecosystem level carbon biosphere-atmosphere exchange were examined over a two-year study period from January of 2015 to December of 2016 at a managed forest site in southern Sweden. The energy balance was initially evaluated for closure, and to see which energy components had the largest influence on this system. Initial energy balance closure before energy storage correction was 0.75 and improved by about twenty percent after energy storage was modeled. Shortcomings in the energy balance were outlined by looking at closure during varying atmospheric conditions. The best EBR values prior to correction happened during daytime hours when friction velocities were higher than 0.8 m s-1 and windspeeds were higher than 4 m s-1 for stable-neutral and unstable conditions. Closure of the energy balance was then used to validate later calculations of evapotranspiration. Substantiated evapotranspiration fluxes were used to determine the nature of the water balance and water use efficiency. Looking at the water balance revealed the extent of the drought that occurred in 2016, which had less precipitation and higher temperatures than 2015. Temporal investigation of the individual water balance components described when the drought was most severe and which components were most affected. This decrease in incoming water had the largest effect on soil water storage and ground water storage. Diurnal patterns for the radiation components, evapotranspiration, CO2 fluxes and ecosystem exchanges were then evaluated. All of which were highest during daytime hours of the growing season. Integrated water use efficiency at the ecosystem level was determined and productivity of carbon sequestration for biological growth was gauged. Water use efficiency and the carbon exchanges indicated high productivity during the study period primarily during daytime hours of the growing season when the forest acted as a carbon sink. During nighttime and non-growing season periods it released more carbon than it took up. Aggregate, annual gross primary production was surprisingly higher in 2016 at 291 g C m-2 year-1, which was unexpected given the environmental strain placed upon this ecosystem due to drought stress. The implications of this suggest that drought stress would need to be more severe to make a lasting, negative effect on forest productivity.