Greenhouse Gas Fluxes and Carbon Sequestration in Young Norway Spruce Stands : The Effects of Fertilization

Abstract: The enormous challenge of climate change is discussed and debated today because of its major impact on life on Earth. The forests have an important role to play as the plants absorb carbon dioxide (CO2) from the atmosphere through their photosynthesis and the growing tree retain carbon (C). Hence, the larger the growth the greater the carbon storage and climate benefit. The demand for wood and wood products is increasing as well as the ongoing debate about forest management. Therefore, alternative management methods to increase wood production is of interest and the effects these methods could have on climate change mitigation. In this context this Thesis deals with the effect of fertilization on carbon balance and growth in young forest as well as flows of the greenhouse gases, CO2, methane (CH4), and nitrous oxide (N2O) from forest land. In addition, it deals also with the reliability and comparability of different measurement methods which are compared with respect to the carbon balance.The studies have been carried out in a young mixed stand of Norway spruce (Picea abies (L.) Karst) and birch (Betula pendula and B.pubescens) on a storm-felled (Gudrun 2005) area in southern Sweden, Kronoberg county. Part of the area was fertilized with 150 kg N ha-1 everysecond year from 2014 and forward, while the other part was kept unfertilized. In the unfertilized part a dose experiment was set up where 0,150, 300, and 450 kg N ha-1 were added to investigate the impact of the different fertilizer levels on forest floor greenhouse gas fluxes. Chamber measurements of forest floor fluxes, eddy-flux measurements of stand net-fluxes and tree measurements of height, diameter and birch leaf biomass were conducted in different, occasionally overlapping, periods in the years 2013-2021.The results show that even if the flows of CO2 from the forest floor increase initially after a first standard fertilization, the effect decreases quickly. The net fluxes show that the stands become carbon sinks already eight years after the storm with a net uptake of about 18 ton CO2 ha-1 yr-1 of. The forest floor fluxes of CH4 and N2O also show a short-term effect of fertilization, however the levels are very low compared to CO2. The fertilization induced increase of total tree biomass growth increased with time. The results show that 12 and 15 years after regeneration, the fertilization compared to the control has increased the tree growth by 3.4 and 6.3 m3 ha-1 yr-1 and carbon storage by 4.7 and 8.7 ton C ha-1 yr-1 respectively.Comparison of measurement results of the Eddy-flux technique's netflows and chamber measurements of soil respiration together with tree growth shows the importance of calibrating the measurement methods when the results are later to be used in modeling future climate scenarios.