Carbon stocks and fluxes in a young Siberian larch (Larix sibirica) plantation in Iceland

Abstract: The understanding of how forests function with respect to carbon (C) balance and its interaction with the climate system is a fundamental question in climate change research. Another important question is how large the sink for CO2 is in northern forests and how it varies with forest type, management, stand age and with external factors, such as climate variability. According to the Kyoto Protocol, C sequestration of all afforested areas since 1990 is to be deducted from the national GHG emissions. It is however highly uncertain when such young plantations become net sinks for CO2, especially if some major site preparation has been applied. The main aim of the present thesis was to evaluate the effect of afforestation on ecosystem C dynamics. For this, eddy covariance measurements of net ecosystem exchange (NEE) were made during three years over a Siberian larch (Larix sibirica) plantation with typical site conditions for afforestation in Iceland; previously grazed heathland that was site-prepared and planted in 1991-1992 and was therefore a ‘Kyoto-forest’. Additional studies took place in a comparable treeless sites and in different age-classes of Siberian larch, Sitka spruce (Picea sitchensis) and lodgepole pine (Pinus contorta), which are all commonly used in afforestation in Iceland. The young Siberian larch plantation acted as a relatively strong sink for CO2, with NEE of -375, -566 and -245 g CO2 m-2 for years 2004, 2005 and 2006, respectively. The observed annual variation in NEE was more related to variation in the carbon efflux (Re) than in the carbon uptake (GPP). Air temperature and soil water potential showed the strongest correlation to annual changes in Re. The GPP was lower in 2005, when the plantation experienced severe spring frost damage, but that was also the year with the highest NEE. Stock-change measurements supported the results of the eddy covariance flux measurements, indicating a mean annual sequestration of -673 g CO2 m-2, year-1. They indicated largest change in belowground C-stocks, in the fine root biomass and soil organic matter, not in aboveground C-stocks that generally is the main focus of carbon inventories. The chronosequences in 10-50 year old plantations showed similar increase in soil C stocks following afforestation as the main study site.