Carbon Exchange Processes In A Old-Growth Undisturbed Boreal Forest In Northern Sweden

Abstract:

It has been a common and long lasting view that old-growth forests are carbon neutral, i.e., the uptake of CO2 by gross photosynthesis is equal to the release of the same amount through ecosystem respiration. This hypothesis was originally developed by Odum based on theoretical reasoning on the balance between stability and diversity in ecosystems and how this relationship shifted with succession over time. At that time, the theory was underpinned by a relatively scarce empirical material but later supported by the observed decline in net primary productivity (NPP) with increasing stand age. More recently, based on direct measurements of net ecosystem exchange (NEE) Luyssaert et al. showed that old-growth forests still were significant sinks for atmospheric carbon dioxide thus challenging the hypothesis that old forests are in balance with respect to uptake and emission of CO2. Most of the forests in Luyssaert et al. analyses were temperate and semi-arid boreal forests and only few were located in the humid boreal zone. In order to shed light on this issue we initiated in 2011 carbon exchange studies in an old (>200 years) undisturbed humid boreal forest in Northern Sweden using EC measurements of NEE and chamber measurements of soil effluxes. The results indicate that the forest is a small sink of CO2 in the order of 20 g C m-2 y-1. The forest floor vegetation contributes significantly to GPP, in the order of 25-40%, depending on time of season. The nighttime ecosystem respiration showed a weak increase with air temperature up to about 15 ºC and then it started to decrease. The reason for this decrease that occurred at a relatively low temperature is still unclear. The small annual sink of -20 g C m-2 observed here is similar in magnitude to changes in soil carbon content on nearby much older forests located on small islands which has not been disturbed for several hundreds of years. Our study thus confirms that old forests can continue to take up carbon although of relatively minor magnitude.