Carbon dioxide and water vapour exchange from understory species in boreal forest

Although recent eddy covariance measurements in boreal forests provide CO2 and energy exchange data for the whole ecosystem, very little is known about the role of the understory vegetation. We conducted chamber flux measurements in an Alaskan black spruce forest in order to compare CO2 and water vapour exchange among patches of understory vegetation dominated by feathermoss (Hylocomium), peatmoss (Sphagnum), vascular plants (mainly low shrubs), or lichens. We found large differences among understory vegetation types with respect to midday net CO2 exchange and its seasonal pattern of variation. Sphagnum and vascular-plant plots showed net CO2 uptake, with most uptake on days of high light availability. In contrast, Hylocomium and lichen plots lost CO2 during the middle of the growing season, but showed net uptake at the end of the season when the soil had cooled down. Spatial variation in net CO2 exchange was related more to biotic variables like soil organic matter than to environmental variables. The differences among vegetation types with respect to water vapour fluxes were smaller, because evapotranspiration was more constrained by climatic variables like solar radiation. Net CO2 uptake in Hylocomium plots was negatively related to evapotranspiration, because Hylocomium photosynthesis was very sensitive to evaporative stress, whereas evapotranspiration and net CO2 uptake in Sphagnum were not limited by moisture conditions. These differences suggest that species composition of the understory should be taken into account when discussing understory contributions to CO2 and water vapour exchange. Author Keywords: Climate change; CO2; Evapotranspiration; Hylocomium splendens; Sphagnum capillifolium; Understory vegetation