Forest floor carbon exchange of a boreal black spruce forest in eastern North America

Abstract. This study reports continuous automated measurements of forest floor carbon (C) exchange over feathermoss, lichen, and sphagnum micro-sites in a black spruce forest in eastern North America during snow-free periods over three years. The response of soil respiration (R_s-auto) and forest floor photosynthesis (P_ff) to environmental factors was determined. The seasonal contributions of scaled up R_s-auto adjusted for spatial representativeness (R_s-adj) and P_ff (P_ff-eco) relative to that of total ecosystem respiration (R_e) and photosynthesis (P_eco), respectively, were also quantified.

Shallow (5 cm) soil temperature explained 67–86% of the variation in R_s-auto for all ground cover types, while deeper (50 and 100 cm) soil temperatures were related to R_s-auto only for the feathermoss micro-sites. Base respiration was consistently lower under feathermoss, intermediate under sphagnum, and higher under lichen during all three years. The R_s-adj/R_e ratio increased from spring through autumn and ranged from 0.85 to 0.87 annually for the snow-free period. The R_s-adj/R_e ratio was negatively correlated with the difference between air and shallow soil temperature and this correlation was more pronounced in autumn than summer and spring.

Maximum photosynthetic capacity of the forest floor (P_ff-max) saturated at low irradiance levels (~200 μmol m⁻² s⁻¹) and decreased with increasing air temperature and vapor pressure deficit for all three ground cover types, suggesting that P_ff was more limited by desiccation than by light availability. P_ff-max was lowest for sphagnum, intermediate for feathermoss, and highest for lichen for two of the three years. P_ff normalized for light peaked at air temperatures of 5–8°C, suggesting that this is the optimal temperature range for P_ff. The P_ff-eco/P_eco ratio varied from 13 to 24% over the snow-free period and reached a minimum in mid-summer when both air temperature and P_eco were at their maximum. On an annual basis, P_ff-eco accounted for 17–18% of P_eco depending on the year and the snow-free season totals of P_ff-eco were 23–24% that of R_s-adj.