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

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/Re) ratio increased from spring through autumn and ranged from 0.85 to 0.87 annually for the snow-free period. The R(s-adj/Re) 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(ffmax)) saturated at low irradiance levels (similar to 200 mu mol 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(ffmax) 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 degrees 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).