Seasonal patterns and environmental control of carbon dioxide and water vapour exchange in an ecotonal boreal forest

Abstract

Summary Carbon dioxide, water vapour, and sensible heat fluxes were measured above and within a spruce dominated forest near the southern ecotone of the boreal forest in Maine, USA. Summer, mid‐day carbon dioxide uptake was higher than at other boreal coniferous forests, averaging about – 13 μmol CO2 m–2 s–1. Nocturnal summer ecosystem respiration averaged ≈ 6 μmol CO2 m–2 s–1 at a mean temperature of ≈ 15 °C. Significant ecosystem C uptake began with the thawing of the soil in early April and was abruptly reduced by the first autumn frost in early October. Half‐hourly forest CO2 exchange was regulated mostly by the incident photosynthetically active photon flux density (PPFD). In addition to the threshold effects of freezing temperatures, there were seasonal effects on the inferred photosynthetic parameters of the forest canopy. The functional response of this forest to environmental variation was similar to that of other spruce forests. In contrast to reports of carbon loss from northerly boreal forest sites, in 1996 the Howland forest was a strong carbon sink, storing about 2.1 t C ha–1.

Sugars as source indicators of biogenic organic carbon in aerosols collected above the Howland Experimental Forest, Maine

“…The Howland Experimental Forest (Maine, USA) is located in a transitional northern hardwood-boreal conifer forest (45.2 o N, 68.7 o W, 60 m above sea level) with natural stands about 20 m tall (Hollinger et al, 1999) …”

Relationships between large-scale circulation patterns and carbon dioxide exchange by a deciduous forest

“…The net CO 2 exchange at the site has experienced distinct interannual variations [Goulden et al, 1996;Barford et al, 2001;Urbanski et al, 2007]. While variations in CO 2 exchange at short-term scales (hourly to monthly) were simulated well as prompt responses to the weather patterns and local conditions at Harvard Forest [e.g., Urbanski et al, 2007] and other northern midlatitude forests [e.g., Hollinger et al, 1999], interannual variations and long-term trend in CO 2 exchange are often poorly represented by current models [Hanson et al, 2004;Siqueira et al, 2006;Richardson et al, 2007;Urbanski et al, 2007]. The annual CO 2 exchange at Harvard Forest have been found to be sensitive to climatic factors during and before the growing season [Goulden et al, 1996;Barford et al, 2001;Urbanski et al, 2007].…”

Forest canopy uptake of atmospheric nitrogen deposition at eastern U.S. conifer sites: Carbon storage implications?

“…If the canopy foliage C/N ratio should drop significantly due to retained N, detrimental effects on forest growth may eventually be observed. Erisman et al [1998] [Birdsey et al, 1996], to 1500-2500 kg C ha -• yr -1, based on tower measurements [Goulden et al, 1996;Hollinger et al, 1999], and more ]. Thus estimation of the magnitude of C storage resulting from canopy retained N is of interest.…”