Net ecosystem production in a temperate pine plantation in southeastern Canada

Eddy covariance measurements of carbon dioxide (CO2) and water vapour fluxes were made from June 2002 to May 2003 over a 65-year-old temperate conifer plantation forest in southern Ontario, Canada. This site is part of a newly initiated long-term tower flux measurement program over a chronosequence of white pine plantation forests, known as the Turkey Point Flux Station. Net ecosystem productivity (NEP) showed a strong dependence on environmental variables such as temperature, light regime, and vapour pressure deficit. In the summer, saturation water deficit exerted a strong control on photosynthetic uptake through decreased bulk surface conductance. There was a linear relationship between monthly carbon uptake and water loss. For each kilogram of water evaporated from the stand, approximately 2.15 g of carbon was sequestered. Annual NEP was 196 g C m(-2) from June 2002 to May 2003. Annual gross ecosystem productivity (GEP) was 1442 g C m(-2) and annual ecosystem respiration (R) was 1247 g C m(-2). A cross-site comparison of this site with 19 other, planted and natural, temperate conifer forests showed that NEP at the Turkey Point plantation was relatively low compared to other plantation forests of similar age group in Europe and North America. This analysis also showed that GEP and R of plantation forests was significantly higher than that of natural forests. In plantation forests, GEP and NEP linearly decreased with forest age, while R decreased little. Natural stands showed an increase in GEP and R with stand age, except for an old growth stand in western USA. This showed that photosynthesis, rather than respiration, plays a dominant role in net carbon uptake of both plantation and natural forests. In both, plantation and natural, stands there was a weak or no relationship between NEP and annual temperature and precipitation, while annual GEP and R was positively correlated to these environmental variables. The differences in carbon uptake among plantations and between plantation and natural temperate conifer forests were due more to physical and physiological differences among stands (e.g. stand age, tree density, leaf area index, site history, and adopted management practices in case of plantation forests), rather than differences in environmental variables. (C) 2004 Elsevier B.V. All rights reserved.