A comparison was made of annual net ecosystem productivity (NEP) of a closed canopy Sitka spruce forest over 2 years, using either eddy covariance or inventory techniques. Estimates for annual net uptake of carbon (C) by the forest varied between 7.30 and 11.44 t C ha À1 year À1 using ecological inventory (NE-P eco ) measures and 7.69-9.44 t C ha À1 year À1 using eddy covariance-based NEP (-NEE) assessments. These differences were not significant due to uncertainties and errors associated with estimates of biomass increment (15-21%) and heterotrophic respiration (12-19%). Carbon-stock change inventory (NEP DC ) values were significantly higher (27-32%), when compared to both NEP eco -and -NEE-based estimates. Additional analyses of the data obtained from this study, together with published data, suggest that there was a systematic overestimation of NEP DC -based assessments due to unaccounted decomposition processes and uncertainties in the estimation of soil-C stock changes. In contrast, there was no systematic difference between NEP eco and eddy covariance assessments across a wide range of forest types and geographical locations.

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-The impact of water availability on the photosynthetic performance of three year old, commercially obtained, Sitka spruce (Picea sitchensis) seedlings under exposed and shaded conditions was evaluated to provide a physiological understanding of the factors controlling seedling performance under conventional and continuous cover forestry (CCF) management scenarios. Decreases in photosynthesis in response to water deficits, under exposed and shaded conditions, were associated with reductions in both stomatal (G s ) and mesophyll conductance (G m ), and an increase in the proportion of electrons consumed in non-photosynthetic pathways. After re-watering, photosynthesis of plants subjected to higher irradiances was inhibited for up to 6 days due to high photorespiratory activity and damage to photosystem II. Waterlogged seedlings grown under both exposed and shaded conditions showed smaller decreases in photosynthesis that were also associated with an altered G s and G m , but no changes in chlorophyll fluorescence related parameters were observed. We conclude that the performance of seedlings will be more susceptible to management-related or environmentally-induced water deficits in exposed sites typical of temperate latitudes and may, therefore, be improved in CCF systems.continuous cover forestry / irradiance / water availability / photosynthesis Résumé -Interactions entre intensité lumineuse et disponibilité en eau sur la performance photosynthétique de plants de Picea sitchensis : implications sur le développement des plants selon leur différentes pratiques culturales. L'impact de la disponibilité en eau sur la performance photosynthétique de jeunes plants de Picea sitchensis, âgés de 3 ans d'origine commerciale sous conditions ensoleillées et ombragées fut évalué pour comprendre physiologiquement les facteurs contrôlant la performance des plants dans les conditions conventionnelles et de couvert forestier continu (continuous cover forestry, CCF). Les diminutions de la photosynthèse en réponse aux déficits hydriques sous conditions de plein ensoleillement et d'ombre étaient accompagnées de réductions des conductivités stomatique (G s ) et mésophyllienne (G m ) et d'une augmentation de la proportion d'électrons consommés par les activités non-photosynthétiques. Après réhydratation, la photosynthèse chez les plants soumis aux hautes intensités lumineuses restait limitée jusqu'à 6 jours à cause de l'importante activité photorespiratoire et des dommages subits par le PSII. En conditions inondées, les plantules cultivées sous ensoleillement et à l'ombre présentèrent de moindres réductions de leur photosynthèse qui étaient associées à des modifications de G s et G m , mais sans changement des paramètres de la fluorescence chlorophyllienne. Nous en concluons que la performance des plants est plus sensible aux déficits hydriques liés à la pratique culturale ou aux conditions environnementales dans les sites ensoleillés typiques des latitudes tempérées et peut donc être amélioré par l'emploi de systèmes CCF. c...