Seasonal variation of carbon dioxide exchange rates above and below a boreal jack pine forest

Abstract

Fluxes of carbon dioxide were measured above and below a boreal jack pine stand with the eddy covariance method. Our goal was to examine the seasonal variation of the net carbon dioxide exchange between the ecosystem and the atmosphere and to understand the relative contributions of the vegetation and rhizosphere to these fluxes. Over the course of the growing season the jack pine stand was a small, net sink of carbon dioxide—47 g C m −2 were assimilated during a 115 day period between spring (day 144) and autumn (day 259). On a daily basis, the magnitude of the net uptake of CO 2 by the forest was relatively small, less than 4 g C m −1 day −1. We also observed many days when carbon dioxide was lost from the forest stand (up to 2 g C m −2 day −1). Day to day sums of net carbon dioxide exchange exhibited little seasonality, despite the seasonality experienced by controlling environmental and biological variables. The aseasonality of net canopy CO 2 exchange occurred because temporal variations associated with canopy photosynthesis and plant and microbial respiration offset one another over the growing season. Canopy photosynthesis rates of the jack pine stand were evaluated using CO 2 flux measurements made above and below the canopy. Daily totals of canopy photosynthesis were a linear function of available solar energy and exhibited a quadratic dependence on temperature, with optimum photosynthesis occurring between 15 and 25°C. We also observed that the net daily uptake of carbon dioxide was independent of soil moisture in the upper 0.15 m of the soil profile, despite the wide range of soil moisture that was experienced. Finally, the net gain of carbon by a jack pine canopy came with a relatively large respiratory expense. On average, 68% of the carbon gained by photosynthesis was consumed by plant respiration, over the course of a day.