PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Carbon dioxide efflux from stems and soil during a cottonwood dry-down experiment under low and high atmospheric CO₂.

Johnsen, Kurt¹, Butnor, John¹, Maier, Chris¹, Murthy, Rameesh², ^{1 2}

ABSTRACT- Cottonwood (Populus deltoides) trees, located in Biosphere 2 (Oracle, AZ), grew from cut stumps from April 15, 2000. Trees were grown in two compartments under low (430 ppm) or high (1200 ppm) atmospheric CO₂. Beginning in November 2000, the trees were subjected to increased water stress via step-wise increases in soil and atmospheric drought. During the dry-down, a sub-set of trees was continuously measured for both stem and soil CO₂ efflux; soil chambers were placed at the base of the tree and between rows. There was high among-location variation in soil CO₂ efflux that precluded response surface analyses; however, over the course of the dry-down, soil CO₂ efflux rates declined by 14% and 39% under low and high atmospheric CO₂, respectively. The greater reduction under elevated CO₂ was due to a more rapid and severe dry-down of the soil due to greater leaf areas in the high atmospheric CO₂ stands. Despite the high among-location variation, both tree position and treatment effects were strong. Under low atmospheric CO₂, soil CO₂ efflux at the base of the trees averaged 5.03 mmol CO₂ m² s^-1 while inter-row CO₂ efflux was 3.94 mmol CO₂ m² s^-1. Values in the high CO₂ compartment were 7.27 (tree-base) and 6.91 mmol CO₂ m² s^-1 (inter-row). Thus, an increase in atmospheric CO₂ of 300% was associated with an increase in soil CO₂ efflux of 68%.

KEY WORDS: Populus deltoides , drought, respiration