The increase in atmospheric carbon dioxide concentration ([CO 2]) and consequent increase in air temperature is expected to have significant effects on plant growth and nutritive value. Studies examining the effects of elevated [CO 2] on plants under field conditions have been limited by the inherent difficulty to modify air composition in open air. Here we describe an efficient and inexpensive open-top chamber (OTC) system designed to study the effects of elevated atmospheric [CO 2] and temperature on perennial alfalfa-timothy ( Medicago sativa L.)-( Phleum pratense L.) mixture. The design and construction of these OTCs are described in detail, along with cost estimation for each component. Eight OTCs, each with 1.2 m 2 of ground area (four with elevated [CO 2] and four with ambient [CO 2]) were fabricated and four control plots of the same dimension were established to assess the chamber effects on plant responses to CO 2. The [CO 2] in elevated-CO 2 chambers fell 93% of the time within 20% of the targeted 600 mol mol -1 CO 2, based on 10 min averages. The CO 2 consumption in elevated-CO 2 chambers averaged 3.0 kg CO 2 m -2 d -1. To ensure that the environment within OTCs was similar to the surrounding field, growing conditions were determined in all chambers and control plots. Adequate light transmission was observed compared to control plots (93%) and the temperature increase was 0.7°C on average. After two growing seasons of continued use, this system has proven its effectiveness for studying the effects of CO 2 and climate change in the field at low cost.
