Totally closed, transparent chambers or mini-greenhouses are being used to contain samples of ecosystems up to chaparral-sized systems (Paul Miller, personal communication). Most of these attempts, however, have been on crops (Parsons et al., 1980; Jones et al. , 1984a) or smaller stature ecosystems such as Arctic tundra (Oechel and Strain, 1985).

   Two types of closed systems are portrayed in Figures 15.5 and 15.6. These facilities are designed and operated to provide accurate flexible control of dry-bulb temperature, CO₂ concentration, and vapor pressure of the air. In contrast to the open-top systems, which allow a differential between external and internal variables, these closed systems are designed to maintain small or no differentials. Refrigerated coils, condensing and humidifying systems, and CO₂ adding or scrubbing circuits are used in an attempt to obtain and maintain given levels of environmental control.

   Specific methods and equipment for controlling chamber conditions vary but are generally based on: (1) sensors that detect air and leaf temperature, CO₂, humidity, wind velocity, and irradiance; (2) feedback mechanisms such as thermostats or loops in computer logic that compare sensed with desired conditions; and (3) control devices such as heaters, coolers, fans, solenoid valves, etc. that are regulated by computer control to produce the desired treatment conditions.

   Recent experiments using controlled environment mini-greenhouses have focused on short and long-term effects of elevated CO₂ and temperature increase on productivity, phenology, and plant-to-plant competition (Jones et al., 1984b, 1985; Tissue and Oechel, 1987; Overdieck and Reining, 1986).