Cool Forest is a living pop-up laboratory that investigates trees as atmospheric infrastructure and as an atmospheric commons capable of mitigating urban heat island effects and improving air quality under climate change. Conceived as both installation and experiment, the project examines how trees function as active agents within urban climates through intertwined biological, chemical, and atmospheric processes. A planting matrix of tree species is augmented by a bespoke, multi-modal sensor network measuring air temperature, humidity, heat index, soil moisture, and sap flux. These measurements are communicated in real time through spatial actuators, living vegetation, and a web-based interface, forming a cybernetic feedback system. Empirical results demonstrate microclimatic cooling and species-specific physiological responses to heat stress, reframing landscape architecture as the design of sensing ecologies and adaptive atmospheric infrastructure.