Major components of the heat tolerance testing system. A. The mobile supply unit is housed in a rugged and weather - proof enclosure. (1) Main switch, (2) power supply cable, (3) ventilation hole with fan, (4) connection panel for the eight exposure chambers, (5) RJ45 network port for connecting the notebook from which the complete system can be controlled, (6) venting hole (passive). B. Two exposure chambers in upright position during a controlled heat exposure. Heat exposure of a small Pinus cembra L. seedling (1) in dark - mode (2) and under natural solar irradiation (3). The handle (4) contains a hose pump and an electronic interface for controlling temperature and humidity inside the exposure chamber. C. Side view of an exposure chamber. (1) Sunscreen, (2) external fan for cooling purposes, (3) transparent bottom with opening slit, (4) heating wires preventing condensation inside the chamber. D. View into the inside of the heat exposure chamber. The main fan (1) transports air from the exposure room to the heating and humidifying system behind the separating plate (2) from where it is delivered back by 12 small fans (3). (4) Mobile temperature sensor, (5) humidity sensor (heated), (6) stationary temperature sensor.