A whole‐tree chamber system for examining tree‐level physiological responses of field‐grown trees to environmental variation and climate change

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Abstract
A whole‐tree chamber (WTC) system was installed at Flakaliden in northern Sweden to examine the long‐term physiological responses of field‐grown 40‐year‐old Norway spruce trees [Picea abies (L.) Karst.] to climate change. The WTCs were designed as large cuvettes to allow the net tree‐level CO2 and water fluxes to be measured on a continuous basis. A total of 12 WTCs were used to impose combinations of atmospheric carbon dioxide concentration, [CO2], and air temperature treatments. The air inside the ambient and elevated [CO2] WTCs was maintained at 365 and 700 µmol mol−1, respectively. The air temperature inside the ambient temperature WTCs tracked air temperature outside the WTCs. Elevated temperatures were altered on a monthly time‐step and ranged between +2.8 and +5.6 °C above ambient temperature. The system allowed continuous, long‐term measurement of whole‐tree photosynthesis, night‐time respiration and transpiration. The performance of the WTCs was assessed using winter and spring data sets. The ability of the WTC system to measure tree‐level physiological responses is demonstrated. All WTCs displayed a high level of control over tracking of air temperatures. The set target of 365 µmol mol−1 in the ambient [CO2] chambers was too low to be maintained during winter because of tree dormancy and the high natural increase in [CO2] over winter at high latitudes such as the Flakaliden site. Accurate control over [CO2] in the ambient [CO2] chambers was restored during the spring and the system maintained the elevated [CO2] target of 700 µmol mol−1 for both measurement periods. Air water vapour deficit (VPD) was accurately tracked in ambient temperature WTCs. However, as water vapour pressure in all 12 WTCs was maintained at the level of non‐chambered (reference) air, VPD of elevated temperature WTCs was increased.