Controlled environment chambers for investigating tree response to elevated CO_2 and temperature under boreal conditions

摘要

A closed CO_2 and temperature-controlled, long-term chamber system has been developed and set up in a typical boreal forest of Scots pine (Pinus sylvestris L.) near the Mekrijarvi Research Station (62 deg 47'N, 30 deg 58'E, 145 m above sea level) belonging to the University of Joensuu, Finland. The main objectives of the experiment were to provide a means of assessing the medium to long-term effects of elevated atmospheric CO_2 concentration (EC) and temperature (ET) on photosynthesis, respiration, growth, and biomass at the whole-tree level and to measure instantaneous whole-system CO: exchange. The system consists of 16 chambers with individual facilities for controlling CO_2 concentration. temperature, and the combination of the two. The chambers can provide a wide variety, of climatic conditions that are similar to natural regimes. In this experiment the target CO_2 concentration in the EC chambers was set at a fixed constant of 700 #mu#mol mol~(-1) and the target air temperature in tile ET chambers to track the ambient temperature but with a specified addition. Chamber performance was assessed oil the base of recordings covering three consecutive years. The CO_2 and temperature control in these closed chambers was in general accurate and reliable CO_2 concentration in the EC chambers was within 600-725 #mu#mol mol~(-4) for 90 % of the exposure time during the "growing- season" (15 April - 15 September) and 625-725 #mu#mol mol~(-1) for 88 % of the time in the "off-season" (16 September - 14 April), while temperatures in the chambers were within +- 2.0 deg C of the ambient or target temperature in the "growing season" and within +- 3.0 deg C in the "off season". There were still some significant chamber effects. Solar radiation in the chambers was reduced by 50-60 % for 82 % of the time in the "growing season" and 55-65 % for 78 % of the time in the "off season", and the relative humidity of the air was increased by 5-10 % for 72 % of the time in the "growing season" and 2-12 % for 91% of the time in the "off season". The crown architecture and main phenophase of the trees were not modified significantly by enclosure in the chambers, but some physiological parameters changed significantly, e.g., the radiant energy-saturated photosynthesis rate, transpiration rate, maximum photochemical efficiency of photosystem 2, and chlorophyll content.