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Microclimatic performance of a free-air warming and CO2 enrichment experiment in windy Wyoming, USA

LeCain, Daniel, Smith, David, Morgan, Jack, Kimball, Bruce A., Pendall, Elise, Miglietta, Franco
PLoS one 2015 v.10 no.2
seasonal variation, summer, prairies, carbon dioxide, ecosystems, leaves, electricity, energy, infrared heaters, monitoring, relative humidity, winter, microclimate, soil, climate change, temperature, plant physiology, canopy, experimental design, free air carbon dioxide enrichment, growing season, wind speed, heat transfer, Wyoming
In order to predict and plan for global changing climate experiments are being conducted in many countries, but few have monitored the effects of the climate change treatments (warming, elevated CO2) on the ecologically critical experimental plot microclimate. During three years of an eight year study with year-round feedback-controlled infra-red heater warming (1.5/3.0 oC day/night) and growing season free-air CO2 enrichment (600 ppm) in the mixed-grass prairie of Wyoming, USA, we monitored soil, leaf, canopy-air, above-canopy-air temperatures and relative humidity of control and treated experimental plots and evaluated ecologically important temperature differentials. Leaves were warmed somewhat less than the target settings (1.1 & 1.5 oC day/night) but soil was warmed more creating an average that matched the target settings extremely well both during the day and night plus the summer and winter. The site typically has about 50% bare or litter covered soil, therefore soil heat transfer is more critical than in dense canopy ecosystems. The Wyoming site commonly has strong winds (5 ms-1 average) and significant daily and seasonal temperature fluctuations (as much as 30 oC daily) but the warming system was nearly always able to maintain the set temperatures regardless of abiotic variation. The within canopy-air was only slightly warmed and above canopy-air was not warmed by the system, therefore convective warming was minor. Elevated CO2 had no direct effect nor interaction with the warming treatment on microclimate. Relative humidity within the plant canopy was only slightly reduced by warming. Power required to achieve the target warming varied greatly during a 24 hour period, but during a year one of our plots (7.1 m2) with six 1000W IR-heaters used about 20,000 kWh of electricity. This study demonstrates the importance of monitoring the microclimate in manipulative field global change experiments so that critical physiological and ecological conclusions can be determined. Highly variable energy demand fluctuations showed that passive IR heater warming systems will not maintain desired warming for the much time. Planned studies, especially in sparse canopy grasslands, can look to our results for guidance on experimental design.