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

Authors: Lecain, Daniel; Smith, David; MORGAN, JACK - Retired ARS Employee; Kimball, Bruce; PENDALL, ELISE - University Of Sydney; MIGLIETTA, FRANCO - Institute Of Biometeorology

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2014
Publication Date: 2/6/2015
Publication URL: http://handle.nal.usda.gov/10113/60350
Citation: Lecain, D.R., Smith, D.P., Morgan, J., Kimball, B.A., Pendall, E., Miglietta, F. 2015. Microclimatic performance of a free-air warming and CO2 enrichment experiment in windy Wyoming, USA. PLoS One. 10(2):e0116834. DOI: 10.1371/journal.pone.0116834.

Interpretive Summary: Climatic changes of rising carbon dioxide (CO2) in the atmosphere and predicted increases in temperature can have consequences for agriculture and for managers of these lands. Several elevated CO2 and elevated temperature experiments have been conducted, but few have monitored microclimate attributes such as temperature and humidity to assist in experimental interpretations. Here, we monitored plant, soil, within plant-air, above canopy-air temperatures and relative humidity for three years in an elevated CO2 (Free Air Carbon dioxide Enrichment or FACE methodology) and temperature study in Wyoming. Both the warming and CO2 enrichment systems used a feedback-controlled integrative system and were of a "free-air", lowly intrusive design of the current best technology. The warming system increased plant and soil temperatures as designed and performed well during day and night, as well as during growing and non-growing seasons. The CO2 enrichment system performed well during the growing season (not used during the non-growing season). CO2 demand varied greatly over very short periods due to wind speed, demonstrating that passive-non-feedback controlled systems are inadequate. Our findings will be useful to scientists designing similar multi-factor global change experiments.

Technical Abstract: 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.