1. Effects of Burning and Thinning Treatments on Sunfleck Duration and Below- canopy Reference Evapotranspiration in an Old-growth Mixed Conifer Forest Siyan Ma 1, Jiquan Chen 1, Malcolm North 2, James Innes 2 1 Department of Earth, Ecological, and Environmental Sciences, The University of Toledo, Toledo, OH 43606, USA 2 USFS Sierra Nevada Research Center, Department of Environmental Horticulture, University of California, Davis, CA 95616, USA Treatments BeforeAfter Control Prescribe dBurning & Overstory Thinning Prescribe dBurning & Understory Thinning Prescribe dBurning & no Thinning UnBurne d & Overstory Thinning UnBurne d & Understory Thinning Sunfleck Duration (mins) Below-canopy ET 0 (mm) Abstract In the forests, canopy cover is the primary regulator of energy flow and, consequently, influences microclimatic variability. We used hemispherical photos to quantify below-canopy sunfleck duration and evaluate the influences of prescribed burning and thinning treatments on below-canopy potential reference evapotranspiration (ET 0 ). ET 0 was estimated using Penman-Monteith equation based on our automatic microclimate data. We concluded that with 25.8% canopy removal following thinning treatments, the patterns of daily sunfleck durations were changed more likely from the flat shape to the bell-shaped pattern. Burning and Thinning increased sunfleck duration, especially between April and July. As a result, only canopy removal could increase ET 0 up to 39% in the months between the wet spring and the dry summer. Acknowledgements We thank Rhonda Roberts, Eric Huber, David Stern, Josh Reed, Nathan Williamson, and many others who helped with maintaining climate stations and collecting field data. The USDA Forest Service, Sierra Nevada Research Center and the University of Toledo provided financial support for the project. Many thanks to Jyh- min Chiang introduced the software of Gap Light Analyzer and related information. Simon Fraser University, Burnaby, British Columbia, Canada, and Institute of Ecosystem Studies, Millbrook, New York, USA for providing free software Gap Light Analyzer (GLA Version 2.0). Thanks John Rademacher and Mary Bresee for generously providing this nicely colored poster template. Burning and Thinning Designs in Teakettle Experimental Forest 1.Annual sunfleck duration has two typical patterns: (1) flat and (2) bell-shaped. 2. After treatments, the patterns of sunfleck durations were changed more likely from the flat pattern to the bell-shaped pattern. 3. With 25.8% canopy removal following thinning treatments, sunfleck duration increases, especially between April and July. 4. As a result, only canopy removal could increase ET 0 up to 39% in the months between the wet spring and the dry summer. Conclusions Methods Based on the primary treatment designs, 18 automatic climate stations were installed on the central grid points. At each station, we measured air temperature, relative humidity, soil heat flux, and wind speed at the height of 2 m. A standard methods – Peman-Monteith equation (Allen et al. 1998) was used to calculate the below- ground reference evapotranspiration (ET 0 ). The reference ET 0 is based on the concept of reference vegetation, which is a hypothetical crop with an assumed height of 0.12 m having a surface resistance of 70 s m-1 and an albedo of 0.23, closely resembling the evaporation of an extension surface of green grass of uniform height, actively growing and adequately watered. Hemispheric photos were taken at each microclimate station. Gap Light Analyzer (GLA, 2.0) were used to compute the daily sunfleck duration. Day of year