Results and Discussion The effect of clouds on photosynthesis in Engelmann Spruce (Picea engelmanniii) saplings at treeline Junior Neal, Nicole Hughes, Charlene Miller, Kaylyn Carpenter, Timothy Keidel Department of Biology, High Point University Results and Discussion As demonstrated in previous studies, cloudy days are characterized by dramatically reduced sunlight intensity relative to sunny days (Fig. 2). However, as expected, this did not result in a decrease in daily photosynthetic yields in P. engelmannii relative to sunny days (Fig. 3). This is consistent with our hypothesis that conifer needle and shoot morphology allows individuals to utilize a greater proportion of diffuse light during cloudy conditions. Our data also illustrate physiological drought effects experienced by the plants in the later summer months. The summer of 2012 was one of the driest on record in the Medicine Bow mountains, and corresponded with record low precipitation and frequent forest fires in the area. During the wetter months of early summer (June), photosynthesis remained relatively constant throughout the day, indicating relatively stable stomatal conductance. However, in the later months of July and August, as drought ensued, photosynthesis decreased throughout the day into late afternoon, most likely due to leaf desiccation and decreased stomatal conductance (data not shown). Abstract Conifer shoot morphology is characterized by a 3-dimensional cluster of needle-shaped leaves, occurring in a dense, spiraling pattern around the shoot. While this specific form of plant morphology results in substantial self-shading under collimated light (sunny) conditions), under diffuse light (cloudy) conditions, incident light per unit leaf area may increase, potentially offsetting photosynthetic losses rendered by diminished photon flux. However, this idea has yet to be tested in the field. In this study, we compared photosynthesis of Engelmann spruce (Picea engelmannii) shoots during cloudy and sunny conditions at the alpine-treeline ecotone, near Centennial WY. We demonstrate for the first time that conifers exhibit similar photosynthetic rates under sunny and cloudy conditions, supporting the hypothesis that conifer shoot morphology may be an adaptation to maximize photosynthesis in environments typified by diffuse light. Materials and Methods Study was conducted at a field site situated at the alpine-treeline ecotone in the Snowy Range of the Medicine Bow mountains, Wyoming, USA (Fig. 1) (41° 21’01.94 “N 106° 16’ 17.34 “W; elevation 10,532ft). Individuals studied ranged in height from14cm-1.88m. The Li-Cor LI-6400XT Portable Photosynthesis System was used for measuring CO₂ gas exchange of individual, current year shoots. Measurements were taken at regular intervals throughout the day, on both clear and cloudy days, between June and August, 2012. Environmental parameters at the field site were recorded using Hobo data loggers and micrometeorological sensors. Objectives Fig. 1. Native range of Picea engelmannii. Location of field site indicated by a star. Fig. 2. Photosynthetically active radiation (PAR) on a typical sunny (solid) versus cloudy (dashed) day. Record PAR (photosynthetically active radiation) on cloudy and sunny days. Measure photosynthesis of P. engelmanii shoots on cloudy vs. sunny days during summer, 2013. Acknowledgements Funding for this project was provided by a RUI grant to N. Hughes and High Point University from the National Science Foundation. Fig. 3. Photosynthetic gas exchanged measured on clear (blue line) versus cloudy (red line) days during June, July, and August 2012. Points represent means of 10-20 individuals, +/- SD. Picea engelmannii