CANCELLED - The elephant in the room -- leaf angle of forest canopies: measurements and implications for the light environment and photosynthesis

Background/Question/Methods

Life on Earth depends on the conversion of solar energy to chemical energy by plants through photosynthesis, and the fundamental role of leaves is to intercept sunlight for photosynthesis. This role creates a key challenge: leaves need to be angled in ways that capture as much sunlight as possible under the constraints of heat stress, water loss, and competition. Despite the importance of leaf orientation, until recently we have lacked data and frameworks to describe and predict leaf angle dynamics and their impact from leaves to the globe because of the difficulty in measuring leaf angles in complex canopies accurately and rapidly. New techniques to measure leaf angles are emerging, opening the opportunities to understand the interspecific, intraspecific, seasonal, and interannual variations of leaf angle and their implications to plant biology and earth system science. Here we present results from model simulations of canopy photosynthesis and radiative transfer with varying canopy leaf angle. We also present new observations of the seasonality and inter-specific variations in leaf angle.

Results/Conclusions

We show that variations in leaf angle result in up to 20% changes in canopy photosynthesis, temperature, and stomatal conductance with the simulations from two models. Leaf angle is an essential driver of the remote sensing signals (e.g., reflectance, solar-induced chlorophyll fluorescence, and canopy temperature) collected at the global scale. Our field observations show that leaf angle varies seasonally in a temperate forest, and the magnitude and direction of changes vary at different heights of the canopy. We suggest three directions for future research: incorporation of leaf angles as a leaf and canopy structural trait into the global spectrum of plant function and structure; investigations on how leaf angles respond to environmental changes (CO2, temperature, water) that can shed light on the future trajectories of vegetation change and surface energy balance; and development of terrestrial ecosystem models and land surface models that consider leaf angle as an important parameter that influences carbon, water, energy fluxes, and competition.