Continuous, long-term, high-frequency thermal imaging of vegetation: Uncertainties and recommended best practices

Leaf temperature is an elementary driver of plant physiology, ecology and ecosystem productivity. Individual leaf temperature may deviate strongly from air temperature, and may vary throughout the canopy. Measurements of leaf temperature, conducted at a high spatial and temporal resolution, can improve our understanding of leaf water loss, stomatal conductance, photosynthetic rates, phenology, and atmosphere-ecosystem exchanges. However, continuous high-resolution measurement of leaf temperature outside of a controlled environment is difficult and rarely done. Here, thermal infrared cameras are used to measure leaf temperatures. We describe two long-term field measurement sites: one in a temperature deciduous forest, and the other in a subalpine conifer forest. The considerations and constraints for deploying such cameras are discussed and the temperature errors are typically +/–1 °C or smaller (σ = 0.60 °C, 2σ = 1.20 °C). Lastly, we compare leaf temperature by species and height at hourly to multi-seasonal timescales and show that on average, leaf temperature is warmer than air temperature in a temperate forest. Leaf temperature can be uniform or heterogeneous across a scene, depending on canopy structure, leaf habit, and meteorology. With this data, we verify that leaf temperature follows classic expectations, yet exhibits noteworthy departures that require additional study and theoretical consideration.