Overstory and understory leaves warm faster than air in evergreen needleleaf forests

• Leaf temperature was estimated with a multi-layer energy balance model. • Leaves were warmer than air regardless of canopy position and climate. • Emitted radiation was the dominant cooling force, not transpiration. • Sensible heat flux prevents needle leaves from warming further. The limited homeothermy hypothesis states that leaves maintain their temperature within an optimal range for photosynthesis by increasing transpiration during warm conditions. Under limited homeothermy, plants may offset thermal stress caused by climate change. If this hypothesis is true, we should observe: 1) leaf temperature increasing slower than air temperature and 2) leaves cooler than air during warm conditions. We tested these predictions with an energy balance model for evergreen needleleaf forest sites in the National Ecological Observatory Network. A key feature of our model was its vertical stratification of the canopy, which allowed us to analyze vertical gradients in canopy temperature. This feature is especially important given that prior work has focused on the tops of forest canopies. Our results do not support limited homeothermy at any canopy position. In all canopy strata, leaf temperature increased faster than air and periods with leaves cooler than air were rare. In such cases, cooling was due to emitted radiation, not transpiration. But, when water was abundant, transpiration could produce mildly homeothermic behavior. We attribute these results to the needle-like shape of leaves in our study sites. This leaf shape increases boundary layer conductance and causes heat gain from surrounding air to overpower heat loss from transpiration when leaves are cooler than air. Our results indicate that needleleaf forests cannot avert thermal stress in a warming world. Thermal limits on photosynthesis and non-linear increases in respiration with temperature may weaken the role of evergreen forests as a global carbon sink.