Thermal imaging of increment cores: a new method to estimate sapwood depth in trees

The cells in tree sapwood form a network of interconnected conduits which enables the transport of water and nutrients from the tree roots to the canopy. Sapwood depth must be assessed when tree water use is estimated from sap flow velocities. However, current approaches to assess sapwood depth are either not applicable universally, or require expensive instruments, the application of chemicals or laborious field efforts. Here, we present a new method, which estimates sapwood depth by thermal imaging of increment cores. Using a low-cost thermal camera for mobile devices, we show that the sapwood–heartwood boundary is detectable by a sharp increase in temperature. Estimated sapwood depths agree with dye estimates (R² = 0.84). We tested our approach on a broad range of temperate and tropical tree species: Quercus robur, Pinus sylvestris, Swietenia macrophylla, Guazuma ulmifolia, Hymenaea courbaril, Sideroxylon capiri and Astronium graveolens. In nearly all species, the methods agreed within 0.6 cm. Thermal imaging of increment cores provides a straightforward, low-cost, easy-to-use, and species-independent tool to identify sapwood depth. It has further potential to reveal radial differences in sapwood conductivity, to improve water balance estimations on larger scales and to quickly develop allometric relationships.