Almond tree canopy temperature reveals intra-crown variability that is water stress-dependent

Tree water status is often characterized by measuring a few leaves and it is not known to what extent such measurements represent the tree as a whole. We present an assessment of the intra-crown temperature variability and its relationship with water status in two almond cultivars. High-resolution imagery was acquired on 30 June 2009 at 11:30, 14:30, and 16:30 h (solar time) with a thermal camera on-board an aircraft over an almond orchard in Kern County, CA, USA. Ten irrigation levels were applied, ensuring a wide variability in water status, and each was replicated eight times. Stem water potential and stomatal conductance were measured on trees of various irrigation regimes at each flight. Significant variation in canopy temperature was found within each crown, probably reflecting differences in stomatal conductance in different parts of the tree crown. The intra-crown standard deviation of canopy temperature (intra-crown σTc) increased from fully irrigated trees to intermediate irrigation levels, diminishing afterwards in the most stressed treatments. Mean canopy temperature was well correlated with stomatal conductance and stem water potential (R2 above 0.65). In trees that had similar mean canopy temperature, intra-crown σTc correlated well with tree water status. Our results quantified in detail the spatial variability in surface temperatures that exists within almond tree crowns and suggest that the intra-crown temperature variation may be a useful indicator of the onset of tree water stress.