Using an infrared camera, we measured the leaf temperature across different canopy positions of a 23-m-tall deciduous forest tree (Fagus sylvatica L.) including typical sun and shade leaves as well as intermediate leaf forms, which differed significantly in specific leaf area (SLA). We calculated a temperature index (IG) and a crop water stress index (CWSI) using the surface temperatures of wet and dry reference leaves. Additional indices were computed using air temperature plus 5 °C (IG + 5, CWSI + 5) as dry references. The minimum temperature of the wet leaf and the maximum temperature of the dry leaf proved to be most suitable as reference values. We correlated the temperature indices with leaf area-related conductance to water vapor (gL) using porometry at the leaf level and using xylem sap flow at the branch level. At the leaf and at the branch level, IG and CWSI were equally well suited as proxies of gL, whereas the relationships of IG + 5 and CWSI + 5 with gL were only weak or even insignificant. At the leaf level, the correlations of IG and CWSI with gL were significant in all parts of the crown. The slopes of gL vs. IG and CWSI did not differ significantly among the crown parts; this indicates that they were not influenced by SLA or irradiance. At the branch level, close correlations (r > 0.8) were found between temperature indices and gL across the crown. These results demonstrate that satisfactory relationships between temperature indices and gL can be established in tall trees even in those canopy parts that are exposed to relatively low levels of irradiance and exhibit relatively low values of gL.