Thermographic analysis of leaf water and energy information of Japanese spindle and glossy privet trees in low temperature environment

a r t i c l e i n f o Keywords: Japanese spindle Glossy privet Thermography Hand heating Sap warming Water and thermal status Thermography has been used in many fields to perform non-invasive temperature measurements of natural objects. In this paper, thermography was used to determine the temperature of leaves, stems and branch kerfs of Japanese spindle (Euonymus japonicus Thunb.) and glossy privet (Ligustrum lucidum Ait.) in the city of Jinan in China during winter. The temperatures of the leaves, stems and branch kerfs were monitored as the temperature decreased after the sample was subjected to hand heating or after the branch was cut. Differences in the specific heats and the latent heats of the leaves, branches and stems with different water contents and transpiration capacities were confirmed. The significant temperature difference obtained after hand heating between different leaf sections with varied water contents made it easy to obtain the thermal images, which were clear and exhibited reduced systematic errors. After hand heating, a significantly higher temperature was found at the major vein system of both Japanese spindle and glossy privet. This increased temperature difference made it possible to detect the water and the thermal state of these leaves. Therefore, it was possible to detect scorched area of the leaves, the twig dieback and the sap warming phenomenon in the leaves using thermography. In addition, the leaf bending phenomenon observed in Japanese spindle leaves during the deep freezing process indicates that the leaf scorch symptoms result from water stress and a lack of sap warming. The specific heat and the latent heat of the melting and evaporating process of water lay the foundation of transpiration cooling (Clements, 1934; Rosenberg, 1974). With the highest specific heat, water tends to stabilize temperature, and this process is reflected in the relatively uniform temperature of islands and lands near a large body of water (Kramer, 1983; Rosenberg, 1974). Transpiration causes the temperature of trees to vary less than the air temperature (Gates, 1968). The lower temperature of deeper soil water and soil water under the coverage of crown self increases the effectiveness of the transpiration cooling of trees (Rosenberg, 1974). The soil temperature amplitude decreases with increasing depth during both summer and winter. At a depth of 40 cm, the temperature wave is significantly damped, particularly in winter, whereas no significant diurnal/annual wave is found at a depth of 80 cm (Rosenberg, …