Heat treatment is an effective method to improve the dimensional stability and biological durability of wood. The heat treatment technology and the heat treated wood products were recently introduced to Turkey. However, only a few studies have been reported on the effect of natural weathering on the properties of heat treated wood in both Turkey and the world. In this study, heat treated alder (Alnus glutinosa L.) wood stakes were subjected to a field trial in a test site of northern region of Turkey, which exhibits favorable conditions for degradation of wood. The alder wood specimens were heat treated at temperatures of 150, 180 and 200°C for periods of 2, 6 and 10 hours. The heated samples were exposed to the field soil for 3 years. At the end of the three years the mass loss was calculated. The color stability of the samples of untreated, only heat-treated and heat-treated and weathered samples, was determined according to the CIE L*a*b* system. The bending strength (modulus of rupture: MOR) of the test and control samples were also determined according to the ASTM D 143 standard method. Each sample was visually inspected for decay index according to EN 252 during weathering. Results indicated that the mass loss values were extended with increasing treatment temperature and duration. After heat treatment it was observed that MOR values were reduced by up to 50% compared with normal untreated wood. At the end of the 3 years, MOR values of the field test samples were clearly lower than the only heated ones. Color changes (∆E) values of the samples have varied between 4 and 35 after heat treatment. Heat treatment increased the wood color stability. However, ∆E values of the field test samples were clearly much lesser for the only heated samples having varied between 6 and 19. Index of decay (ID) of heat treated wood samples was fairly lower than that of the controls, especially at temperatures of 180 and 200°C. The lowest ID value (30%) was obtained in the samples treated at 200°C for 10 hours.

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