Climate change is a natural phenomenon with far-reaching impacts. Due to global warming, forest vegetation patterns in the Mediterranean region can be affected and the extent of forested areas can be altered. The purpose of this study was to investigate the impact of slightly decreased forest density on physical water quality parameters by employing 18% thinning with a paired watershed methodology in a broadleaf forest ecosystem. After a 70-month monitoring period that started in December 2005, calibration equations were established between control and treatment watersheds for streamwater parameters including pH, color, turbidity, electrical conductivity (EC), suspended sediment concentration (SSC), water and air temperatures. After 18% standing timber volume was harvested from the treatment watershed, streamwater was also sampled for the same parameters in both watersheds during the 46-month treatment period between January 2012 and October 2015. Changes in the mean monthly values of streamwater parameters were determined as the differences between measured and estimated values derived from the calibration equations. Results showed that 18% forest thinning caused 7.3 μS cm−1 increase in the overall average monthly EC, 2.8 NTU in turbidity, 15.1 mg L−1 in SSC, 0.3 °C increase in the air temperature, and 1.2 °C in the maximum air temperature, whereas it caused 0.5 °C decrease in overall average monthly minimum temperature and 1.3C.P·U in overall mean monthly color. In contrast, forest harvest did not have significant impact on overall average monthly pH and streamwater temperature values. Results of 18% forest harvest indicated that even a small decrease in the forest cover can significantly affect selected physical water quality parameters and hence aquatic life in the forested watersheds.

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