Oak trees play a significant naturalistic and economic role. Common oaks represent the most valuable forest-creating species in Poland. In the recent years, on the area of the whole country, one can observe a visible deterioration of the health conditions in oak tree stands. It is difficult to indicate the main factor responsible for this phenomenon. Because of the wide range of this observation, it can be assumed that this process is caused by many biotic and abiotic, as well as anthropogenic factors. Studies were out in the Sulęcin Forest District. At the turn of July and August in the years 2009 and 2010, an assessment of the losses in the majority of trees showed a medium degree (49%). Another group of trees showed 36% of leaf loss, while 14% of oak trees did not show any losses at all. A very high degree of defoliation was found only in 1% of trees. The presented data indicate that an average loss in the assimilation apparatus of oak trees amounted to 29.15%.

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