In Turkey, continental tropical air movements originating from North Africa and Middle East become dominant, especially in summers. This change in climate brings about dry and hot climatic conditions which generally continue for a long time throughout the summer in the regions outside of the Black Sea Region and North-East Anatolia Region. Ecological processes, such as evapotranspiration and photosynthesis, generally depend on the meteorological conditions that can limit the lives of trees. From this perspective, meteorological conditions become effective in the vegetation period affecting the lives of trees. Thornthwaite has developed the precipitation efficiency formula which sets forth the relation between potential evapotranspiration and precipitation. In terms of finding out the relations between plants and the area they spread out, it is important to investigate on which conditions the regional changes of precipitation efficiency depend.