Effects of recent climate trends on the distribution of potential natural vegetation in Central Germany.

Introducing climate quotients for the growing season (Q(gs)) provides a way to quantify effects of climate trends with respect to Potential Natural Vegetation (PNV), especially beech forests (Fagus sylvatica L.) in Central Germany. What is crucial in this regard is the great influence of the dominant decrease in the amount of precipitation (up to 40% in the last 50 years) during the growing season versus the dormant season. However, precipitation during the dormant season (which is predominantly increasing: up to 40% in the last 50 years) is also important for replenishing the soil water supply. The Q(gs) values of the Climatic Normal period of 1971-2000 are generally higher (up to 12% in lowland areas) compared with the Climatic Normal period of 1961-1990, the extent of the difference being in general inversely proportional to elevation above sea level. What this means for the area under investigation is that humidity conditions, which generally improve as the elevation above sea level increases, have a positive effect on the site potential. However, a comparison of the climatologically important period of 1991-2003 with the period of 1961-1990 (area-wide increase between 12% and 16%) could not identify this positive effect of elevation on precipitation for the area under investigation. With regard to the recent climate-based trends of PNV, we have shown that all natural spatial units in Central Germany are affected by progressing continentality (i.e., dryness) during the growing season and the resulting deterioration of the site potential. The area of potential beech forest at lower elevation has decreased in favour of oak forest as PNV, while less change is observed in the montane area.