Predicting the distribution of Carpinus betulus in Denmark with Ellenberg's Climate Quotient

In this paper we investigate whether Ellenberg's Climate Quotient (EQ, defined as the mean temperature of the warmest month divided by annual precipitation, multiplied by IOOO), can be used to predict the distribution of Carpinus betulus in Denmark. It has been suggested that the competitive relationship between the two tree species Fagus sylvatica and C.betulus is related to EQ in central Europe. Areas with low EQ have dominance of E. sylvatica, whereas higher EQs are associated with dryer and warmer climates that favour C.betulus. To determine if this holds true, also in northern Europe, we investigate the present distribution of C.betulus in Denmark, based on a comprehensive dataset with presence-absence information of the species. We relate the distribution of C.betulus to 12 climate parameters and indices (including EQ), analyse it in a geographical information system and compare the ecology of C.betulus and E sylvatica in four Danish forests, located in different climatic and floristic regions. The highest density of C.betulus was found in eastern Denmark where EQ is high, i.e. summer temperature is relatively high and precipitation low. In the western and south-western parts of the country, where the climate is slightly more wet and more oceanic, there are fewer populations of C.betulus. Based on present climatic data it seems that the climate of Denmark does not limit the occurrence of C.betulus, except perhaps for a small area in western Jutland. We believe that climate changes in the late Holocene cannot alone account for the changes in the distribution of C.befulus in Denmark. And future climate change is likely to affect the distribution of C.betulus with generally better conditions.