The objective of this work has been to evaluate the power potential of fast growth forest species, which could be operated in Cantabria (North Coast of Spain). This region has a forest surface of 145,000 hectares with no plantation. The use of forest species as energy crops implies high density of plantation, and very short rotations (2-3 years). Therefore, it is the tree youthful stage the one that will be the focus of attention on this work. The Eucalyptus is the fast growth species that has demonstrated the greatest biomass production, by hectare, in the region. Two varieties of Eucalyptus have been planted for years in our region in order to supply the paper industry. These species are Eucalyptus Globulus and Eucalyptus Nitens. The great experience accumulated in the use of these forest species and their great productivity, make of them good candidates to join the set of suitable trees for being used as energy crops. In this work a power characterization of the young stages of the species Eucalyptus Globulus and Eucalyptus Nitens has been carried out. The power characterization has been carried out in an adiabatic high accuracy calorimeter, IKA C 5000. The work was made in the four seasons of the year, which has allowed studying the influence of the seasonality in the calorific power of the species. In addition, the influence that the implantation of these species would have on the CO2 emissions and in the greenhouse effect has been evaluated