We examined the diurnal and seasonal patterns of the energy fluxes (net radiation, sensible heat, latent heat, and heat flux) within three conifer forests of different composition (Douglas-fir/western hemlock and jack pine). Measurements at 20- and 40-year old Douglas-fir/western hemlock sites located within the Wind River Valley in southern Washington State, USA were made between June 15 and October 15 of 1998 and 1999. Measurements were also made within a 12-year old jack pine stand located in the Upper Peninsula of Michigan, USA between June 15- October 15 of 2001. At all three sites there was a close link between net radiation (Q) and the sensible (H) and latent (L) heats. Average heat flux (G) at all three sites was negligible. On a monthly basis the energy balance closure was generally highest at the jack pine site (R²=0.95-0.97) and lowest at the 20-year Douglas-fir stand (R²=0.74-0.94). Furthermore, average Q was highest in the jack pine stand (392 W.m²) and lowest in the 20-year stand (212 W.m²). Over the course of the sampling period, H accounted for about 75% of the total available energy (e.g., H + L) in the 20-year stand, as indicated by a Bowen ratio of 3.7. At the 40-year old Douglas-fir stand the Bowen ratio was 2.4, and H accounted for 70% of available energy. The partitioning between H and L was slightly more equal at the jack pine site, with a Bowen ratio of 1.8. Comparative studies, such as this one, can help us more accurately assess how modifications in land-use will affect ecosystem productivity.