Wildland fires propagate by liberating energy contained within living and senescent plant biomass. The maximum amount of energy that can be generated by burning a given plant part can be quantified and is generally referred to as its heat content (HC). Many studies have examined heat content of wildland fuels but studies examining the seasonal variation in foliar HC among vegetation types are severely lacking. We collected foliage samples bi-weekly for five months from two common species in the western USA: lodgepole pine (Pinus contorta Douglas ex Loudon) and big sagebrush (Artemisia tridentata Nutt). We measured HC, live fuel moisture content (LFMC) and biochemical components in the leaf dry mass. Our results showed that HC increased for both species, coinciding with LFMC decrease during the growing season. Measured HC values were higher than the constant value in standard fuel models. Lasso regression models identified biochemical components for explaining temporal HC and LFMC variation in lodgepole pine (HC: R2adj = 0.55, root mean square error (RMSE) = 0.35; LFMC: R2adj = 0.84, RMSE = 10.79), sagebrush (HC: R2adj = 0.90, RMSE = 0.13; LFMC: R2adj = 0.96, RMSE = 7.66) and combined data from both species (HC: R2adj = 0.77, RMSE = 0.33; LFMC: R2adj = 0.61, RMSE = 19.75). These results demonstrated the seasonal change in HC and LFMC resulted from temporal biochemical composition variation in dry mass. This new knowledge about HC seasonal change will ultimately lead to improved predictions of wildland fire spread and intensity.