Forest fires are of increasing concern globally. Fire usually starts as a small flame over a dry patch of forest litter and grows into a large fire with time. The heat feedback from the flame/fire leads to the thermal decomposition of the forest litter bed and the release of gaseous pyrolysate, which in turn burns with the surrounding air to make the flame/fire larger and spread to adjacent locations. In this work, a 1D numerical model is developed to predict the generation of pyrolysate or equivalently, the Mass Loss Rate (MLR) of the forest litter bed when exposed to a prescribed heat flux at the top of the litter bed. The governing equations in the solid phase consist of unsteady equations of conservation of mass, component, and energy and a pyrolysis law. These equations are coupled to the unsteady conservation equations of mass, component, energy, and momentum in the gas phase. The model is validated and used to predict the results of in-house cone calorimeter experiments on the degradation of forest litter beds. The effect of bed density on the thermal decomposition of the fuel bed is analyzed in terms of heat transport into the forest litter bed.