The gasification behavior of six different biomass species (four forest pruning residues including Pinus pinaster, Pinyon pine, Eucalyptus, and white poplar as well as two types of garden prunings) has been investigated in a pilot fluidized bed gasifier to experimentally quantify (i) the yields of pyrolytic gas released from the fuel particle in the absence of oxygen; (ii) the effect of oxygen on the gas yields and the significance of the secondary reactions (gasification of char and reforming/cracking of gas species) during gasification with air; and (iii) the effect of fuel throughput on gasification performance. Continuous steady-state experiments were conducted using air at temperatures 800, 850, and 900 °C, equivalence ratios (ER) in the range of 0.16–0.32, and throughputs between 245 and 426 kg/h/m2, as well as semicontinuous (batch of fuel in a continuous gas stream) devolatilization experiments with N2 (ER = 0). Although significant quantitative differences in the gas yields, char conversion, gas heating value, and gasification efficiency were found for different fuels, the trends with changing temperature and ER followed similar characteristics. P. pinaster gave the best results, showing cold gas efficiencies of up to 70% at 900 °C, while white poplar and garden prunings performed considerably worse. The effect of throughput was analyzed for three out of six fuels, and it was observed to be small for P. pinaster and garden prunings, whereas it was significant for white poplar as a result of the lower bulk density of white poplar compared to the other fuels. The results conclude about gas composition and process efficiency expected from different biomass residues in fluidized bed gasifiers and provide fundamental data to validate a theoretical model, currently under development, to scale up the process.