Broadband spectral tuning of long-wavelength infrared (LWIR) femtosecond lasers without rotating nonlinear crystals has special usefulness in applications of nonlinear integrated photonics and microscopic ultrafast dynamics studies with stringent requirements on beam pointing. Here, we demonstrate, for the first time to the best of our knowledge, a temperature-tuning LWIR femtosecond optical parametric amplifier (OPA), based on a BaGa4Se7 (BGSe) crystal. Broadband spectral tunability from 8.4 to 17.1 µm over a crystal temperature range of 20-140°C at three fixed phase-matching (PM) angles is achieved with mini-watts output power. As a proof of concept, multiple trace gas detections are demonstrated on sulfur hexafluoride, ethane, and acetylene through only temperature variation. Our results validate the feasibility of achieving an ultra-broadband LWIR spectral tuning through temperature variation in a BGSe OPA, which is beneficial for unique applications such as on-chip spectroscopy and microscopic pump-and-probe experiments.