The optimum fire window: applying the fire-productivity hypothesis to Jurassic climate states

Abstract

Abstract. Present day fire frequency has been suggested to relate to a productivity/aridity gradient on a regional and global scale. Optimum fire conditions occur at times of intermediate productivity and aridity, whereas fire is limited on the high productivity (moisture) and aridity (no fuel) endmembers. However, the current global fire activity pattern is biased by the predominant burning of grasslands. Here we test the intermediate fire-productivity hypothesis for a time period on Earth before the evolution of grasses, the Early Jurassic, and explore the fire regime of two contrasting climatic states: the Late Pliensbachian (LPE) cooling Event and the Sinemurian – Pliensbachian Boundary (SPB) warming. Palaeo-fire records are reconstructed from fossil charcoal abundance, and changes in the hydrological cycle are tracked via clay mineralogy, which allows inference of changes in fuel moisture status. Large fluctuations in the fossil charcoal on an orbital eccentricity time scale indicate two modes of fire regime at the time. Wildfires were moisture limited in a high productivity ecosystem during eccentricity minima for both the SPB and LPE. During eccentricity maxima, fires increased, and an optimum fire window was reached, in which heightened seasonality led to intermediate states of productivity and aridity. The LPE experienced more extreme climatic endmembers compared to the SPB, with the fire regime edging closer to ‘moisture limitation’ during eccentricity minima, and more pronounced seasonality during eccentricity maxima, explained by the overall cooler climate at the time. This study illustrates that the intermediate-productivity gradient holds up during two contrasting climatic states in the Jurassic.