Do cone age and heating mode determine the opening of serotinous cones during wildfires? A new bench scale approach applied to Pinus halepensis Mill

Abstract

Serotiny is a well-known fire adaptive trait in some species, as the Mediterranean conifer Pinus halepensis. However, information about cone opening mechanisms during wildfires and consequences on post fire dispersal is scarce. In addition, standardized methods allowing a realistic simulation of heating modes at bench-scale are not available. In this study, we address for the first time the interacting effects of radiation, convection and direct flame on the opening and seed release of serotinous cones, following a novel repeatable methodology. Using a Mass Loss Calorimeter (MLC) device and a wide range of heat exposures (between 5 and 75 kW m^-2) with or without ignition, we intended to simulate realistic cone heating during surface and crown fires in laboratory conditions. Additionally, we included the effect of contrasting serotinous cone ages interacting with heating mode and considering the random individual variation. The proposed methodology has shown a high potential to simulate the complex process of crown fires in relation to cone opening under controlled conditions, detecting a threshold of heat exposure (25-30 kW m^-2) for cone opening. We confirmed that heating mode had a highly significant effect in cone opening, interacting with cone age, while cone age effect on its own was marginal. Particularly, ignition significantly increased the efficacy of cone opening and seed release. Moreover, young and old cones behave differently in seed release, both in surface and crown fire simulations. Implementing and adjusting this methodology in other species will allow more realistic and reliable quantitative comparisons than previously attained.

Keywords: Crown fires; Fire traits; Flammability; Mass Loss Calorimeter; Seed dispersal; Surface fires.