Fire intensity effects on serotinous seed survival

Abstract

In fire-prone environments, some species store their seeds in canopy cones (serotiny), which provides seeds protection from the passage of fire before stimulating seed release. However, the capacity of serotinous cones to protect seeds under high intensity fire is uncertain. Beyond simply “high” versus “low” fire intensity or severity, we must understand the influence of the specific characteristics of fire intensity—heat flux, exposure duration, and their dynamics—on serotinous seed survival. In this study, we tested serotinous seed survival under transient levels of radiant heat to understand the distinct and combined impacts of radiative heat flux and duration of exposure on the survival of seeds from two serotinous obligate seeder species: yellow hakea (Hakea nodosa R.Br.) and heath-leaved banksia (Banksia ericifolia subsp. ericifolia). We found differing impacts of fire intensity treatments on seed survival. Static levels of radiative heat (17 kW/m2) at long durations (600 s) reduced seed survival by 75.7% for yellow hakea and 1.5% for heath-leaved banksia compared to the control. However, dynamic heat (a short 120-s period of 40 kW/m2 followed by a slow decline) with an identical total duration (600 s) did not have comparable reductions in seed survival. This is despite both treatments having comparable radiant exposure (10,200 kJ/m2 for the former and 10,236 kJ/m2 for the latter). Both species demonstrated remarkable capacity to withstand heat treatments, particularly dynamic fire intensity—both high (40 kW/m2) and low (19 kW/m2). While almost all fire exposure treatments reduced survival from the control, most seeds remained viable and germinated upon release. Our study highlights the importance of examining dynamic rather than static fire effects on vegetation, to accurately replicate the conditions of a fire front. Serotinous seeds demonstrate good capacity to tolerate intense fire. Nonetheless, the combined effects of high heat flux at prolonged durations reduces seed survival. We suggest overly prolonged passing fire fronts may cause seed death and are a risk to obligate seeder species that rely solely on seeds for persistence post-fire.