Seed survival in small myrtaceous capsules subjected to experimental heating

Mature capsules of four small-fruited Myrtaceae (Eucalyptus regnans, Leptospermum laevigatum, L. myrsinoides and Kunzea ambigua) were heated in a muffle furnace over a range of temperatures (200-750° C) and for various lengths of time (15-300s). In addition, the rise in intracapsular temperature with time was measured at 250° C and the lethal seed temperature for K. ambigua determined by heating loose seed in a controlled-temperature water bath. Encapsulated seed survived heating for only short periods event at the lower range of experimental temperatures, with no seed surviving for more than 2 min at 200° C and the highest temperature survived being 650° C for 15 s by L. laevigatum. The species were ranked E. regnans, K. ambigua, L. myrsinoides and L. laevigatum in increasing order of insulating capacity of their capsules, based on survival times of encapsulated seed and the rate of increase of intracapsular temperatures. Seed of K. ambigua was killed when heated in water for a few seconds at 90-100° C. This result agrees closely with the threshold lethal temperature derived for all species by superimposing seed survival versus time and intracapsular temperature versus time curves for capsules heated at 250° C. These results demonstrate that despite their in situ efficacy during fire, small myrtaceous capsules hre mediocre seed insulators. They also suggest that in the field, survival times for encapsulated seed are likely to be in the order of seconds rather than minutes, which points to brief flame residence times in individual tree or shrub canopies. This work has the potential to be developed as a simple but powerful method for the measurement and mapping of fire intensities.