An extension of the Mutch hypothesis: does age influence relative flammability in fire adapted and non-fire adapted pines?

Background/Question/Methods

Flammability varies among plant species. Several hypotheses have been put forward about the benefits of plants developing increased flammability, including: elimination of competitors, creation of optimal conditions for germination, and promotion of quicker burning fires to preserve saplings and underground organs.  If true, these hypotheses suggest enhanced flammability would only be an advantage when individuals reach maturity; thus one would expect to see enhance flammability in fire-adapted adults compared to juvenile individuals of fire adapted species.  I investigated the relationship between reproductive status and flammability in pine species, asking the questions: 1) can differences in flammability be detected in needles from pre- and post-reproductive pines? 2) do age differences in flammability differ between fire adapted and non-fire adapted pines?  and 3) do physical characteristics explain differences between fire and non-fire adapted pines?  Downed needles from pre-reproductive and post-reproductive individuals were collected from two fire adapted pine species (Pinus palustris and P. jeffreyi) and two non-fire adapted species (P. monophylla and P. albicaulis).  Additionally, a subset of needles from fire adapted pines was cut to mimic the length of the non-fire adapted species.  Ten 25 gram sample of each treatment were dried and burned.  Maximum temperature and smoldering time were recorded.

Results/Conclusions

As expected fire adapted and non-fire adapted pines differed in both smoldering time (p <0.001) and maximum temperature (p <0.001), with non-fire adapted pines burning longer and hotter. Effects of age were greater on fire adapted pines than non-fire adapted ones for both smoldering time and, especially, maximum temperature.  Lastly, when needles from fire adapted were cut to match the length of needles in non-fire adapted pines, significant differences were observed in both smoldering time (p <0.001) and maximum temperature (p =0.005) relative to unmanipulated needles.  In both smoldering time and maximum temperatures, cut needles performed intermediately to intact fire adapted and non-fire adapted pines.  These results indicate that age of the individual does affect flammability of these fire adapted pines, with needles from mature individuals burning quicker and hotter than immature sources.  Difference between the shorter needles (non-fire adapted and cut treatments) and intact fire adapted needles reinforce the importance of needle length on burning characteristics.  In the field, quicker burning could help spread fire to eliminate neighbors as well as reduce residence time of flames, which impacts survival of individuals, lending support for hypotheses about the benefits of enhanced flammability.