Fire refugia: The mechanism governing animal survivorship within a highly flammable plant

Identifying factors that influence the survival of individuals during disturbance is critical to understanding patterns of species reassembly within ecological communities. Although most studies of recovery of populations post-burning acknowledge the potentially important contribution of animals surviving in situ, few have measured the effectiveness of refugia. This paper tests the hypothesis that some plants with tightly packed leaf-bases provide a refuge for invertebrates during fire (even when the plants themselves burn) by using the highly flammable grass tree (Xanthorrhoeaceae: Xanthorrhoea). Invertebrates were sampled from four unburnt and five experimentally burnt grass trees (Xanthorrhoea preissii Endl.). Also collected were invertebrates fleeing during burning. The dataset comprises 949 specimens, representing 81 species from 18 orders, of which 749 individuals were from unburned plants. Slaters (Isopoda), silverfish (Thysanura), spiders (Araneae) and bugs (Hemiptera) dominated assemblages of the unburnt grass trees. Despite grass trees burning at temperatures of up to 515 degrees C, some invertebrates survived in situ. Species-specific microhabitat preferences within the plant appeared to influence survivorship. Species collected in the crown of unburned plants were found more often alive on burnt plants than species typically inhabiting the dead skirt of decaying leaves (thatch). We contend that the mechanism causing differential mortality is fire temperature. In the dead skirt, temperatures reached 225.33 +/- 66.57 degrees C. In contrast, a region of mild temperature (25.00 +/- 3.54 degrees C) persisted throughout burning near the apical meristem (within the crown). We conclude that grass trees are a potential reservoir from which invertebrates might re-colonize recently burnt areas. However, owing to species-specific microhabitat preferences and differential mortality across microhabitats, the invertebrate assemblage remaining in situ will be restricted taxonomically compared with the original grass tree fauna. Moreover, different fire regimes might mediate the effectiveness of grass trees as refugia. Finally, we argue that in situ survival of invertebrates within plants with tightly packed leaf-bases is an unrecognized global phenomenon applicable to a wide array of plant taxa.