Plant species and populations have different strategies of responding to fires. Some species can survive fires (resprouters), disperse their seeds in the soil before fires (geosporous), others die (non-sprouters) or disperse seeds postfires (serotinous). Based on these strategies, there are four major fire-related functional plant types.
Drs Byron Lamont and Ed Witowski from Curtin University (Australia) and University of the Witwatersrand (South Africa) investigated the mathematical, biological (e.g. dispersal) and environmental drivers of postfire seedling-adult plant distributions. The researchers found that seedlings of non-sprouters that store seeds in the soil prefire were close to the adult plant but seedlings of resprouter adults were four times further away. After an adult plant dies, the soil beneath usually provides good conditions for new seedlings, so dispersing seeds close by is the best strategy. If the adult plant survives, dispersing seeds nearby would lead to competition between the adult and seedling so the further away the seedlings are dispersed, the better. Lamont and Witowski have previously worked on fire-adapted plant strategies, such as seed dispersal and storage.
Lamont and Witowski carried out field work at two locations within the South Eneabba Nature Reserve (Western Australia). One site was a species-rich scrub-heathland and the other site supported low heath vegetation. The researchers selected 19 species of plants with the “wandering quarter method” and paired them in four functional type combinations (resprouters, R; non-resprouters, NR; geosporous, serotinous).
They measured the height, crown diameter, base stem diameter, distance to the closest conspecific adult and seedling. Prefire measurements were estimated based on plant remnants and surrounding plants. Most plants belonged to Proteaceae and Myrtaceae and overall, 370 measures were recorded.
The number of seedlings per adult plant was estimated from the fieldwork. The researchers hypothesised that plants which rely on wind-dispersal have shorter S-A prefire than postfire, whilst S-A distance will be greater for ant-dispersed species. They also expected that the shortest S-A distance would be for non-sprouters which store seeds in soil prefire.
Lamont and Witowski found that the seedling to adult distance (S-A) is generally the direct function of interseedling distance (S-S) and interadult distance (A-A) and inverse function of seedling to parent ratio (S/A) between species and functional types. The resprouters’ crowns, weight of seeds and rootstocks were four, over four and 80 times larger than non-resprouters respectively. Most non-sprouting seedlings were beneath or close to the adult plants whilst resprouters were further away from the adults. Resprouter seedlings tended to be randomly scattered, up to 8 m away from the adults.
“Our results highlight how fire-response type (NR [non-resprouter] vs R [resprouter]) is a fundamental dichotomy in the morphology, fecundity and population dynamics of plants in fireprone ecosystems”, Lamont and Witowski wrote.
Non-resprouters are generally taller plants with lots of viable seeds whilst resprouters have large below-ground bud banks and fewer seedlings per parent.
“The pattern is clearly adaptive: NR seedlings can take advantage of the ‘regeneration niche’ created by their parents. Many are located near where their parents established, the parent has died and even left a legacy of microhabitat conditions and symbiotic microbes conducive to growth and free of competitors arising from soil-stored seeds in some cases”, the researchers explain.
“Conversely, the parents of R survived the fire, and, while they might have a role as a ‘nurse’ plant sometimes, eventually, if not immediately, they will compete with their offspring for resources.”
This study has offered mathematical and biological explanations of post-fire seedling distributions which allows land managers and conservationists better predict how a community might be shaped by wildfires in the future.