Simulation of the Effect of Spatial and Temporal Variation in Fire Regimes on the Population Viability of a Banksia Species

Abstract

Populations of plants that rely on seeds for recovery from disturbance by fire (obligate seeders) are sensitive to regimes of frequent fire. Obligate seeders are prominent in fire‐prone heathlands of southern Australia and South Africa. Population extinction may occur if there are successive fires during a plant’s juvenile period. Research on the population biology of obligate seeders has influenced the management of fire in these heath and shrublands, but work on the effects of the spatial variability of fires is lacking. We hypothesize that extinction maybe avoided under an adverse fire frequency if fires are patchy. We present a model that simulates the effects of spatial and temporal variations in fire regimes on the viability of a plant population in a grid landscape. Seedling establishment, maturation, senescence, and seed dispersal determine the presence or absence of plants in each cell. We used values typical of serotinous Banksia species to estimate probability of extinction in relation to fire frequency and size. We examined the sensitivity of predictions to dispersal, senescence, fire frequency, spatial burning pattern and size variance, and the size of the grid. Simulations 200 years in length indicated that extinction probability was lowest when mean fire frequency was intermediate and mean fire size was large. When fire frequency was high, extinction probability was high irrespective of fire size. Senescence was more important than high‐frequency fire as a cause of extinction in cells. Interactions between dispersal, fire frequency, and size were complex, indicating that extinction is governed by intercell connectivity. The model indicates that fire patchiness cannot be assumed to ensure avoidance of extinction of populations. Conservation of populations is most likely when fire patchiness is relatively low—when the size of fires is moderate to large and when burned patches are contiguous.

The post-fire effects on the outcrossing rate of a Brazilian savannah shrub, Helicteres sacarolha A.St.-Hil

“…Despite the fact that flower density and outcrossing rate increased after 1994 fire, repeated fire may decrease the population size of H. sacarolha if some plants are killed by fire, as the data suggest. A simulation of the effect of spatial and temporal variation in fire regimes on the population viability of a Banksia species showed that when fire frequency is high, the population extinction probability increases (Bradstock et al 1996). Sampson et al (1994) found low outcrossing rates for populations of Banksia brownii and suggested that frequent events of fire may have decreased plant density and outcrossing rate within population.…”

Ecological Implications of Fine-Scale Fire Patchiness and Severity in Tropical Savannas of Northern Australia

“…Price et al (2003) assessed fine-scale patchiness through field measurements in northern Australia and stated that a key issue with the above studies is that the scale of most remotely sensed imagery used, predominantly from the MODIS and Landsat sensors, cannot detect fine-scale patches (≈1m scale). Although the importance of fire patchiness has been widely questioned and recognised, evidence regarding its ecological significance is largely anecdotal (Bradstock et al 1996, Ooi et al 2006, Parr and Andersen 2006.…”

Soil temperatures during autumn prescribed burning: implications for the germination of fire responsive species?

“…Short fire intervals (as experienced in frequent fires) are thought to result in the decline or disappearance of some species, most commonly obligate seeding shrubs, and the increase in other species with short maturation times, e.g. grass species (Gill 1981;Bradstock et al 1996;Keith 1996). If the fire interval is shorter than the time to maturation, species will not add seed to the soil seed bank and over time, soil seed bank stores will be depleted and those species will be lost from the sites.…”