Response of Resprouting Shrubs to Repeated Fires in the Dry Sclerophyll Forest of Gilbraltar Range National Park
Fire regimes affect survival and reproduction of shrub species in fire-prone vegetation such as occurs in Gibraltar Range National Park. The influence of fire regimes on resprouting shrubs is known for a range of species in coastal regions of Australia but is poorly known in montane sclerophyll communities. The fire responses of three Proteaceae shrubs ('Banksia spinulosa', 'Hakea laevipes', 'Petrophile canescens') and a grasstree ('Xanthorrhoea johnsonii') were measured after the wildfire of 2002 to determine whether: 1) storage organ size was related to post-fire growth and flowering response, 2) fire frequency influences postfire mortality and if survival was related to the size of plant; 3) fire frequency influences the resprouting ability of plants, and 4) fire frequency affects pyrogenic flowering in the post-fire environment. We found the size of storage organs was positively related to post-fire sprouting in the three shrubs and to flowering in the grasstree. However, high fire frequency only affected the survival of Banksia spinulosa and decreased flowering in 'Xanthorrhoea johnsonii'. Survival in all species ranged between 83 and 99% and it appears that the intervals between fires (7-22 years) had been sufficient for most adult plants to regain the ability to resprout. The ability of juvenile plants to develop the ability to resprout needs to be tested on seedlings that established after recent fires.
Fire severity, feedback effects and resilience to alternative community states in forest assemblages
Alternative stable state theory is often used to explain the occurrence of flammable vegetation adjacent to less flammable vegetation where fire regimes mediate the shift between states. In 2002 an extreme landscape scale fire burnt extensive areas of forests in eastern Australia, including rainforests that are rarely severely burnt. This unique event allowed us to test long-held assumptions that predict landscape scale change after major disturbance. We tested three assumptions for detecting alternative community states; (1) that the scale of the event was large enough to remove canopy dominants, (2) fire feedbacks, both positive and negative are present, and (3) shifts in the floristic composition of communities are detected. We also examined whether high severity fires resulted in a community shift from less flammable to more flammable vegetation (e.g. from rainforest to wet sclerophyll vegetation), by examining floristic composition of vegetation communities (rainforest, wet sclerophyll forest, and dry sclerophyll forest) when burnt at different fire severities (high and low). Conversely, we tested whether there was a state shift from flammable to less flammable vegetation communities in sclerophyll forests long unburnt. In addition, we determined if there was any evidence that antecedent fire regimes and fire severity influenced flammability feedbacks. Severe fire caused significant ongoing disruption to forest canopies and fire effects were still detectable some 7 years after fire. Whilst some pyrogenic environmental feedbacks were detected from historic fire regimes, we found no shifts in the floristic composition or pyrogenic traits of forests burned at high severity. Medium-term (30 year) fire exclusion did not result in the sclerophyll forest becoming more pyrophobic although some fire-cued species senesced in the absence of fire. Contrary to expectation, we found no evidence that the floristic composition of less flammable vegetation burnt at high severity became more similar to flammable vegetation burnt at low severity or that were unburnt. Conversely, with more than 30 years of fire exclusion there was no evidence that the sclerophyllous communities became more floristically similar to rainforest. We have shown that species assemblages in warm-temperate rainforest were resilient to a catastrophic fire event and propose that these forests are unlikely to represent alternative community states driven by fire alone.
Post-fire response of shrubs in the tablelands of eastern Australia: do existing models explain habitat differences?
Fire is an important ecological factor that influences the distribution and abundance of plant populations of shrub species in fire-prone habitats. Comprehensive information about the fire-response syndromes and post-fire recruitment of seedlings in tableland habitats of eastern Australia is poorly known. In particular, data on shrubs occurring in grassy habitats are lacking for temperate regions of Australia. The post-fire response and recruitment patterns of shrub taxa were recorded from research burns and wildfires on the New England Tablelands over 4 years in the following four habitats: grassy woodlands and open forests, shrubby open forests, wet heaths and rocky outcrops. The ratio of obligate seeder to resprouter species differed among habitats, with the highest ratio occurring on rocky outcrops (90 : 10) and the lowest in grassy forests (19 : 81). Post-fire recruitment of seedlings was also highest on rocky outcrops whereas seedlings were rarely observed in the wet heaths and grassy forests. The following six models that explain these patterns were reviewed: fire and grazing frequency, soil nutrients and texture, habitat openness and environmental variability. No one model could uniformly explain differences in fire response across all habitats but a combination of disturbance-frequency and regeneration-niche models may provide a mechanism for the patterns observed. Field and laboratory experiments are needed to examine allocation to persistence (resprouting) and reproduction in species with different fire-response syndromes. These experiments also need to examine both disturbance-frequency and regeneration-niche factors in manipulative experiments.
Landscape patterns of woody plant response to crown fire: Disturbance and productivity influence sprouting ability
1. - The relationship between environment and the ability of plant species to resprout has been explored more in terms of disturbance frequency than of resource gradients, and has rarely been examined in non-Mediterranean landscapes. 2. - The fire response of 296 non-eucalypt woody taxa was recorded in five habitats in the New England Tablelands (NET) Bioregion of eastern Australia: grassy woodlands, dry sclerophyll forests, rocky outcrops, wet heaths and wet sclerophyll forests. We then tested whether there was a dichotomy of response to crown fire, whether the proportion of resprouters differed among habitats, and if disturbance frequency or resource-productivity models could account for landscape patterns of resprouting. 3. - There was a continuum of sprouting ability but most species could be classified as obligate seeders (killed by fire) or resprouters. Habitats differed in the proportion of resprouting species, with rocky outcrops having the lowest proportion and grassy forests and wet heaths the highest. This pattern was consistent at the congeneric and confamilar phylogenetic levels of comparison. 4. - Resource/productivity models better explained landscape patterns of resprouting than disturbance frequency models. There was a strong positive relationship between resprouting and increasing soil fertility and moisture gradients. Species richness and obligate seeder richness increased with climate variability and landscape heterogeneity. 5. - Landscape resprouting patterns were explained by a resource-competition model where resprouters are favoured because of their ability to persist in more competitive environments. Overall, we suggest that disturbance frequency has larger effects on species richness at the low end of the productivity gradient than at the high end.
Fire season and intensity affect shrub recruitment in temperate sclerophyllous woodlands
The season in which a fire occurs may regulate plant seedling recruitment because of: (1) the interaction of season and intensity of fire and the temperature requirements for seed release, germination and growth; (2) post-fire rainfall and temperature patterns affecting germination; (3) the interaction of post-fire germination conditions and competition from surrounding vegetation; and (4) the interaction of post-fire germination conditions and seed predators and/or seedling herbivores. This study examined the effects of different fire intensities and fire seasons on the emergence and survival of shrubs representing a range of fire response syndromes from a summer rainfall cool climate region. Replicated experimental burns were conducted in two seasons (spring and autumn) in 2 consecutive years and fuel loads were increased to examine the effects of fire intensity (low intensity and moderate intensity). Post-fire watering treatments partitioned the effects of seasonal temperature from soil moisture. Higher intensity fires resulted in enhanced seedling emergence for hard-seeded species but rarely influenced survival. Spring fires enhanced seedling emergence across all functional groups. Reduced autumn recruitment was related to seasonal temperature inhibiting germination rather than a lack of soil moisture or competition. In Mediterranean-type climate regions, seedling emergence has been related to post-fire rainfall and exposure of seeds to seed predators. We think a similar model may operate in temperate summer rainfall regions where cold-induced dormancy over winter exposes seeds to predators for a longer time and subsequently results in recruitment failure. Our results support the theory that the effect of fire season is more predictable where there are strong seasonal patterns in climate. In this study seasonal temperature rather than rainfall appears to be more influential.
Resprouting as a key functional trait: how buds, protection and resources drive persistence after fire
Resprouting as a response to disturbance is now widely recognized as a key functional trait among woody plants and as the basis for the persistence niche. However, the underlying mechanisms that define resprouting responses to disturbance are poorly conceptualized. Resprouting ability is constrained by the interaction of the disturbance regime that depletes the buds and resources needed to fund resprouting, and the environment that drives growth and resource allocation. We develop a buds-protection-resources (BPR) framework for understanding resprouting in fire-prone ecosystems, based on bud bank location, bud protection, and how buds are resourced. Using this framework we go beyond earlier emphases on basal resprouting and highlight the importance of apical, epicormic and below-ground resprouting to the persistence niche. The BPR framework provides insights into: resprouting typologies that include both fire resisters (i.e. survive fire but do not resprout) and fire resprouters; the methods by which buds escape fire effects, such as thick bark; and the predictability of community assembly of resprouting types in relation to site productivity, disturbance regime and competition. Furthermore, predicting the consequences of global change is enhanced by the BPR framework because it potentially forecasts the retention or loss of above-ground biomass.
Trade-offs in resource allocation that favour resprouting affect the competitive ability of woody seedlings in grassy communities
1. Differences in the competitive ability of plant functional groups at early life-history stages can have important consequences for community structure. In particular, trade-offs in allocation to roots by woody plant seedlings may influence competitive ability with grasses in fire-prone vegetation. 2. We followed post-fire survival of seedlings of facultative resprouter and obligate seeder (firekilled) shrubs for 3 years in adjacent communities with a grassy/graminoid ground stratum (54 plots, 20 m²) or a non-graminoid ground stratum(54 plots, 20 m²). 3. The competitive effect of a grass (Poa) on seedlings of three congeneric pairs of resprouters and obligate seeder shrubs was tested in a factorial experiment where nutrients and the grass competitor were manipulated. The effects of grass (+,-) and nutrients (+,-) on the growth response, biomass allocation and root carbohydrate storage were measured after harvest at 26 weeks and the relative neighbour effect calculated. 4. Post-fire shrub seedling survival was high with about 50% (2163 seedlings) surviving over 3 years, but this varied between habitats and functional groups. In the grassy/graminoid ground layer communities 27% of shrub seedlings survived, whereas in the habitats with a more open ground stratum 55% of seedlings survived. In grassy habitats, obligate seeder survival was lower (23% survival) than that of resprouter seedlings (35% survival). Similarly, in open habitats, obligate seeder seedling survival was lower (51%) than that of resprouter seedlings (64% survival). 5. Growth of both resprouters and obligate seeders in our manipulative experiment was strongly reduced in the presence of a grass competitor. Moreover, the addition of nutrients increased the relative difference in mass and height between those seedlings exposed to a grass competitor and those grown without a competitor. Resprouter species allocated more to roots under competition and were less affected by grass competition than obligate seeders. 6. Synthesis. The results of seedling survival and of the experiment on the effects of grass competition on woody plant seedlings suggest that early life-history trade-offs in allocation influence seedling survival. Allocation to resprouting appears to enhance the ability of shrub seedlings to survive grass competition. We propose that grass competition across productivity gradients plays an important role in influencing landscape-level distribution patterns of woody resprouters.
Post-fire recovery of woody plants in the New England Tableland Bioregion
The resprouting response of plant species to fire is a key life history trait that has profound effects on post-fire population dynamics and community composition. This study documents the post-fire response (resprouting and maturation times) of woody species in six contrasting formations in the New England Tableland Bioregion of eastern Australia. Rainforest had the highest proportion of resprouting woody taxa and rocky outcrops had the lowest. Surprisingly, no significant difference in the median maturation length was found among habitats, but the communities varied in the range of maturation times. Within these communities, seedlings of species killed by fire, mature faster than seedlings of species that resprout. The slowest maturing species were those that have canopy held seed banks and were killed by fire, and these were used as indicator species to examine fire immaturity risk. Finally, we examine whether current fire management immaturity thresholds appear to be appropriate for these communities and find they need to be amended.
Fire severity and nutrient availability do not constrain resprouting in forest shrubs
Plants often survive disturbances such as fire by resprouting which involves having both protection traits and carbohydrate storage capacity. Protection traits not only act directly to insulate meristems but also prevent combustion of carbohydrate stores. Rapid stem growth also allows replenishment of carbohydrate stores ensuring persistence through another event. Resource availability may, however, constrain the ability to develop resilience to high-severity fires through either nutrient limitation or light limitation. We tested whether fire severity influenced resprouting ability of woody plants in two contrasting environments, low nutrient dry sclerophyll forest and more fertile wet sclerophyll forest. We tested which fire protection and growth traits were associated with resprouting ability (27 species) and resprouting vigour (16 species). Fire severity did not limit the ability of most species to resprout in either forest type. There was no generalized protection syndrome for surviving top kill, but combinations of bud protection and growth together with storage capacity appear to drive resprouting ability. In nutrient-limited forests, low specific leaf area (SLA) may reduce stem growth in resprouters, causing more reliance on bud protection through bark thickness. Conversely, in the more fertile forests, where light becomes limiting with time-since-fire, high SLA appears to increase the capacity for rapid stem growth with less emphasis on developing thicker bark. These different syndromes appear to be adaptive as fire severity did not influence survival in either forest type.
Mapping of Fire Severity and Comparison of Severity Indices Across Vegetation Types in Gibraltar Range National Park, Australia
The effect of fire on natural resources is termed "fire severity" and is related to the energy output of the fire. Recently the term "burn severity" has been introduced to identify the impacts of fire on soil and plants when the fire has been extinguished. This study addresses the assessment of a large wildfire in Gibraltar Range National Park, Australia, through remote sensing of fire severity and explores the spatial relationships between, fire severity and biophysical factors. Burn severity indices were developed from Landsat TM satellite images using pre-fire and post-fire images. Reflectance values computed from Landsat Enhanced Thematic Mapper (ETM) images acquired before and after the fire were used to estimate the Normalised Burn Ratio (NBR), which incorporates the near and mid infrared bands. Spatial distribution of ANBR data were calibrated with field observations and threshold values of burn severity were used to classify fire severity into 5 severity classes per vegetation type. ANBR values were extracted from different representative fire severities and spatial relationships were developed between ANBR and vegetation type, fuel type, fire danger index, time since fire, fire frequency, slope and rockiness in order to account for variables influencing fire severity patterns. General linear models and tests of significance were used to ascertain whether the effects of individual factors were statistically significant. The various models tested showed that no single factor (weather, fuel or landscape) accounted for the burn severity pattern. Fire weather and vegetation type Were found to be the key factors in the models.