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.
A new approach and case study for estimating extent and rates of habitat loss for ecological communities
The pace and magnitude of biodiversity loss has led to wide recognition that efforts to conserve individual species must be complemented by assessment and planning at community and ecosystem levels. Emerging protocols for assessing the conservation status of communities include as central criteria the current extent, historical reduction and contemporary rate of decline in geographic distribution. Estimation of these parameters is confronted by methodological challenges, data limitations and uncertainties that may vary from case to case. We describe an approach to these issues comprising five steps: (1) classification of the community using an analysis of data from systematic ground surveys; (2) mapping to produce a contemporary baseline distribution (1980s); (3) interpolation to produce a historical distribution (pre-European settlement); (4) interpretation of satellite imagery to update the distribution (various dates up to 2004) and (5) assessment of change in extent over historical and contemporary time scales incorporating plausible bounds of uncertainty around best estimates. The bounds can be based on areas for which image interpretation produces uncertain diagnosis of clearing and differences between credible alternative base maps of the same area. We demonstrated the approach using a case study of Coolibah–Black Box Woodland, a declining semi-arid woodland community in Australia and found that 61% (plausible range 50–67%) of the woodland community had been cleared since European settlement and that during 1998–2004 the community continued to decline on average by 135.3(±21.7) km² each year, or 1.7(±0.3)% of each previous year's distribution, apparently accelerating in recent years. Strengths of the approach include the use of biological data (cf. remote sensing) to distinguish the target assemblage from others, the use of historical and contemporary base lines to examine change over different time scales, and the use of bounded estimates to incorporate uncertainty into the assessment.
Vegetation, terrain and fire history shape the impact of extreme weather on fire severity and ecosystem response
Questions: Do endogenous (landscape/vegetation) or exogenous (weather) factors control fire severity? During severe fire weather, is there convergence in fire severity across rain forest, forests and heathlands such that all locations burn with similarly high severity? Are there long-term effects of fire severity in temperate crown-fire ecosystems? Location: Montane rain forests, eucalypt forests and heaths in the temperate climate zone of eastern Australia (Washpool/Gibraltar Range National Park). Methods: The immediate and longer-term effects of fire weather and landscape (terrain, previous fire history and vegetation type) factors on fire severity and ecosystem response were measured using remote sensing and ground measures of microclimate, productivity and plant resprouting at 45 sites. Results: Fire weather strongly interacted with terrain, antecedent fire history and vegetation type, resulting in complex mosaics of mixed fire severity rather than convergence to uniform fire severity. Vegetation type influenced the effects of time-since-fire and fire frequency on fire severity, suggesting differential fire feedbacks. High fire severity left a long-term imprint on total reflectance, ground temperatures and productivity of the vegetation, but these effects were not uniform across vegetation types. The abundance of resprouting species was not strongly affected by fire severity. Conclusions: There was evidence for strong weather control of fire severity but fire history, terrain and vegetation shape the immediate effect due to the contrasting pyrogenic vs pyrophobic nature of the vegetation mosaic. The short-term dominance of weather as a driver of fire severity is only weakly related to the longer-term ecosystem response because of the strong resprouting ability of the canopy dominants, even in rain forest. The forest complexes of eastern Australia appear highly resilient to high fire severity in both structure and floristics, which may influence long-term feedbacks.
Evidence for bark thickness as a fire-resistance trait from desert to savanna in fire-prone inland Australia
Although bark thickness and fire-activity are correlated in many ecosystems worldwide, substantial data-gaps remain, especially for dryland biomes, preventing generalisation of this relationship at the global scale. We examined bark thickness trends in trees and shrubs across a large-scale fire-rainfall gradient from desert to dry savanna in northern Australia. Bark thickness increased with local fire activity but was unrelated to mean annual rainfall or the mean normalised difference vegetation index (surrogates of resource-productivity). In high-fire regions, thin-barked trees were restricted to localised low-fire patches. Thicker bark was associated with sites characterised by flammable 'Triodia' hummock grassland (spinifex). Within this flora, bark thickness reflected a trade-off in trait allocation to fire resistance versus fire resilience. For trees, thicker bark (fire resistance) was strongly associated with epicormic resprouters. In contrast, fire-resilient species that were either basal resprouters or reseeders had thinner bark. With increasing aridity there was a shift in dominance from epicormic resprouters to thinner-barked shrub and mallee species that either basally resprout or are killed by fire. Pairwise congeneric species comparisons showed a consistent relationship of thicker bark under high fire activity. This same pattern also emerged from a multi-species comparison within the dominant tree genus 'Eucalyptus'. Overall, for this system, species with thick bark at the sapling stage dominate where fire is frequent. Thus, we confirm that fire can be a major driver of plant traits in fire-prone drylands.
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.
Fire, aridity and seed banks: What does seed bank composition reveal about community processes in fire-prone desert?
Questions: The relationship between fire, aridity and seed banks is poorly understood in plant community ecology. We tested whether there was a close correspondence between the seed bank and standing vegetation composition with time-since-fire in a desert. We also examined whether longer-lived species showed seed limitation relative to more ephemeral species, as this could influence grass-woody ratios in a major biome. Location: Dune hummock grasslands/shrublands of central Australia. Methods: The effects of time-since-fire on floristic and functional group composition were examined by comparing plots unburned since 1984 against plots that had been burned in 2002. Three methods were used to quantify seed abundances: a germination trial using heat and smoke application, a flotation method, and a sieving method. Results: Seed bank densities were very low (<3000m⁻²). Species similarity between the seed bank and standing vegetation was high at sites recently burned (0.86) and low in sites long-since burned (0.52). The relative abundance of ephemeral species in the seed bank peaked in recently burned plots, but the relative abundance of seeds of woody species did not match the pattern of abundance in the standing vegetation. Remarkably, the dominant perennial grasses and woody species were either absent from the seed bank or present at extremely low abundances. Discussion: Differences in the relative abundance of ephemeral species between standing vegetation and seed bank relate to the post-fire succession process. The small soil pool of seed from woody species may be explained by allocation to belowground carbohydrate storage over seed production. Field observations suggest, however, that production of strongly dormant seed can be prolific and that high levels of seed predation make this system strongly seed-limited. The discovery of this seed bank syndrome indicates that shifts in grass-woody ratios can be driven by the juxtaposition of unpredictable seed rain and fire events in these desert dunes. However, estimates of grass-woody ratios due to changing fire regimes will be difficult to predict.
Woody-grass ratios in a grassy arid system are limited by multi-causal interactions of abiotic constraint, competition and fire
Predicting changes in vegetation structure in fire-prone arid/semi-arid systems is fraught with uncertainty because the limiting factors to coexistence between grasses and woody plants are unknown. We investigated abiotic and biotic factors influencing boundaries and habitat membership in grassland ('Triodia' or 'spinifex' grassland)-shrubland ('Acacia aneura' or 'mulga' shrubland) mosaics in semi-arid central Australia. We used a field experiment to test for the effects of: (1) topographic relief (dune/swale habitat), (2) adult neighbour removal, and (3) soil type (sand/clay) on seedling survival in three shrub and two grass species in reciprocal field plantings. Our results showed that invasion of the shrubland (swale) by neighbouring grassland species is negated by abiotic limitations but competition limits shrubland invasion of the grassland (dune). All species from both habitats had significantly reduced survival in the grassland (dune) in the presence of the dominant grass ('Triodia') regardless of soil type or shade. Further, the removal of the dominant grass allowed the shrubland dominant ('A. aneura') to establish outside its usual range. Seedling growth and sexual maturation of the shrubland dominant ('A. aneura') was slow, implying that repeated fire creates an immaturity risk for this non sprouter in flammable grassland. By contrast, rapid growth and seed set in the grassland shrubs (facultative sprouters) provides a solution to fire exposure prior to reproductive onset. In terms of landscape dynamics, we argue that grass competition and fire effects are important constraints on shrubland patch expansion, but that their relative importance will vary spatially throughout the landscape because of spatial and temporal rainfall variability.
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.
Water relations of woody plants on contrasting soils during drought: does edaphic compensation account for dry rainforest distribution?
The occurrence of dry rainforest in climates considered drier than the recognised limit for rainforest has been explained by the edaphic compensation hypothesis, which proposed that finer-textured soils facilitate the occurrence of rainforest at climatic extremes.We Tested this by examining the effect of soil type on the water relations and plant traits of four dry rainforest species, during a severe drought and subsequent non-drought periods. We predicted plants growing in sandy soils would exhibit higher levels of water stress (lower predawn water potential and stomatal conductance) and possess morphological and physiological traits that more typically reflect drought resistance (late leaf fall in deciduous species, low specific leaf area, vertical leaf angles and stomata that close at low water potential) than those growing in loam soils. During drought, levels of water stress were similar across soil types, while post-drought plants on sandy soils were less stressed. Soil type did not cause shifts in drought tolerance traits, suggesting there has been no ecotypic differentiation of dry rainforest species across soil types for these traits. Hence, we found no support for the edaphic compensation hypothesis in adult plants; future studies should consider other life-cycle stages, such as seedlings.