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.
Soil temperature and depth of legume germination during early and late dry season fires in a tropical eucalypt savanna of north-eastern Australia
Temperatures that significantly increase seed germination of some tropical legumes (i.e. 80–100°C) were documented in the topsoil during the passage of early (May) and late (October) dry season fires in a tropical eucalypt savanna of north-eastern Australia. Elevated temperatures penetrated at least 30 mm into the soil during the higher-intensity, late dry season fires, but were only detected at 10 mm during the early dry season fires. The depth from which germination of two native legume forbs 'Galactia tenuiflora' and 'Indigofera hirsuta' occurred was positively related to the temperature elevation in the topsoil and was greater after late compared with early dry season fires. A broader range in germination depth, resulting in higher seedling densities, was recorded for 'I hirsuta' after late dry season fires. These results suggest that seedling emergence of native leguminous forbs is likely to occur at a greater density after late rather than early dry season fires in tropical eucalypt savannas of north-eastern Australia. Therefore, the season of burning, as a result of its relationship to fire intensity, can influence species composition through its effect on seed germination.
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.
Response of Montane Wet Sclerophyll Forest Understorey Species to Fire: Evidence from High and Low Intensity Fires
On the New England Tablelands wet sclerophyll forests typically form the ecotone between rainforest and dry sclerophyll forest. Currently there are few data on the response of wet sclerophyll plant species to fire. We compared the fire-response traits of woody understorey and sub-canopy species in wet sclerophyll forest after high and low intensity fires. The majority of species (>80%) resprouted after fire and the prevalence of resprouting did not differ with fire intensity. Obligate seeders were rare in these communities (<10% of species), and similar numbers of rainforest and sclerophyllous species were killed by fire. Resprouting from basal stems and root suckering were the most common mechanisms of vegetative regeneration; however, these traits may have arisen more in response to canopy disturbance than fire regime. We found that most rainforest taxa resprouted but lacked post-fire seedling recruitment, whereas most resprouting sclerophyllous taxa recruited from seed after fire. This dichotomy in seedling recruitment could reflect the productivity and disturbance gradients across the ecotone. We propose that gap-phase recruitment is favoured towards the rainforest margin and fire-related recruitment is more prevalent at the eucalypt forest edge.
Perennial grassland dynamics on fertile plains: Is coexistence mediated by disturbance?
The response of grasslands to disturbance varies with the nature of the disturbance and the productivity of the landscape. In highly productive grasslands, competitive exclusion often results in decreased species richness and grazing may allow more species to coexist. Once widespread, grasslands dominated by 'Dichanthium sericeum' (Queensland bluegrass) and 'Astrebla spp'. (Mitchell grass) occur on fertile plains but have been reduced in extent by cultivation. We tested the effects of exclusion of livestock grazing on these grasslands by comparing the floristic composition of sites in a nature reserve with an adjacent stock reserve. In addition, sites that had been cultivated within the nature reserve were compared with those where grazing but no cultivation had occurred. To partition the effects of temporal variation from spatial variation we sampled sites in three different years (1998, 2002 and 2004). Some 194 taxa were recorded at the nature reserve and surrounding stock routes. Sampling time, the occurrence of past cultivation and livestock grazing all influenced species composition. Species richness varied greatly between sampling periods relating to highly variable rainfall and water availability on heavy clay soils. Native species richness was significantly lower at previously cultivated sites (13–22 years after cultivation), but was not significantly influenced by grazing exclusion. After 8 years it appears that reintroducing disturbance in the form of livestock grazing is not necessary to maintain plant species richness in the reserve. The highly variable climate (e.g. droughts) probably plays an important role in the coexistence of species by negating competitive exclusion and allowing interstitial species to persist.
Fire-related cues break seed dormancy of six legumes of tropical eucalypt savannas in north-eastern Australia
This paper describes an assessment of the effect of exposure to fire-related cues (heat shock, smoke and nitrate) and the interactions between the cues on seed dormancy release of tropical savanna legumes in north-eastern Australia. Ten legume species were tested, comprising both native and exotic species. The ten species responded variously to the treatments. Brief exposure to temperatures between 80 and 100°C was found to break the seed dormancy of the native ephemeral herbs 'Chamaecrista mimosoides', 'Crotalaria calycina', 'Crotalaria montana', 'Indigofera hirsuta' and 'Tephrosia juncea', as well as the exotic ephemeral herb 'Crotalaria lanceolata'. Exposure to 80°C combined with treatment with a nitrate solution produced an additive effect on the germination of 'Chamaecrista mimosoides' and 'Crotalaria lanceolata'. However, the four species with the heaviest seeds, two exotic ephemeral herbs ('Chamaecrista absus' and 'Crotalaria pallida') and two native perennials ('Galactia tenuiflora' and 'Glycine tomentella') displayed no significant increase in germination with exposure to fire-related cues. Exposure to 120°C for 5 min produced seed mortality in all species tested. Two of the largest seeded species, 'Crotalaria pallida' and 'Galactia tenuiflora', displayed the lowest tolerance to heat shock, with seed mortality after exposure to 100°C for 5 min. These data indicate that fire can promote the germination of some tropical savanna legumes. As a proportion of seeds of each species displayed no innate dormancy, some germination may occur in the absence of fire, especially of exotic species.
Seed traits and seed bank longevity of wet sclerophyll forest shrubs
In wet sclerophyll forests seedling recruitment either occurs after intermittent fire events or continuously during intervals between fires in gaps created by small-scale disturbances. The dormancy and dispersal characteristics of seeds will influence how plant species exploit these contrasting recruitment opportunities. For example, long-lived seed banks may be crucial for persistence of species that are unable to recruit during intervals between fires if the length of fire intervals exceeds the life span of standing plants (senescence risk). To better understand mechanisms of population persistence during prolonged absence of fire in montane wet sclerophyll forests, we studied seed bank dynamics in four understorey species. We chose two species thought to have fire event-driven recruitment, 'Banksia integrifolia' subsp. monticola (Proteaceae) and 'Goodia lotifolia' (Fabaceae), and two species that are thought to have canopy gap-phase recruitment, 'Trochocarpa laurina' (Ericaceae) and 'Tasmannia stipitata' (Winteraceae). We measured seed rain, seed bank density and used seeds buried in nylon mesh bags to estimate rates of seed decay in the soil over time. All species produced a substantial seed crop on an annual basis. The annual seed crop in three species ('G. lotifolia', 'T. stipitata' and 'T. laurina') was released in a dormant state and developed a persistent seed bank, while one species ('B. integrifolia') lacked dormancy and rapidly germinated under laboratory and field conditions. Seed bank characteristics of 'G. lotifolia' appear to promote episodic recruitment after large landscape-scale fires, those of 'B. integrifolia' appear to promote more continuous recruitment in response to smaller fires and other disturbances that avoid widespread mortality of established plants, while seed bank characteristics of 'T. stipitata' and 'T. laurina' may facilitate both episodic and continuous recruitment under respective types of disturbance. The four species appeared to have varied vulnerabilities and mechanisms for reducing immaturity risk and senescence risk to persistence of their populations under recurrent disturbance. Dormancy, seed bank longevity and seed rain are likely to be useful syndromes for predicting the response of wet sclerophyll forest understorey species to changed disturbance regimes.
Dense regeneration of floodplain 'Eucalyptus coolabah': invasive scrub or passive restoration of an endangered woodland community?
Clearing of native vegetation and changes to disturbance regimes have resulted in dense regeneration of native trees and shrubs in parts of Australia. The conversion of open vegetation to dense woodlands may result in changes to the composition of plant communities and ecosystem function if structure, composition and function are tightly linked. Widespread clearing of the floodplain tree 'Eucalyptus coolabah' subsp. 'coolabah' (coolibah), in New South Wales, Australia, has led to state and federal listings of coolibah woodland as an endangered ecological community. Dense regeneration of coolibah in the mid 1970s, however, also resulted in its listing as an 'invasive native species' in NSW, meaning it can be legally cleared under certain conditions. Dense regeneration could be a novel state dissimilar to the threatened community or it could represent the next generation of coolibah woodlands and may contribute to passive restoration of heavily cleared landscapes. This study investigated if dense stands are distinct from remnant woodland by comparing floristic composition of the ground-storey community and top-soil properties of four coolibah vegetation states: derived grassland, derived degraded grassland, dense regeneration and remnant woodland. Ground-storey composition was found to overlap broadly among states regardless of tree density. Most species were common to all states, although dense regeneration contained characteristic woodland species that were absent from grasslands. The carbon : nitrogen ratio of the soil was significantly higher in dense regeneration and remnant woodland than in either of the grassland states, indicating that the woody states are broadly similar in terms of nutrient cycling. The study demonstrates that structurally different vegetation states (grasslands, woodlands and dense regeneration) are not associated with distinct plant communities. The results also suggest that grazing management has a more pronounced effect on ground-storey composition of plant communities than tree density and that well managed derived grasslands and dense regeneration are floristically similar to remnant woodlands. Since dense regeneration and remnant woodlands are not floristically distinct from one another, dense regeneration could contribute to the conservation of endangered coolibah woodlands in cleared agricultural landscapes.
How do drought and fire influence the patterns of resprouting in Australian deserts?
Rainfall is the key driver of woody cover and life-history attributes in arid grassy biomes where disturbance is mostly rare and of low intensity. However, relatively little is known about the causes of woody community assembly in arid systems that are subject to periodic intense fire disturbance. In the central Australian desert region, grassland and shrubland fire can occur following above average rainfall. Patterns of species regeneration response (resprouting vs. reseeding) are poorly documented in this region. We tested the effects of rainfall and fire on species' resprouting response across the latitudinal rainfall-fire gradient using constrained ordination of 385 sites and general linear models. A resprouting response was significantly greater in grassland habitat as well as at the high end of the rainfall-fire gradient. The frequency of epicormic stem resprouting also increased along the rainfall-fire gradient. We attribute this pattern to the combined effects of frequent fire and rapid gap closure on seedlings of slow-growing, fire-killed woody species in higher rainfall grasslands. In addition, we also demonstrated that rapidly maturing fire-recruiting species are similarly favoured by high fire disturbance. In arid grassy ecosystems, unlike in mesic savanna, flammable grassland supports a mix of resprouting and recruitment functional types, and habitat membership cannot be predicted by resprouting capacity. Regions, such as central Australia, that are characterised by grassland-shrubland mosaics of high and low fuel biomass, respectively, pose specific challenges to fire ecology research that are possibly best dealt with by focussing modelling at the habitat scale.
Resprouting as a key functional trait in woody plants: challenges to developing new organizing principles
Most global biomes are shaped by disturbances, such as fire or herbivory, that damage or kill the aboveground biomass of plants. In many of these biomes, however, the damaged plants do not die; rather, they persist through sprouting. In disturbance-prone environments, resprouting from meristems stabilizes plant populations where disturbance may cause demographic bottlenecks. The advantage of resprouting is that it confers persistence under disturbance; however, sprouting has disadvantages such as potentially reducing sexual reproduction and limiting gene flow (Bond & Midgley, 2001; Lamont & Wiens, 2003). Understanding resprouting is critically important for understanding long-term vegetation dynamics, extinction risks, carbon balance and woody plant management. A Vegetation Function Working Group (Working Group 67; http://www.vegfunction.net/wg/67/67_Sprouting.htm) was established in 2009 to identify challenges encountered in developing coherent models of the functional role of resprouting in fire-prone environments, with an emphasis on savanna and Mediterranean biomes. Fire is the most pervasive disturbance and is integral to the evolutionary ecology of these biomes. This broad disciplinary group met at the International Ecological Conference (INTECOL) in Brisbane in September 2009 and reconvened in July 2010 at The University of New England, Armidale, Australia, to review progress. In this report we highlight major challenges encountered in developing unifying models of the functional role of resprouting.