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.
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.
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.
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.
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.
Fire regime (recency, interval and season) changes the composition of spinifex ('Triodia' spp.): dominated desert dunes
Between 2000 and 2002, central Australia experienced the largest fire season in three decades when ~500 000 km² burned. The effects of these and preceding wildfires in the 1980s on spinifex ('Triodia' spp.) sand-ridge plant communities were examined at 38 sites in central Australia. We used both multivariate and univariate techniques to assess floristic differences among sites of contrasting time-since-fire, fire season and fire interval. Time-since-fire had a consistent floristic influence across the landscape, with increased abundances of ephemeral grasses and forbs and 'Triodia' seedlings, and species richness soon after fire but decreasing long after fire. Fire season had little effect on most functional groups of plants, although seedlings of woody species were significantly more abundant following summer than winter fires. Likewise, recent short fire intervals appeared to have little impact on the population dynamics of most functional groups, although some transient effects were observed on abundances of ephemeral forbs, 'Triodia' seedlings and herbaceous clonal species. Long-term woody species abundances appeared to be affected by short fire intervals in the 1980s when repeated fires seemed to stimulate recruitment of some resprouting species. The present study highlighted the relative stability of spinifex vegetation types in the face of landscape-scale pyric perturbation, but emphasised that localised shifts in the composition and structure of the plant community may occur under certain fire regimes.
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.
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.
Plant trait-environmental linkages among contrasting landscapes and climate regimes in temperate eucalypt woodlands
Ecological sorting of species along climate and landscape gradients is a fundamental global pattern. However, the extent to which functional traits reflect floristic turnover in response to interactions between climate and landscape gradients is rarely assessed. We tested whether floristic variation among sites within a bioregion was more strongly correlated with soil fertility or climate. We then examined the relationship between floristic composition, environment and the co-variation of selected vegetative and regenerative functional traits. This allowed us to assess the ecological sorting of species along soil fertility and rainfall gradients and to detect any resource compensation effects via interactions between these factors. Floristic differences were equally associated with soil fertility and climate contrasts but species’ trait patterns were more strongly associated with soil fertility than rainfall. No interactive effects, which would suggest resource compensation, were detected. Instead, more fertile sites consistently had more forbs, annuals and grasses in comparison with less fertile sites which were dominated by woody species and had a higher abundance of graminoids. Three broad mechanisms for sorting of species based on trait patterns are proposed (1) differences in the fundamental regenerative and growth niche, (2) resource competition during establishment and (3) disturbance-mediated sorting.
Resprouting responses of 'Acacia' shrubs in the Western Desert of Australia: fire severity, interval and season influence survival
The hummock grasslands of arid Australia are fire-prone ecosystems in which the perennial woody plants mostly resprout after fire. The resprouting ability among these species is poorly understood in relation to environmental variation; consequently, little is known about the impacts that contemporary fire regimes are having on vegetation within these systems. We examined the resprouting ability of adults and juveniles of four widespread Acacia species ('A. aneura', 'A. kempeana', 'A. maitlandii', 'A. melleodora') by experimentally testing the effects of fire severity, interval and season. We found that fire severity and season strongly affected survival, but the magnitude of the effects was variable among the species. Unexpectedly, a short fire interval of 2 years did not have a strong negative effect on resprouting of any species. Fire severity had variable effects among the four species, with those species with more deeply buried buds being more resilient to high-severity soil heating than those with shallow buds. Season of fire also strongly affected survival of some species, and we propose that seasonal variation in soil heating and soil moisture mediated these effects. The species by environment interactions we observed within one functional group (resprouters with a soil-stored seed bank) and in one genus suggest that modelling landscape response to fire regimes will be complex in these arid ecosystems. We predict, however, that the dominant resprouting acacias in hummock grasslands of central Australia are highly resilient to a range of fire regimes.