Peter J. Clarke

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.

Germination and dormancy of grassy woodland and forest species: effects of smoke, heat, darkness and cold

The germination requirements of a broad spectrum of common species found in grassy woodlands and forests in the New England region of northern New South Wales were tested in a series of replicated growth-cabinet experiments. The effects of dark/diurnal light and smoke/no smoke were measured on 65 species in an orthogonal experiment, 21 of which were retested after storage for 12 months. The effect of storage for several years was also assessed under a diurnal light regime. In addition, the effects of preimbibition heat (80°C), and chilling on germina- tion were also measured. A single temperature regime (15°C night/25°C day) was used in all treatments for compar- ative purposes. Most species had high viability and germinability under a diurnal light regime. Small shrub species included, however, a large proportion of species with entrenched dormancy. Light enhanced germination of 21 species significantly, whereas dark stimulated germination of only eight species. Heat and cold treatments also stim- ulated some germination but more often inhibited germination or produced no effect. Smoke stimulated germination, relative to other cues, in only one species (Ajuga australis) and more often inhibited germination or produced no effect. The relationship between variation in germination (stimulation, no effect or inhibition) of species in six growth-form classes was tested by using contingency tables for each treatment. No significant relationship between growth form and the effects of light, smoke or chilling was detected. Preimbibition heat effects were, however, sig- nificantly different among growth forms. Subshrubs showed a higher than expected proportion of species with a heat stimuli while herbaceous species showed a higher than expected proportion of species inhibited by preimbibition heat. Germinability generally increased in herbaceous species when stored at ambient temperatures while it remained relatively constant in woody species. Conversely, viability decreased in herbaceous species but remained relatively more constant in woody species. The effects of seed storage and high germinability suggest that most perennial herbaceous species have transient or short-term persistent seed banks. Germinability, with and without cues, was also negatively correlated with increasing seed size and larger growth forms. These traits might be related to the need for woody species, with soil-stored seed banks, to spread establishment risks in an environment where herbaceous competition and herbivory are likely to be important selection factors.

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.

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.

Do Forest Gaps Influence the Population Structure and Species Composition of Mangrove Stands in Northern Australia?1

Abstract Gap-phase regeneration of tropical mangroves was studied to test if there were significant environmental, structural, and floristic differences between canopy gaps and adjacent forests with intact canopies. Twenty-five canopy gaps in the low to mid intertidal zone were sampled at two study areas in northern Australia. Canopy gaps at the more equatorial study area were larger and had a higher surrounding canopy. Soil analyses showed that sediment texture exerts strong control on conductivity, pH, organic matter, total P, total N and total S. Some soil factors differed between gaps and intact forest, and PAR was significantly higher in gaps. Despite this, propagule and young seedling abundances did not differ between gaps and adjacent forests due to the dominance of viviparous species. Enhanced recruitment of seedlings to the sapling stage was detected in many canopy gaps, although saplings were also present under mangrove forest canopies. Nevertheless, the overall floristic composition, species richness, and species rank abundance were similar between canopy gaps and forests with intact canopies. There appears to be little evidence for specialized gap colonists in tropical mangrove forest as distinct from other tropical forests. Instead, gap-phase regeneration of mangroves appears to follow a simple “direct replacement” model in which a species population is replaced by members of the same species in forest gaps. The scope for gap-dependent specialization in mangroves via seed dormancy and fast growth rates, common in rain forest pioneers, may be constrained by the intertidal environment in which mangroves occur.

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.

Habitat insularity and fire response traits: evidence from a sclerophyll archipelago

Abstract. Rock outcrops are landscape features that may form habitat islands in a matrix of more widespread vegetation. The patterns of floristics, reproduction, gender, life span, growth forms, and fire response traits were compared between rock outcrops and matrix sclerophyll vegetation to test for insularity in taxon composition and functional traits. The outcrops and matrix had similar reproduction, gender, life span, and growth form traits, being dominated by co-sexual sclerophyll shrubs. The outcrops, however, were dissimilar in species composition and functional traits forming an archipelago of habitat islands in a forest matrix. Rank abundance curves were less even on rock outcrops than in adjacent forests, being dominated by shrubs that were killed by fire (obligate seeders). The ratio of shrubs killed by fire (obligate seeders) to resprouters was 70:30 on the outcrops compared with 38:62 in the matrix. Evidence for functional convergence in fire response traits comes from 27 genera, in 17 families, which have congeners in each habitat. Most shrub congeners on or near rocky outcrops were killed by fire whereas related taxa in the forests resprout after fire. Functional convergence can be related to disturbance frequency and/or differences in regeneration niche among habitats. A resprouting response appears to be related to more frequent fires in the matrix as outcrops experience fires less often. The dominance of obligate seeding shrubs on high rainfall outcrops may also be related to better resources in an environment where allocation to growth rather than storage could be advantageous. In drier and shadier habitats, however, resprouting may be promoted over seedling recruitment as the risks of recruitment failure are higher.

Composition of grazed and cleared temperate grassy woodlands in eastern Australia: patterns in space and inferences in time

Abstract A regional vegetation survey of the temperate grassy woodlands (temperate savanna) in Australia was designed to assess the effects of clearing and grazing on the composition of vegetation remnants and the adjacent pasture matrix. Vegetation was sampled across a range of habitats using 77 0.1024-ha quadrats; the relative abundance of species was recorded. Classification analysis clustered the sites into three main groups that corresponded to intensity of grazing/clearing followed by groups based on underlying lithology (basalt, metasediment, granites). Using Canonical Correspondence Analysis, exogenous disturbance and environmental variables were related to the relative abundance of species; grazing intensity had the highest eigenvalue (0.27) followed by tree canopy cover (0.25), lithology (0.18), altitude (0.17) and slope (0.10). Based on two-dimensional ordination scores, six species response groups were defined relating to intensity of pastoralism and nutrient status of the landscape. Abundance and dominance of native shrubs, sub-shrubs, twiners and geophytes were strongly associated with areas of less-intense pastoralism on low-nutrient soils. The strongest effects on species richness were grazing followed by canopy cover. Continuously grazed sites had lower native species richness across all growth forms except native grasses. There was no indication that intermediate grazing intensities enhanced forb richness as a result of competitive release. Species richness for all native plants was lowest where trees were absent especially under grazed conditions. Canopy cover in ungrazed sites appeared to promote the co-existence of shrubs with the herbaceous layer. Predicted declines in forb richness in treeless, ungrazed, sites were not detected. The lack of a disturbance-mediated enhancement of the herbaceous layer was attributed to habitat heterogeneity at 0.1 ha sampling scale. Nomenclature: Harden 1990–1993.

A synthesis of postfire recovery traits of woody plants in Australian ecosystems

Postfire resprouting and recruitment from seed are key plant life-history traits that influence population dynamics, community composition and ecosystem function. Species can have one or both of these mechanisms. They confer resilience, which may determine community composition through differential species persistence after fire. To predict ecosystem level responses to changes in climate and fire conditions, we examined the proportions of these plant fire-adaptive traits among woody growth forms of 2880 taxa, in eight fire-prone ecosystems comprising ~87% of Australias land area. Shrubs comprised 64% of the taxa. More tree (>84%) than shrub (~50%) taxa resprouted. Basal, epicormic and apical resprouting occurred in 71%, 22% and 3% of the taxa, respectively. Most rainforest taxa (91%) were basal resprouters. Many trees (59%) in frequently-burnt eucalypt forest and savanna resprouted epicormically. Although crown fire killed many mallee (62%) and heathland (48%) taxa, fire-cued seeding was common in these systems. Postfire seeding was uncommon in rainforest and in arid Acacia communities that burnt infrequently at low intensity. Resprouting was positively associated with ecosystem productivity, but resprouting type (e.g. basal or epicormic) was associated with local scale fire activity, especially fire frequency. Although rainforest trees can resprout they cannot recruit after intense fires and may decline under future fires. Semi-arid Acacia communities would be susceptible to increasing fire frequencies because they contain few postfire seeders. Ecosystems dominated by obligate seeders (mallee, heath) are also susceptible because predicted shorter inter-fire intervals will prevent seed bank accumulation. Savanna may be resilient to future fires because of the adaptive advantage of epicormic resprouting among the eucalypts. The substantial non-resprouting shrub component of shrublands may decline, but resilient Eucalyptus spp. will continue to dominate under future fire regimes. These patterns of resprouting and postfire seeding provide new insights to ecosystem assembly, resilience and vulnerability to changing fire regimes on this fire-prone continent.

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.