Survival of serotinous seedbanks during bushfires: Comparative studies of Hakea species from southeastern Australia

Abstract The study assessed the survival, during bushfires, of seedbanks of six serotinous Hakea species found in the Sydney region of southeastern Australia. The survival of seeds was examined when fruits were heated in a muffle furnace at ambient temperatures ranging from 200–800°C for 1 min. For each species, fruit weight and dimensions of fruit walls were measured to characterize insulation. A field experiment was performed to examine the survival of the serotinous seedbank of Hakea dactyloides in a bushfire. Ambient and internal fruit temperatures were recorded during the fire. The viability of seeds from fruits exposed to the fire was tested and compared with an unburnt sample. Viability of seeds within fruits exposed in the furnace varied according to species. Seeds of large fruited species such as Hakea constablei and Hakea propinqua survived, whereas those of the small fruited species Hakea teretifolia and H. dactyloides suffered significant mortality. The threshold temperature for death in four species was linearly related to the thickness of lower and lateral fruit walls, and to dry weight of fruits. Internal and external temperatures of fruits decreased with increasing height on experimental H. dactyloides plants in the field. High levels of mortality (relative to the unburnt control) corresponded with fire temperature maxima greater than 400°C (external) and greater than 60°C (internal). In general, these temperatures occur when shrub crowns burn. A high risk of death for H. dactyloides, H. teretifolia and H. sericea seeds will result because fruits of these species have thin walls.     

Population structure and reproductive status of two Banksia shrubs at various times after fire

Populations of the obligate-seeder, Banksia ericifolia, were even-aged. Seedling recruitment occurred only after fire. Mean genet size (height + canopy diameter; H+D) increased progressively with elapsed time since fire in stands last burnt 2–23 years before 1981. Populations of a co-occurring resprouter, B. oblongifolia, were mixed-aged. Genet size varied significantly between stands, but this variation was not explained by regressions of H+D on years since fire. In addition B. oblongifolia seedlings were recruited both after fire and in patches of heath unburnt for 16 years.Most flower and seed production in B. oblongifolia occurred in the stands last burnt less than 10 years previously. More than 30% of genets had not produced cones since the last fire, irrespective of how many years had elapsed. In contrast, few B. ericifolia genets had produced cones five years after fire, but by 16 years after fire nearly 100% had. Overall, about 51% of B. ericifolia inflorescences and about 28% of B. oblongifolia inflorescences set seed. The number of seeds in seed-bearing cones was not significantly different between species.Resprouting B. oblongifolia genets began flowering sooner after fire, but B. ericifolia subsequently overtook them in accumulating a bank of serotinous seeds. In the stand unburnt for 23 years the largest B. ericifolia genets had more than twice as many cones as the largest co-occurring B. oblongifolia. However, when accumulated cone production was compared for genets of equal H+D over all stands, there was no difference between species.We thank the New South Wales National Parks & Wildlife Service for permission to do this work in Ku-ring-gai Chase National Park. We are grateful to Don Adamson, Lynn Day, David Haig and James Sim for constructive comments on earlier drafts.     

Post‐fire seed predation: Does distance to unburnt vegetation matter?

Human‐induced changes to fire regimes result in smaller, more patchy fires in many peri‐urban areas, with a concomitant increase in potential edge effects. In sclerophyll vegetation, many structurally dominant serotinous plants rely on the immediate post‐fire environment for recruitment. However, there is little information about how fire attributes affect seed predation or recruitment for these species. We examined the influence of distance to unburnt vegetation on post‐dispersal seed predation for five serotinous species from sclerophyll vegetation in the Sydney region, south‐eastern Australia; Banksia serrata L.f., Banksia spinulosa Sm. var. spinulosa, Hakea gibbosa (Sm.) Cav., Hakea teretifolia (Salisb.) Britten (all Proteaceae) and Allocasuarina distyla (Vent.) L. Johnson (Casuarinaceae). We used cafeteria trials and differential exclusion of vertebrates and invertebrates to test whether rates of seed removal for these five species differed among (i) unburnt, (ii) burnt‐edge (approx. 10 m from unburnt vegetation) and (iii) burnt‐interior (approx. 100 m from unburnt vegetation) locations. When all animals had access to seeds, seeds were removed at lower rates from burnt‐interior areas than from other locations. Vertebrates (small mammals) showed this pattern markedly the first time the experiment was run, but in a repeat trial this effect disappeared. Rate of seed removal by invertebrates differed among plant species but we did not detect any such differences for removal by vertebrates. Overall rates of seed removal also differed significantly between the two fires studied. Our results indicate that small mammal seed predation can be substantial for large‐seeded serotinous shrubs, and that differences in the perimeter: area ratio, severity or size of a fire are likely to affect seed predation.     

Persistence of resprouting species after fire in natural and post‐mine restored shrublands in southwestern Australia

Questions: Is post‐fire persistence of resprouting species lower in restored sites, and is survival related to lignotuber size? Location: Southwestern Australia, Eneabba, 300 km north of Perth. Methods: Post‐fire persistence of 10 lignotuberous shrub species was compared between three sites restored 8–24 years ago after mineral‐sand mining and three surrounding natural shrubland sites (8–24 years since previous fire). Results: Overall persistence of species was 11–93% in restored sites (mean 52%) and 79–100% in natural sites (mean 96%). Persistence increased with time since rehabilitation for five species with <25% of individuals in three species surviving in the youngest stand. For equivalent crown size, average lignotuber circumferences were 50% smaller at restored sites and this probably accounted for their higher post‐fire mortality. Apart from differences in the age of plants, restored sites had lower soil penetrability than natural sites, which may have restricted rootstock development. A tradeoff favoring a higher crown volume to lignotuber size ratio was apparent in nine of the ten species with greater crown volumes (by 37%) and smaller lignotubers (by 36%) in restored sites. Two resprouting species for which crown seed store was quantified had much higher fecundity in restored sites. Conclusions: Fires reduced resprouter persistence in restored sites owing to poor development/insufficient size of lignotubers. Further management after fires is required, including application of resprouter seeds/seedlings on restored topsoil, transplanting adult resprouters (where viable) from natural areas ahead of the mining front. Low intensity/patchy fires are recommended on long unburnt sites. Resprouter survival would have likely been much greater in the first place if a deeper sandy soil profile was rehabilitated, thereby providing a more suitable medium for lignotuber development.     

Mortality of resprouting chaparral shrubs after a fire and during a record drought: physiological mechanisms and demographic consequences

We examined postfire regeneration of chaparral shrubs during an intense drought. This study focused on the demography and physiology of shrub species that resprout from a basal lignotuber following fire. We found significant levels of resprout mortality when intense drought occurred in the year following fire during the period of shrub recovery. Three of the seven sampled resprouting species had the greatest or near greatest levels of mortality ever recorded when compared to previous studies. Most shrub mortality occurred during the drought after individuals had resprouted (i.e. individuals survived fire, resprouted and then subsequently died). Physiological measurements of species with high mortality suggested that resprout stems were highly embolized and xylem hydraulic conductivities were close to zero during the peak of the drought. In addition, lignotubers of two of the three species experiencing high mortality were depleted of starch. Population densities of most shrub species declined after the drought compared with their prefire levels, with the exception of one drought tolerant obligate seeding species. Resprouting shrub species may deplete their carbohydrate reserves during the resprouting process, making them particularly vulnerable to drought because of the need to transpire water to acquire the CO₂ that is used to supply energy to a large respiring root system. Drought appears to interact with fire by altering postfire shrub recovery and altering species abundances and composition of chaparral communities.     

Non‐fire induced seed release ina weakly serotinous pine: climatic factors,maintenance costs or both?

The advantages of canopy seed retention (serotiny) for plants inhabiting fire‐prone ecosystems are well documented. However, very few species are completely serotinous and non‐fire induced opening of serotinous fruits is commonly observed (weak serotiny). Two non‐mutually exclusive causes are envisaged to contribute to this process: mechanical changes in serotinous fruits mediated by climatic conditions (e.g. drought) or the costs of maintenance for the plant of these long‐lasting structures. However, their relative contribution to the spontaneous opening of serotinous fruits remains elusive as well as the consequences for the build‐up of the canopy seed bank and inter‐individual differences in serotiny. In this study we monitored the dynamics of cone production and cone opening in the weakly serotinous Pinus halepensis for five years (2004–2008), including two severe drought episodes (2005, 2006). Drought decreased the production of conelets, increased the abortion of immature cones, reduced the seed quality in the cohorts of cones produced during these years, and increased the opening of serotinous cones. During the first drought episode, a higher proportion of serotinous cones opened in those pines bearing a larger crop of younger cones. This suggests that not only passive changes induced by drought but also competition among cones for resources (e.g. water) might be involved in this process. The opening of serotinous cones in pines bearing more cones made inter‐individual differences in the size of the canopy cone bank to narrow or even to reverse from 2004 to 2008. These results may help to understand the decrease in serotiny when pines grow and accumulate more cones and the large inter‐individual variability in the degree of serotiny observed in P. halepensis forests. In addition, the negative effects of drought episodes for the size of the canopy cone bank and the seeds contained can be an unexplored cause of post‐fire regeneration constraint.     

Patch to landscape patterns in post fire recruitment of a serotinous conifer

Obligate seeding species are highly specialized to fire disturbance and many conifers such as cypress, which are adapted to high intensity stand-replacing fires, have canopy seed banks stored in serotinous cones. Resilience of these trees to fire disturbance is a function of disturbance frequency and one focus of this study was to determine the effect of patch age on postfire recruitment. A second focus was to determine the extent to which fire induced a landscape level change in the location of the forest boundary. Prior to a fire in 1994, a large Cupressus sargentii forest was a mosaic landscape of different aged patches of nearly pure cypress bordered by chaparral. Patches less than 60 years of age were relatively dense with roughly one tree every 1–2 m² but older patches had thinned to one tree every 3–15 m². Older trees had substantially greater canopy cone crops but the stand level seed bank size was not significantly correlated with stand age. Fire-dependent obligate seeding species are sensitive to fire return interval because of potential changes in the size of seed banks – facing both a potential `immaturity risk' and a `senescence risk'. At our site, C. sargentii regeneration was substantial in stands as young as 20 years, suggesting that fire return interval would need to be shorter than this to pose any significant risk. Reduced seedling recruitment in stands nearly 100 years of age may indicate risk from senescence is greater, however, even the lowest density seedling recruitment was many times greater than the density of mature forests – thus this cypress would appear to be resilient to a wide range of fire return intervals. Changes in landscape patterning of forest and chaparral are unlikely except after fire. Factors that inhibit tree establishment within the shrubland, as well as factors that affect shrub establishment within the forest border likely affect the `permeability' of this ecotone. After the 1994 fire this boundary appeared to be stable in that cypress recruited best within the shadow of burned canopies and cypress were weak invaders of adjacent shrublands.     

The direct regeneration hypothesis in northern forests

Question: Can the direct regeneration hypothesis (DRH) be used to predict post‐disturbance regeneration after fire, wind disturbance, and clearcutting in northern forests? Do life‐history traits such as regeneration strategy and shade tolerance influence post‐disturbance regeneration success of tree species? Location: Northern forests in North America. Methods: A meta‐analysis was conducted by collecting published data on pre‐ and post‐disturbance stand compositional characteristics in the northern forests. For each tree species, compositional difference (CD) was calculated as the difference between basal area proportions of the post‐ and pre‐disturbance stands, but for post‐disturbance stands <25 years of age, post‐disturbance proportions were calculated based on relative stem density. Results: Species response to disturbances was best explained by regeneration strategy, while disturbance type had no effect on CD. The proportion of broadleaf trees with either strong or weak vegetative reproduction ability increased after all disturbances. Serotinous species had CD values not significantly different from zero after fire, while CD for semi‐serotinous species was negative. The post‐disturbance proportions of non‐serotinous conifers decreased after all forms of disturbance. Conclusions: All disturbances promote broadleaf trees, regardless of regeneration strategy (suckering, sprouting, or seeding). The DRH is supported for conifers with serotinous cones after fire. Fire causes local extinction of non‐serotinous conifers, while wind and clearcutting only decrease the proportion of non‐serotinous conifers because of partial survival of seed sources and advanced regeneration. This study suggests that increasing stand‐replacing disturbances associated with global climate change will promote broadleaf trees in northern forests.     

Seed bank patterns in Restionaceae and Epacridaceae after wildfire in kwongan in southwestern Australia

Abstract. Post-fire seed germination, seedling mortality and seed banks were investigated in scrub-heath (kwongan) in SW Australia. Study species included herbaceous and woody, obligate seeders and resprouters in two non-bradysporous but significant plant families (Restionaceae and Epacridaceae). In all species, seedlings were recruited only in the first autumn-spring after fire and occurred in similar densities as the estimated germinable annual seed input. Seedlings were absent from unburnt vegetation. Although most species retained some residual seeds after fire, tests (excised embryo culture) indicated that a negligible number of seeds were germinable. Regardless of fire response or species, there appeared to be a large loss of seeds each year and in most cases, only a small proportion of the annual seed production was used in post-fire recovery of plants. Based on seedling: parent ratios, all species had the capacity to reconstitute parent densities from germinants in the first year after fire, but high seedling mortality and no further recruitment resulted in less seedlings than replacements for four resprouter Restionaceae and three Epacridaceae (all obligate seeders) at the end of the third year after fire.     

What predicts the richness of seeder and resprouter species in fire‐prone Cape fynbos: Rainfall reliability or vegetation density?

In ecosystems subject to regular canopy fires, woody species have evolved two general strategies of post‐fire regeneration. Seeder species are killed by fire and populations regenerate solely by post‐fire recruitment from a seed bank. Resprouter species survive fire and regenerate by vegetative regrowth from protected organs. Interestingly, the abundance of these strategies varies along environmental gradients and across regions. Two main hypotheses have been proposed to explain this spatial variation: the gap dependence and the environmental‐variability hypotheses. The gap‐dependence model predicts that seeders are favoured in sparse vegetation (vegetation gaps allowing effective post‐fire recruitment of seedlings), while resprouters are favoured in densely vegetated sites (seedlings being outcompeted by the rapid crown regrowth of resprouters). The environmental‐variability model predicts that seeders would prevail in reliable rainfall areas, whereas resprouters would be favoured in areas under highly variable rainfall that are prone to severe dry events (leading to high post‐fire seedling mortality). We tested these two models using distribution data, captured at the scale of quarter‐degree cells, for seeder and resprouter species of two speciose shrub genera (Aspalathus and Erica) common in fire‐prone fynbos ecosystems of the mediterranean‐climate part of the Cape Floristic Region. Contrary to the predictions of the gap‐dependence model, species number of both resprouters and seeders increased with values of the Normalized Difference Vegetation Index (a widely used surrogate for vegetation density), with a more marked increase for seeders. The predictions of the environmental‐variability hypothesis, by contrast, were not refuted by this study. Seeder and resprouter species of both genera showed highest richness in environments with high rainfall reliability. However, with decreasing reliability, seeder numbers dropped more quickly than those of resprouters. We conclude that the environmental‐variability model is better able to explain the abundance of woody seeder and resprouter species in Southern Hemisphere fire‐prone shrublands (fynbos and kwongan) than the gap‐dependence model.     

Biogeography of seeder and resprouter Erica species in the Cape Floristic Region—Where are the resprouters?

The genus Erica L., with more than 600 species, and a high number of endemics, represents the most remarkable example of floristic diversity in the Cape Floristic Region (CFR). It is largely confined to nutrient-poor, acidic, sandy soils, being one of the most characteristic element of fynbos. The ability to survive fires, resprouting from a lignotuber, is a common trait among Euro-mediterranean Erica species. In contrast, resprouting is fairly uncommon among ericas in the CFR (less than 10%). Most of them are killed by fire, regenerating only but readily by seed germination. An extensive survey on the resprouting ability of South African Erica species was carried out and the pattern of geographical distribution of resprouters and seeders in the CFR was determined. The geographical distribution of these two regeneration classes was related to a climatic gradient of seasonality along the CFR. A pattern of higher proportions of resprouter species towards the mediterranean, strongly seasonal northwestern CFR and the non-seasonal eastern CFR and summer rainfall area outside the CFR was identified. The number of resprouter species reaches a maximum in the eastern CFR and is lower in the southwestern CFR despite the overall higher concentration of species in this subregion. Summer drought strongly influences the effectiveness of post-fire regeneration and growth (i.e. new recruits plus survivors) of Erica species, and is the major selective force accounting for the pattern of distribution of seeders and resprouters in the CFR. A mild mediterranean climate with reliable autumn-winter rains and a short summer drought, typical of the mountain areas of the southwestern CFR, favours recruitment of seeders but hampers recruitment of resprouters. Resprouter species persist and become dominant under harsh conditions for recruitment (severe summer drought) and would coexist with seeders under situations of no summer stress. Diversification is associated with seeder lineages. Hence, number of seeder species will be higher than number of resprouters, especially in the southwestern CFR, where favourable conditions for recruitment allow a massive concentration of seeder species, many of them narrow endemics.     

Fire interval effects on persistence of resprouter species in Mediterranean-type shrublands

The capacity of many plant species to resprout in fire-prone shrublands is thought to engender persistence, yet management concerns exist for the long-term persistence of some resprouting species given anthropogenic impacts including shortened fire intervals, long periods of fire exclusion, and/or fires of increasingly high severity. We explored the potential demographic effects of different fire interval regimes on lignotuberous resprouter species using the last fire interval for 36 sites (33 experimental fires, 3 wildfires) in biodiverse SW Australian shrublands, spanning an interval range of 3–42 years. Mortality and regrowth 1 year following the last fire was assessed for >7,000 tagged individuals from 20 shrub and sub-shrub species. Using generalized linear mixed effect models, we estimated the influence of fire interval (and selected fire and environmental covariates) on mortality and regrowth rates across all species, and individually for the four most common species. The overall model, as well as the models for three of the four most common species (Banksia attenuata, Melaleuca leuropoma, and M. systena, but not Hibbertia hypericoides) supported the hypothesis of increased mortality at short and long fire intervals, most likely due to total non-structural carbohydrate (TNC) and bud-bank limitation, respectively. However, no relationship between regrowth rate and fire interval was detected, suggesting that increased mortality at short (3–5 year) fire intervals may not be due solely to resource (TNC) limitation. Results show that lignotuberous resprouters are potentially vulnerable to population decline through attrition of mature plants under both shortened and lengthened fire interval regimes.     

High and dry: post‐fire tree seedling establishment in subalpine forests decreases with post‐fire drought and large stand‐replacing burn patches

Aim Climate warming and increased wildfire activity are hypothesized to catalyse biogeographical shifts, reducing the resilience of fire‐prone forests world‐wide. Two key mechanisms underpinning hypotheses are: (1) reduced seed availability in large stand‐replacing burn patches, and (2) reduced seedling establishment/survival after post‐fire drought. We tested for regional evidence consistent with these mechanisms in an extensive fire‐prone forest biome by assessing post‐fire tree seedling establishment, a key indicator of forest resilience. Location Subalpine forests, US Rocky Mountains. Methods We analysed post‐fire tree seedling establishment from 184 field plots where stand‐replacing forest fires were followed by varying post‐fire climate conditions. Generalized linear mixed models tested how establishment rates varied with post‐fire drought severity and distance to seed source (among other relevant factors) for tree species with contrasting post‐fire regeneration adaptations. Results Total post‐fire tree seedling establishment (all species combined) declined sharply with greater post‐fire drought severity and with greater distance to seed sources (i.e. the interior of burn patches). Effects varied among key species groups. For conifers that dominate present‐day subalpine forests (Picea engelmannii, Abies lasiocarpa), post‐fire seedling establishment declined sharply with both factors. One exception was serotinous Pinus contorta, which did not vary with either factor. For montane species expected to move upslope under future climate change (Larix occidentalis, Pseudotsuga menziesii, Populus tremuloides) and upper treeline species (Pinus albicaulis), establishment was unrelated to either factor. Greater post‐fire tree seedling establishment on cooler/wetter aspects suggested local topographic refugia during post‐fire droughts. Main conclusions If future drought and wildfire patterns manifest as expected, post‐fire tree seedling establishment of species that currently characterize subalpine forests could be substantially reduced. Compensatory increases from lower montane and upper treeline species may partially offset these reductions, but our data suggest important near‐ to mid‐term shifts in the composition and structure of high‐elevation forests under continued climate warming and increased wildfire activity.     

Seed germination and life history syndromes in the California chaparral

Syndromes are life history responses that are correlated to environmental regimes and are shared by a group of species (Stebbins, 1974). In the California chaparral there are two syndromes contrasted by the timing of seedling recruitment relative to wildfires. One syndrome, here called the fire-recruiter or refractory seed syndrome, includes species (both resprouting and non-resprouting) which share the feature that the timing of seedling establishment is specialized to the first rainy season after fire. Included are woody, suffrutescent and annual life forms but no geophytes have this syndrome. These species are linked by the characteristic that their seeds have a dormancy which is readily broken by environmental stimuli such as intense heat shock or chemicals leached from charred wood. Such seeds are referred to as “refractory” and dormancy, in some cases, is due to seed coat impermeability (such seeds are commonly called hardseeded), but in other cases the mechanism is unknown. Seeds of some may require cold stratification and/or light in addition to fire related stimuli. In the absence of fire related cues, a portion or all of a species’ seed pool remains dormant. Most have locally dispersed seeds that persist in the soil seed bank until the site burns. Dispersal of propagules is largely during spring and summer which facilitates the avoidance of flowering and fruiting during the summer and fall drought. Within a life form (e.g., shrub, suffrutescent, etc.), the seeds of these species have less mass than those of species with non-refractory seeds and this possibly reflects the environmental favorableness of the postfire environment for seedling establishment. Regardless of when fire occurs, germination is normally delayed until late winter or early spring. In the absence of fire, or other disturbance, opportunities for population expansion are largely lacking for species with this syndrome. The other syndrome, here called the fire-resister or non-refractory seed syndrome, includes species that are resilient to frequent fires (mostly by vegetative resprouting), but require fire-free periods for recruiting new seedlings. Included are shrubs, subshrubs, suffrutescents, lianas, geophytes and annuals. All are linked by the characteristic that their seeds germinate in the absence of cues related to wildfires. In many cases no form of seed dormancy is present and the seeds germinate soon after dispersal; consequently these species do not accumulate a persistent seed bank. Germination and seedling establishment is independent of fire and thus opportunities for population expansion are also independent of fire. The demographic pattern of seedling recruitment varies with the life form. For shrubs, seedling recruitment may be restricted to sites free of fire for periods of a hundred years or more. Recruitment appears to require relatively mesic conditions and this may account for the patchy distribution of these species within the matrix of relatively arid sites. Finding such sites has selected for propagules specialized for wind or animal dispersal; the majority are bird dispersed. These shrub species all disperse fruits in fall and winter and this may have been selected to take advantage of migratory birds as well as to time dispersal to the winter rains typical of the mediterranean-climate. Germination typically occurs within several weeks of the first fall or winter rains. Maturation of flowers and fruits during the summer and fall drought may account for the distribution of these species on more mesic sites. Seed mass of these species is large and this may have been selected to provide an advantage to seedlings establishing under the canopy of this dense shrub community.     

Canopy seed bank structure in relation to: fire,tree size and density

To assess the canopy seed bank structure of Pinus halepensis, we measured the level of serotiny and the seed bank size and density of trees in unburned stands and post-fire regenerated stands in Israel. We analysed the effects of tree size, tree density and fire history on the level of serotiny. The level of serotiny decreased with an increase in tree height. The high level of serotiny in short trees could be explained by selection to increase regeneration chances after burning at pre-mature age. Also, limitation of long-distance seed dispersal opportunities in short trees may favour high serotiny levels. The level of serotiny was higher in post-fire stands than in unburned stands, suggesting a fast selection for serotiny by fire. Unburned stands had a higher total stand seed density than post-fire regenerated stands, but the proportion of seeds in serotinous cones of the total stand seed density was higher in post-fire regenerated stands. The fact that P. halepensis bears simultaneously serotinous and non-serotinous cones reflects its dual strategy as both a post-fire obligate seeder, mainly from serotinous cones and an early coloniser during fire-free periods, mainly from non-serotinous cones. The relative investment in these strategies is dependent on fire history and varies with tree height. Furthermore, mature brown cones can contribute to post-fire regeneration in case of spring fires, and serotinous cones are known to open partially also in dry spell events. Thus, post-fire regeneration and invasion are strategies, which seem to complement each other.     

Demography of woody plants in relation to fire: Banksia serrata Lf. and Isopogon anemonifolius (Salisb.) Knight

Banksia serrata and Isopogon anemonifolius are serotinous resprouters (single-stemmed tree, multi-stemmed shrub, respectively) found in forests within the Sydney region. Studies were conducted to predict the population dynamics of these species. Seed production and survival and the accumulation of seed-bank within cones were estimated in relation to time since fire. Emergence, survival and development of lignotubers were measured in young juveniles and the time taken to reach adulthood was estimated. This information and published data on survival were used to estimate the amount of recruitment of adults and juveniles necessary for stands to remain in a stable state under frequent (<16 years), high and low intensity fire regimes. The effects of longer intervals (up to 80 years) between fires were also estimated. It was predicted that B. serrata populations will decline in numbers when the interval between high intensity fires is <9 years, while under low intensity fires the critical interval was 12–13 years. In I. anemonifolius the predicted intervals were 14 and 16 years, respectively. When fires are timed so that maximum seed-bank is available (about 30 year interval), it is unlikely that resprouters will dominate communities because the seed-banks and rates of growth of seedlings of obligate seeder shrubs are greater than these resprouters. Populations of these resprouters may be more able to persist than obligate seeders when the fire frequency is either very high (<6 years) or low (>50years), though the density of resprouter populations may slowly decline under such fire regimes.     

Organisation of the soil seed bank in mixed chaparral

Seed densities, estimated from germinations under glasshouse conditions, were determined for fire-treated and control soils sampled from north and south facing stands of mixedAdenostoma fasciculatum-Ceanothus greggii chaparral that last burned 10, 17, 36, 62 and 86 years before 1987. A total of 53 species was recorded. Total germinable seed density was not influenced by aspect nor by an experimental fire treatment, however, densities varied significantly, but not predictably, over the time since fire sequence. Species diversity was significantly lower in the 86 year old stand, but neither diversity or richness was influenced by the fire treatment. Soils from north facing slopes supported a higher species diversity overall, and seed densities of six species were significantly higher on north than on south slopes.We identify two functional groups in the seed bank on the basis of the direct effects of fire on seed densities: a fire-dependent group, comprising 6 species and about 21% of the total seed bank, in which germinable densities increased after the fire treatment, and a fire-independent group, made up of 11 species and almost two-thirds of the seed bank. The fire-independent group is further separated into two: a shrub-centred sub-group (Crassula erecta, Filago californica, Pterostegia drymarioides, Streptanthus heterophyllus andCryptantha intermedia) maintained larger seed banks in older stands and was associated with shrub cover; and an opportunistic sub-group (Camissonia hirtella, Gnaphalium chilense, Mimulus pilosus andSenecio vulgaris) tended to have larger seed banks in younger stands and was associated with canopy gaps.Overall, the soil seed assemblage from this mixed chaparral does not appear to be tightly coupled to fire-generated opportunities for recruitment. The reason for this is that the fire-dependent group represents only 6 species. In addition, among the fire-independent species, the shrub-centred group of 5 annuals made up almost 40% of total soil seed density. This group shows ecological and taxonomic affinities with desert annuals.     

Interactive effects of radiation,temperature and salinity on different life history stages of the Arctic kelp <Emphasis Type="Italic">Alaria esculenta</Emphasis> (Phaeophyceae)

Vegetative resprouting, soil or canopy-stored seed banks, post-fire seed dispersal and germination are the major strategies by which plants regenerate after fires. Post-fire regeneration modes of plants are commonly based on the presence or absence of post-fire recruitment as well as the presence or absence of post-fire resprouting. High temperatures, smoke and ash are characteristics of fire and the post-fire environment. We hypothesized that heat, smoke, ash and pH will have differential effects on seed germination depending on species’ post-fire regeneration strategies: serotinous vs. nonserotinous (which may have soil seed banks) and resprouters vs. nonresprouters (which may be obligate seeders). Here we examined the effects of these factors on the germination of 27 common east Australian species. Most serotinous species supported our hypothesis by showing no effect or reduced germination in response to heat. However, contrary to our prediction, all nonserotinous nonresprouting species also showed no effect or reduced germination in response to heat. Smoke, contrary to our hypothesis, had a negative or no effect on all serotinous and nonresprouting species, but no clear directional effect on serotinous and resprouting species. Supporting our hypotheses, ash and high pH showed positive or nonsignificant effects on the germination of all serotinous resprouting species, and a negative or no effect on nonserotinous resprouting species. However, contrary to our prediction, it had a negative or no effect on the serotinous nonresprouting species and no clear effect on nonserotinous nonresprouting species. We also discovered large differences in germination responses between conspecific populations that varied in their degree of resprouting. Although our data confirmed several of our predictions, the overall conclusion is that the responses of seeds to heat, smoke, ash and pH are not tightly associated with post-fire regeneration functional types. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Resilience of Mediterranean shrubland to a severe drought episode: the role of seed bank and seedling emergence

Extreme climate events, such as severe drought episodes, may induce changes in vegetation if they induce species‐specific adult mortality and changes in the seedling recruitment pattern. In 2005 a severe drought occurred in Doñana National Park (south Spain) causing extensive shrubland mortality. Over the following years we monitored the soil seed bank and seedling emergence via a gradient of canopy dieback induced by the drought episode. The canopy dieback corresponded to an increase in emergence of seedlings of woody species in 2007, probably because of the reduced competition induced by canopy loss. The soil seed bank of woody species sampled in 2008 was less abundant on plots with a higher proportion of dead vegetation, probably because of depletion of the seed bank as a result of the increased germination in the previous year and also as a result of a reduction in seed supply in these sites. Accordingly, in 2009 we detected reduced emergence of woody species on plots that had suffered the greatest shrub mortality. We failed to find any significant changes in patterns of the soil seed bank and seedling emergence of short‐lived herbaceous species, indicating greater resilience in these types of species. This study highlights the resilience of Mediterranean shrublands to climate fluctuations at one extreme of the variability characteristic of these ecosystems. An increase in the frequency of severe drought episodes – increasingly probable under the new climate conditions – does have the potential, however, to induce changes in vegetation, especially in woody communities that need more time to replenish their seed banks.     

Fire and competition in Australian heath: a conceptual model and field investigations

Abstract. We describe a model of heath vegetation, in which species were classified into five functional groups based on characteristics of their propagule pools, post-fire growth, timing and mode of reproduction and competitive status. The model assumes no recruitment without fire and a simple competitive hierarchy based on vertical stature. A critical feature of the model is an initial post-fire window of 5–6 yr in which competition from overstorey species on understorey species is reduced. Understorey functional groups differ in their ability to exploit this window. In the field, we tested five predictions derived from the model: (a) overall species richness of understorey varies inversely with overstorey density as a result of a trend in richness of woody species, but not in herbaceous species; (b) where an overstorey was present in the previous fire interval, post-fire population density is reduced in a functional group of understorey serotinous resprouting shrubs, but not in a group of understorey obligate-seeding shrubs with soil seed banks; (c) in understorey serotinous resprouting shrubs, post-fire regrowth in resprouting individuals is adversely affected by the presence of an overstorey in the preceding fire interval; (d) in understorey serotinous resprouting shrubs, levels of pre-fire propagules are lower in the presence of an overstorey, reducing the density of post-fire recruits; and (e) in understorey serotinous resprouting shrubs, recruitment relative to the pre-fire population is unaffected by overstorey species within the window of reduced competition. Of these, three tests (a,b,d) supported the model, one (e) may support the model, but the results were inconclusive and one (c) did not support the model. Limitations and further applications of the model are discussed. Our results suggest that maintenance of high densities of overstorey populations is in conflict with conservation of some understorey species. Models of the type we propose will help identify and resolve such conflicts and promote the judicious use of fire to maintain full species diversity of plant communities.