Fire, soil fertility and delayed seed release: a community analysis of the degree of serotiny

Delayed seed release (serotiny) is a convergent plant trait in fire-prone regions of the world but explaining the degree of serotiny has remained elusive because of the paucity of community data. Selective forces involving seed predators, fire and soil nutrients have been suggested as factors influencing serotiny. We tested whether protection of seeds and/or synchronized dispersal were associated with different levels of serotiny and if resprouting ability influences selection for strong serotiny. We compared the numbers and abundance of 146 woody species with delayed dispersal among five community types varying in combinations of fire severity, fire frequency, soil fertility and seed predators. The strength of the relationship between levels of serotiny and environmental factors was tested among community types ranging from rainforests to heathlands. Highest levels of serotiny were recorded in low nutrient shrublands with intermediate fire return intervals that burn at high severity, while the lowest were recorded in high nutrient, low flammability forests. Both protection of seeds and synchronized seed release were related to fire effects in nutrient-limited environments. Strong serotiny is prominent in species killed by fire whereas weak serotiny is more common in resprouting species. Recruitment failure in the inter-fire interval appears to drive selection for strong maternal care of seeds and synchronized seed dispersal in fire-prone environments. Weak serotiny is proposed as a bet-hedging strategy that relies on resprouting after fire for population persistence and higher probability of inter-fire recruitment. The spectrum of serotiny (weak to strong) in these communities is proposed to be driven by the interactive effect of both fire and soil nutrients on the selection for delayed seed dispersal.     

Variation in serotiny of three Banksia species along a climatic gradient

The degree of serotiny (i.e. the proportion of follicles remaining closed in each year's crop of cones since the last fire) was measured in Bank-sia attenuata, B. menziesii and B. prionotes at five sites along a climatic gradient extending 500 km north of Perth, Western Australia. The decrease in annual rainfall and increase in average temperature along the gradient paralleled a decrease in plant height and an increase in the degree of serotiny of all species. Extreme serotiny was recorded in the scrub-heath at the xeric end of the gradient whereas two species were essentially non-serotinous in the low woodland at the most mesic site. It is concluded that degree of serotiny is related to the fire characteristics of the site which depend on plant height. In xeric scrub-heath, the entire canopies of the Banksia spp. are consumed by fire which promotes massive release of seed. This facilitates recruitment in an otherwise unpredictable and unreliable seedbed. In mesic woodland, where cones rarely come into contact With flames, seeds are released spontaneously and site conditions are more conducive to recruitment in the inter-fire period.     

Genome-wide association genetics of an adaptive trait in lodgepole pine

Pine cones that remain closed and retain seeds until fire causes the cones to open (cone serotiny) represent a key adaptive trait in a variety of pine species. In lodgepole pine, there is substantial geographical variation in serotiny across the Rocky Mountain region. This variation in serotiny has evolved as a result of geographically divergent selection, with consequences that extend to forest communities and ecosystems. An understanding of the genetic architecture of this trait is of interest owing to the wide-reaching ecological consequences of serotiny and also because of the repeated evolution of the trait across the genus. Here, we present and utilize an inexpensive and time-effective method for generating population genomic data. The method uses restriction enzymes and PCR amplification to generate a library of fragments that can be sequenced with a high level of multiplexing. We obtained data for more than 95,000 single nucleotide polymorphisms across 98 serotinous and nonserotinous lodgepole pines from three populations. We used a Bayesian generalized linear model (GLM) to test for an association between genotypic variation at these loci and serotiny. The probability of serotiny varied by genotype at 11 loci, and the association between genotype and serotiny at these loci was consistent in each of the three populations of pines. Genetic variation across these 11 loci explained 50% of the phenotypic variation in serotiny. Our results provide a first genome-wide association map of serotiny in pines and demonstrate an inexpensive and efficient method for generating population genomic data.     

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire‐prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non‐serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post‐fire communities and ecosystems widespread in the Rocky Mountains.     

Field heritability of a plant adaptation to fire in heterogeneous landscapes

The strong association observed between fire regimes and variation in plant adaptations to fire suggests a rapid response to fire as an agent of selection. It also suggests that fire‐related traits are heritable, a precondition for evolutionary change. One example is serotiny, the accumulation of seeds in unopened fruits or cones until the next fire, an important strategy for plant population persistence in fire‐prone ecosystems. Here, we evaluate the potential of this trait to respond to natural selection in its natural setting. For this, we use a SNP marker approach to estimate genetic variance and heritability of serotiny directly in the field for two Mediterranean pine species. Study populations were large and heterogeneous in climatic conditions and fire regime. We first estimated the realized relatedness among trees from genotypes, and then partitioned the phenotypic variance in serotiny using Bayesian animal models that incorporated environmental predictors. As expected, field heritability was smaller (around 0.10 for both species) than previous estimates under common garden conditions (0.20). An estimate on a subset of stands with more homogeneous environmental conditions was not different from that in the complete set of stands, suggesting that our models correctly captured the environmental variation at the spatial scale of the study. Our results highlight the importance of measuring quantitative genetic parameters in natural populations, where environmental heterogeneity is a critical aspect. The heritability of serotiny, although not high, combined with high phenotypic variance within populations, confirms the potential of this fire‐related trait for evolutionary change in the wild.     

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.     

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.     

Maintenance costs of serotiny do not explain weak serotiny

Considerable variation in the duration of serotiny exists among species of both Australian and South African Proteaceae. ‘Weak’ serotiny (pre‐fire loss after <3 years) could be dictated by the costs (water or carbon) of cone/fruit retention or by benefits accruing from pre‐fire seed establishment. We determined that cones/fruits of a range of Australian and south western Cape Proteaceae species (Leucadendron xanthoconus, Aulax umbellata, L. linifolium, L. gandogeri, Hakea drupacea, H. sericea) are not sealed dead wood, but that they continuously lose H₂O and CO₂. Water loss from cones/fruits was poorly controlled, occurring in both light and dark. The rates of both H₂O and CO₂ loss from mature cones/fruits were negatively correlated with the degree of serotiny (r² = 0.59 and 0.18, respectively, P < 0.001 both). However, the amounts of H₂O and CO₂ lost per weight were small relative to the fluxes from leaves (13–29% for H₂O and 3–10% for CO₂). The [N] and [P] in the cones/fruits and seeds was substantial. Despite 25% of N and 38% of P being recovered from the cones/fruits following maturation, the loss of the cones/fruits and seeds would still incur a substantial nutrient cost. The seed [P] was positively correlated with the degree of serotiny (r² = 0.24, P = 0.001). We suggest that maintenance costs (water and carbon) of serotiny, although exceeding those of soil stored seeds, are relatively low. The correlation between the degree of serotiny and seed [P] indicates that stronger serotiny is required, much like sclerophylly, for survival under low nutrient availability in frequently burnt vegetation.     

Increased canopy seed-storage in post-fire pine invaders suggests rapid selection mediated by fire

Exotic species storing seeds in the canopy (serotinous species) can experience a clear advantage in fire-prone communities that lack native taxa with such fire-resistant traits. In addition, selection in the new environment can potentially increase the frequency of fire-adapted characteristics such as serotiny. We studied the potential role of fire favoring the serotinous, non-native conifer Pinus radiata in NW Patagonia. We characterized the degree of serotiny (percentage of serotinous cones) and the size of the canopy seed bank in the unburned plantation and in stands of trees recruited after a fire 30 years ago as a proxy for invasion potential. Fire had a positive effect, increasing serotiny in post-fire P. radiata stands. Post-fire recruited cohorts showed higher serotiny levels and a larger canopy seed bank compared with plantations. Our study suggests that fire-linked traits like serotiny may be subjected to a rapid, fire-driven selection process in fire-adapted species such as P. radiata invading fire-prone ecosystems. Thus, increased serotiny can lead to higher postfire invasion densities, which in turn create a positive feedback loop in invaded areas under recurrent fires.     

Anatomic basis and insulation of serotinous cones in Pinus halepensis Mill

Pinus halepensis Mill., a widespread, low elevation conifer common in Mediterranean Basin, shows a dual reproductive strategy: post-fire obligate seeder (from serotinous cones) and an early coloniser (from non-serotinous cones). Release of seeds encased in serotinous cones is induced either by fire (pyriscence, serotiny or bradychory) or by drying (xeriscence). Morphological differences in serotinous and non-serotinous cones in natural populations of P. halepensis in Southeastern Italy were analyzed. Relationships between tree size (diameter class) and serotiny were checked by counting and sampling serotinous and non-serotinous cones. The macro and microscopic characteristics that could affect cones’ opening were measured in sampled cones. Protection against high temperatures offered by wood scales was also evaluated by applying different temperatures and time exposures, and following the inner thermal raise. Results showed that non-serotinous cones had bigger resin ducts and more separate scales. Also it was highlighted that ovuliferous scales of serotinous cones were bigger and thicker. These scales had more lamellated (multilayered) sclereid cells, and were significantly thinner with a shorter lumen diameter. Continuous temperature-monitoring heat tests inside cones showed that temperatures close to the cone axis were rather low, so seed germination was not influenced. Results confirm that serotinous cones are more compact, rigid and consistent than non-serotinous cones. These characteristics explain the lower insulation, seed protection and the ease opening of non-serotinous cones as well. In conclusion, opening mechanism of pinecone scales under the effect of fire or dry conditions seem related to anatomic differences and it provides seeds with an efficient protection against heat.     

What are the relative costs,limits and correlates of increased degree of serotiny?

There is variability in the period that serotinous species retain seeds in protective closed cones and in the sizes of the cones. Seed predation and inter‐fire recruitment have been suggested to be reasons for this variability. I argue against these two reasons. Variation in annual rates of increase in fecundity and in numbers of flowering episodes before fire should be correlated with the degree of serotiny. These two attributes are a consequence of cone size, plant architecture and degree of serotiny.     

Delayed seed dispersal in Pinus torreyana (Torrey pine)

Summary Torrey pine exhibits a pattern of seed release intermediate between an open and closed-cone species. It was found that the cones open at maturity, but that seed fall from some cones continues for up to 13 years. Approximately 77% of the total seed crop was present in age classes one or more years past the time of seed maturity.Serotiny is thought to evolve in response to fire, and partial serotiny at the population level has been attributed to circumstances in which the strong selective pressure of fire is relaxed. We hypothesize that delayed seed dispersal in Torrey pine is a stable trait which is the result of a pattern of large relatively rare crown fires separated by intervals longer than one generation. The reduced serotiny observed in other conifers on headlands and islands in California seems consistent with our hypothesis.     

Heritability and quantitative genetic divergence of serotiny,a fire-persistence plant trait

Background and Aims

Although it is well known that fire acts as a selective pressure shaping plant phenotypes, there are no quantitative estimates of the heritability of any trait related to plant persistence under recurrent fires, such as serotiny. In this study, the heritability of serotiny in Pinus halepensis is calculated, and an evaluation is made as to whether fire has left a selection signature on the level of serotiny among populations by comparing the genetic divergence of serotiny with the expected divergence of neutral molecular markers (Q_ST–F_ST comparison).

Methods

A common garden of P. halepensis was used, located in inland Spain and composed of 145 open-pollinated families from 29 provenances covering the entire natural range of P. halepensis in the Iberian Peninsula and Balearic Islands. Narrow-sense heritability (h²) and quantitative genetic differentiation among populations for serotiny (Q_ST) were estimated by means of an ‘animal model’ fitted by Bayesian inference. In order to determine whether genetic differentiation for serotiny is the result of differential natural selection, Q_ST estimates for serotiny were compared with F_ST estimates obtained from allozyme data. Finally, a test was made of whether levels of serotiny in the different provenances were related to different fire regimes, using summer rainfall as a proxy for fire regime in each provenance.

Key Results

Serotiny showed a significant narrow-sense heritability (h²) of 0·20 (credible interval 0·09–0·40). Quantitative genetic differentiation among provenances for serotiny (Q_ST = 0·44) was significantly higher than expected under a neutral process (F_ST = 0·12), suggesting adaptive differentiation. A significant negative relationship was found between the serotiny level of trees in the common garden and summer rainfall of their provenance sites.

Conclusions

Serotiny is a heritable trait in P. halepensis, and selection acts on it, giving rise to contrasting serotiny levels among populations depending on the fire regime, and supporting the role of fire in generating genetic divergence for adaptive traits.     

Serotiny in the South African shrub Protea repens is associated with gradients of precipitation,temperature, and fire intensity

Plant Ecology - Globally, variability in canopy seed retention within closed cones (serotiny) among fire-adapted plant species is often associated with gradients in fire regime. Few studies have...     

Jack pine of all trades: Deciphering intraspecific variability of a key adaptive trait at the rear edge of a widespread fire-embracing North American conifer

Premise

Understanding mechanisms fostering long-term persistence of marginal populations should provide key insights about species resilience facing climate change. Cone serotiny is a key adaptive trait in Pinus banksiana (jack pine), which shows phenotypic variation according to the fire regime. Compared to range-core populations within the fire-prone boreal forest, low and variable serotiny in rear-edge populations suggest local adaptation to uncommon and unpredictable wildfire regime. We assessed environmental/physiological factors that might modulate intraspecific variation in cone serotiny.

Methods

We experimentally subjected closed cones to incrementing temperatures, then tested seed germination to determine whether and how various ecological factors (cone age, branch height, tree size, tree age) are related to cone dehiscence and seed viability in jack pines from rear-edge and range-core populations in eastern Canada.

Results

Cones from rear-edge populations dehisce at a lower opening temperature, which increases with cone age. Cones from range-core stands open at a more constant, yet higher temperature. Cones from rear-edge stands take between 13 and 27 years to reach the level of serotiny achieved at the range core. At the rear edge, seed viability is steady (51%), whereas it decreases from 70% to 30% in 20 years at the range core.

Conclusions

We inferred the mechanisms of a bet-hedging strategy in rear-edge populations, which ensures steady recruitment during fire-free intervals and successful postfire regeneration. This capacity to cope with infrequent and unpredictable fire regime should increase the resilience of jack pine populations as global changes alter fire dynamics of the boreal forest.     

Canopy seed storage in woody plants

The retention of seeds in the plant canopy for one to 30 years or more is termed serotiny. It is well represented floristically and physiognomically in fire-prone, nutrient-poor and seasonally-dry sclerophyll vegetation in Australia, and to a lesser extent, South Africa followed by North America. While the seed-storing structures vary greatly, all will release their propagules following exposure to the heat of a fire (pyriscence). This phenomenon can be contrasted with seed release at maturity (non-storage) and soil storage of seeds. Although the evolutionary requirements for serotiny are clear, its adaptive advantages over other seed storage syndromes are largely the subject of conjecture in the absence of comparative experiments. Nine hypotheses were assessed here. Canopy storage maximises the quantity of seeds available for the next post-fire generation (unlike non-storage). Synchronized post-fire release satiates post-dispersal granivores (unlike non-storage and soil storage) and ensures arrival on a seed bed conducive to seedling recruitment (unlike non-storage). Canopy stored seeds are better insulated from the heat of a fire than non-stored, and probably soil-stored, seeds. Fluctuating annual seed crops, the opportunity for post-fire wind-dispersal, the possible advantages of dense stands of adults, short lifespan of the dispersed seeds and their optimal location in the soil for germination have only a limited role in explaining the advantages of serotiny. It is concluded that canopy seed storage is favoured in regions where seed production is restricted and inter-fire establishment and maturation are unlikely. In addition, these regions have a reliable seasonal rainfall and are subjected to intense fires at intervals occurring within the reproductive lifespan of the species.     

Fruit-seed relations in Hakea: serotinous species invest more dry matter in predispersal seed protection

Abstract The ability to maintain a canopy stored seedbank (serotiny) is characteristic of many woody genera inhabiting fire-prone environments. The relationship between level of serotiny for 94 Hakea (Proteaceae) species in southwestern Australia and follicle mass, density, three wall thicknesses and seed mass was investigated. Two species were non-serotinous (fruits open at maturity), 12 were weakly serotinous (majority of fruits open at maturity), 9 were moderately serotinous (fruits open within five years of maturity) and 71 were strongly serotinous (fruits still closed at least five years after reaching maturity). A positive relationship existed between the level of serotiny and follicle morphology. Strongly serotinous species were more likely to have heavier, woodier and thicker-walled follicles than non- and weakly serotinous species. Moderately/strongly serotinous species invested more energy (six times higher follicle:seed mass ratio) than non weakly serotinous species, consistent with increased protection of the seeds from granivores, pathogens, desiccation and/or heat. Recent work has shown that thicker fruit walls (strongly serotinous species) provide better insulation to seeds from heat, although the need to survive fire is just as critical for thin-walled, weakly serotinous species. Greater protection from granivores may provide a better explanation for the adaptive significance of dense, thick-walled serotinous follicles, as the opportunities for predispersal granivore damage are low among weakly serotinous species.     

Evolutionary history of fire‐stimulated resprouting,flowering, seed release and germination

Fire has shaped the evolution of many plant traits in fire‐prone environments: fire‐resistant tissues with heat‐insulated meristems, post‐fire resprouting or fire‐killed but regenerating from stored seeds, fire‐stimulated flowering, release of on‐plant‐stored seeds, and germination of soil‐stored seeds. Flowering, seed release and germination fit into three categories of response to intensifying fire: fire not required, weakly fire‐adapted or strongly fire‐adapted. Resprouting also has three categories but survival is always reduced by increasing fire intensity. We collated 286 records for 20 angiosperm and two gymnosperm families and 50 trait assignments to dated phylogenies. We placed these into three fire‐adapted trait types: those associated with the origin of their clade and the onset of fire‐proneness [primary diversification, contributing 20% of speciation events over the last 120 million years (My)], those originating much later coincident with a change in the fire regime (secondary diversification, 30%), and those conserved in the daughter lineage as already adapted to the fire regime (stabilisation, 50%). All four fire‐response types could be traced to >100 My ago (Mya) with pyrogenic flowering slightly younger because of its dependence on resprouting. There was no evidence that resprouting was always an older trait than either seed storage or non‐sprouting throughout this period, with either/both ancestral or derived in different clades and times. Fire‐adapted traits evolved slowly in the Cretaceous, 120–65 Mya, and rapidly but fitfully in the Cenozoic, 65–0 Mya, peaking over the last 20 My. The four trait‐types climaxed at different times, with the peak in resprouter speciation over the last 5 My attributable to fluctuating growing conditions and increasing savanna grasslands unsuitable for non‐sprouters. All experienced a trough in the 40–30‐Mya period following a reduction in world temperatures and oxygen levels and expected reduced fire activity. Thick bark and serotiny arose in the Mid‐Cretaceous among extant Pinaceae. Heat‐stimulated germination of hard seeds is ancestral in the 103‐My‐old Fabales. Smoke‐(karrikin)‐stimulated germination of non‐hard seeds is even older, and includes the 101‐My‐old Restionaceae–Anarthriaceae. A smoke/karrikin response is detectable in some fire‐free lineages that prove to have a fire‐prone ancestry. Among clades that are predominantly fire‐prone, absence of fire‐related traits is the advanced condition, associated either with increased fire frequency (loss of serotiny and soil storage), or migration to fire‐free habitats (loss of thick bark, pyrogenic flowering, serotiny or soil storage). Protea (Africa) and Hakea (Australia) illustrate the importance of stabilisation processes between resprouting/non‐sprouting in accounting for speciation events over the last 20 My and highlight the frequent interchange possible between these two traits. Apart from Pinus, most ancestral trait reconstruction relative to fire has been conducted on predominantly Southern Hemisphere clades and this needs to be redressed. Despite these limitations, it is clear that fire has had a profound effect on fire‐related trait evolution worldwide, and set the platform for subsequent evolution of many non‐fire‐related traits. Genetics of the triggering mechanisms remain poorly understood, except the karrikin system for smoke‐stimulated germination. We exhort biologists to include fire‐proneness and fire‐related traits in their thinking on possible factors controlling the evolution of plants.     

Seed bank dynamics of two obligate seeders, <Emphasis Type="Italic">Cistus monspeliensis</Emphasis> and <Emphasis Type="Italic">Rosmarinus officinalis</Emphasis>, in relation to time since fire

Many species in Mediterranean-type ecosystems regenerate after fire by seed germination from soil seed banks. Seed bank dynamics of two of those obligate seeders, Cistus monspeliensis and Rosmarinus officinalis, were investigated in relation to stand age since fire in southwestern Portugal. Soil seed density, annual seed input, annual seed losses through germination and seed persistence were compared between species at stands differing in age since fire (5, 10 and 35 years). Soil seed density and seed input increased over the first decade after fire and were lowest at 35-year-old stands for C. monspeliensis. In R. officinalis, few seeds were produced and found in the soil at early stages, and maximum seed input and soil seed density were attained at 35-year-old stands. Soil seed density was mostly driven by seed production in both species, which is largely dependent on plant traits and population dynamics related to fire. Overall, stand age since fire had a negligible effect on seed germination, seed persistence and viability. Ten to 39% of buried seeds were not recovered after 1 year, and viability of seeds recovered was 97–100% for C. monspeliensis and only 0–3% for R. officinalis. Variation in plant traits within the seeder syndrome was evidenced by this study. R. officinalis evidenced lower seed persistence, lower proportion of viable seed produced and lower density of viable soil seed than C. monspeliensis at any stage after fire. R. officinalis is expected to depend largely on previous year seed production for population replacement after fire.     

Germination response to fire-related factors of seeds from non-serotinous and serotinous cones

Pinus halepensis, a Mediterranean pine tree, is a partially serotinous species: individual trees of this species carry both non-serotinous and serotinous cones. Serotinous cones open mainly after fire, whereas non-serotinous cones open in absence of fire. In this study we addressed the question, whether or not this cone response is linked with the germination response of seeds to fires. Two main factors associated with fire are heating of seeds and soil pH. A combination of high heat and high pH simulates a scenario with fire, whereas low heat and low pH simulates a scenario without fire. We assessed the separate and combined effects of heat and pH on the germination rate and the percentage of germination of seeds from non-serotinous cones and two age classes of serotinous cones of P. halepensis. Heat had no effect on the percentage of germination of seeds from any of the cone types, but did positively affect the germination rates of seeds from both age-classes of serotinous cones. High pH negatively affected the germination rate of seeds from all cone types as well as the percentage of germination of seeds from non-serotinous cones. The combinations of heat and pH had different effects on the three cone types: percentage of germination and rate of germination of seeds from non-serotinous cones was higher in the combination high heat-high pH than in the combination low heat-low pH. In the combination high heat-high pH, seeds from serotinous cones germinated better than seeds from non-serotinous cones. The different germination responses of seeds from non-serotinous and serotinous cones could not be attributed to differences in cone age. Our results indicate that the cone response is linked to the germination response of the seeds in P. halepensis, with seeds from serotinous cones being more tolerant to fire related factors. This revised version was published online in June 2006 with corrections to the Cover Date.