Seed size and seed germination in the Mediterranean fire-prone shrub <Emphasis Type="Italic">Cistus </Emphasis><Emphasis Type="Italic">ladanifer</Emphasis>

The germination response of different sized seeds from individuals of a Mediterranean fire-prone shrub (Cistus ladanifer) was investigated in relation to pre-germination heating. A control (no heating), a low temperature during a short exposure time (50°C during 5 min), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, low intensity pasture fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Mean seed weight only showed a marginally significant positive correlation with the proportion of germinated seeds whatever the pre-germination treatment. These results suggest that seed dormancy is unrelated to seed size and that under the experimental conditions used in this study, the effect of seed size on seed germination is low. Nevertheless, larger seeds could be favoured in natural conditions, especially under the high competition scenario which arise after wildfires. Control seeds showed a negative correlation between seed size and germination velocity suggesting that lighter seeds could take advantage from early germination in recruitment events in the absence of wildfires. Nevertheless, even the lower pre-germination heating treatment turns this correlation in not significant, suggesting a strong selection pressure (unrelated to seed size) for early germination after fire events. In our study, different sized seeds of C. ladanifer seem to perform better under different germination scenarios suggesting that seed size variation could be maintained by the alternation of recruitments without wildfires and recruitments after wildfire events.     

Patterns of Seed Size,Germination and Seed Viability of Tropical Tree Species in Southern India1

Seed weight, days to germination and seed viability were observed for 99 species growing in the Western Ghats of Karnataka, India. Seed size was strongly correlated with days to germination; smaller seeds germinated faster than larger seeds. Species which flowered during the rainy season had lighter seeds than species which flowered during the dry season. It was also found that seed size and viability of seeds were related to the season of fruiting. Species which fruit during the rainy season had heavier seeds and shorter viability than species which fruit during the dry season. These flowering and fruiting patterns and varying seed sizes are argued to be adaptations to the time of dispersal, time of moisture availability in the habitat and seedling survival.     

Seeds photoblastism and its relationship with some plant traits in 136 cacti taxa

Seed germination triggered by light exposure (positive photoblastism) has been determined in quantitative studies for numerous plant families and species. For Cactaceae, positive photoblastism is thought to be associated with life form and seed mass, but this association has never been evaluated. To explore hypotheses on associations between seed mass, seed dispersal, seed dormancy, life form, taxa and plant height with Relative Light Germination (RLG) in Cactaceae, we evaluated the effect of light on seed germination of 136 taxa. The taxa studied are native to several countries: México, Chile, Argentina, Brazil, Perú, USA, and Venezuela. Seed traits contrasted with RLG were life form, seed mass, seed dispersal, seed dormancy, adult plant height and taxon. We found some differences between RLG among taxa; Cacteae, Pachycereeae and Trichocereeae had higher RLG than Notocacteae. RLG was lower for seeds from taller than for shorter taxa, and lower for taxa with heavier seeds than for taxa with lighter seeds. Dispersal syndrome groups varied with RLG. RLG did not differ between cylindrical and globose taxa. Trends found here were in agreement with expectations for small-seeded species to have a light requirement to germinate more often than large-seeded species. This is the first time that cactus height is related to photoblastism. It is possible that seeds from tall plants are larger and thus have the capacity to produce taller seedlings than those from small plants, and that seedlings from large seeds with more resources have the ability to emerge from greater soil depths than those from small seeds.     

The dynamics of three shrub species in a fire-prone temperate savanna: the interplay between the seed bank, seed rain and fire regime

A model was developed to assess how the seed rain and fire regime affect seed bank dynamics and seedling establishment of three native shrub species (Acanthostyles buniifolius, Baccharis pingraea and Baccharis dracunculifolia) with different regeneration strategies, in temperate South American savanna. Seed bank and seed rain were quantified for each species under different fire regimes, and their relative roles in regeneration were evaluated. All species had short-term persistent seed banks and high annual variability in seed production. A high proportion of seeds deposited in the seed rain produced seedlings after fire; few entered the soil seed bank. Fire killed a high proportion of the seeds in the soil seed bank. Seedlings derived from the seed rain had a higher probability of surviving for 2 years than seedlings emerging from the soil seed bank. In the absence of fire, establishment depended on germination both from the seed rain and the soil seed bank, whereas with annual fire, establishment was primarily dependent on germination of seeds arriving in the annual seed rain, regardless of species’ regeneration strategies. These results help to explain changes in the vegetation of South American temperate savannas as a result of changes in fire regime and grazing management during the last 50 years. By revealing the crucial roles of the soil seed bank and seed rain in regeneration, this study provides vital information for the development of appropriate management practices to control populations of shrub species with different regeneration strategies in South American temperate savannas.     

Plant regeneration from seeds responds to phylogenetic relatedness and local adaptation in Mediterranean Romulea (Iridaceae) species

Seed germination is the most important transitional event between early stages in the life cycle of spermatophytes and understanding it is crucial to understand plant adaptation and evolution. However, so far seed germination of phylogenetically closely related species has been poorly investigated. To test the hypothises that phylogenetically related plant species have similar seed ecophysiological traits thereby reflecting certain habitat conditions as a result of local adaptation , we studied seed dormancy and germination in seven Mediterranean species in the genus Romulea (Iridaceae). Both the across‐species model and the model accounting for shared evolutionary history showed that cool temperatures (≤ 15°C) were the main factor that promoted seed germination. The absence of embryo growth before radicle emergence is consistent with a prompt germination response at cool temperatures. The range of temperature conditions for germination became wider after a period of warm stratification, denoting a weak primary dormancy. Altogether these results indicate that the studied species exhibit a Mediterranean germination syndrome, but with species‐specific germination requirements clustered in a way that follows the phylogenetic relatedness among those species. In addition, species with heavier seeds from humid habitats showed a wider range of conditions for germination at dispersal time than species from dry habitats possessing lighter seeds. We conclude that while phylogenetically related species showed very similar germination requirements, there are subtle ecologically meaningful differences, confirming the onset of adaptation to local ecological factors mediated by species relatedness.     

Cryptic seed heteromorphism in Packera tomentosa (Asteraceae): differences in mass and germination

Seed mass variation and heteromorphism may afford plant species differential germination behavior and ultimately seedling success, particularly in disturbed habitats. We asked whether such variation occurs in Packera tomentosa (Michx.) C. Jeffrey (Asteraceae), a clonal species of the southeastern USA. Seed mass was compared within and among genetic individuals differentiated using amplified fragment length polymorphisms. We compared central and peripheral seeds produced by disc and ray florets, respectively, for their morphology, mass, and germination behavior, including response to water availability, aging, and cold stratification. Seed mass was highly variable both within and among individuals and influenced germination behavior. We found cryptic seed heteromorphism in P. tomentosa. Central and peripheral seeds had similar morphologies but dissimilar mass and biomass allocation. We used failure time analysis to detect different germination behavior. Central seeds were heavier, contained larger embryos, and germinated faster and at a higher proportion in most germination studies. Highly variable mass and heteromorphism of seeds may allow persistence of P. tomentosa in its disturbed habitats. Based on our results, some future studies of Asteraceae species with disc and ray florets may need to account for possible differences between seed types, even when morphological differences are not apparent. Evaluation of individual seed mass and maternal differences in germination studies may assist in the detection of cryptic seed heteromorphism, a phenomenon thought to be common, yet rarely documented.     

Novel perennial ryegrass-Neotyphodium endophyte associations: relationships between seed weight,seedling vigour and endophyte presence

Two forage cultivars of perennial ryegrass (Lolium perenne) colonised by the mutualistic fungal endophyte Neotyphodium lolii, strain AR37, were used to investigate relationships between seed weight, seed vigour and endophyte presence. Seed was separated into six fractions according to weight, with each fraction divided into two further groups with the first being subject to accelerated ageing. Seed germination rates and proportions of viable and total endophyte frequencies were assessed for each fraction. Heavier fractions of seed produced a higher number of endophyte infected seedlings sooner than the lighter fractions for both cultivars. The highest proportion of viable endophyte was found in the fastest germinating perennial ryegrass seed for all weight fractions, from both cultivars, indicating a strong relationship between endophyte presence and seed germination rate. For one of the cultivars, after accelerated ageing, as seed weight increased the proportion of viable endophyte increased and the discrepancy between the proportion of endophyte in fresh and accelerated aged seed was reduced. This implies that for this cultivar heavier ryegrass seed provides a more favourable habitat to the dormant endophyte than lighter seed during storage and/or allowed for a greater biomass of endophyte hyphae to proliferate in the seed tissues prior to seed dormancy, thus allowing the fungus to develop more propagules and greater nutritional reserves.     

Germination and emergence of annual species and burial depth: Implications for restoration ecology

Due to the high content of viable seeds, topsoil is usually spread on ground left bare during railway and motorway construction to facilitate the regeneration of vegetation cover. However, during handling of the topsoil, seeds are often buried deeply and they cannot germinate or the seedlings cannot emerge from depth. This study experimentally explores the predictive value of seed mass for seed germination, mortality and seedling emergence at different burial depths for 13 common annual species in semiarid Mediterranean environments. We separate the effect of burial depth on germination and emergence by means of two experiments. In the germination experiment, five replicates of 20 seeds for each species were buried at depths ranging from 0 to 4 cm under greenhouse conditions. Germinated and empty or rotten seeds were counted after 8 weeks. In the emergence experiment, five replicates of four newly-germinated seeds per species were buried at the same depths under controlled conditions and emergence was recorded after 3 weeks. The effect of burial depth on percentage of germination and seedling emergence was dependent on seed size. Although all species showed a decrease in germination with burial depth, this decrease was greater for small-than large-seeded species. Percentage of emergence was positively related to seed mass but negatively related to burial depth. Seed mortality was higher for small-than large-seeded species, but there was no general effect of burial depth on this variable. Thus, the current practice of spreading 30 cm deep layers of topsoil in post-construction restoration projects is unadvisable. In this restoration scenario, thinner layers of topsoil should be used to achieve the maximum potential of the topsoil for germination and seedling establishment.     

Role of heat tolerance and cone protection of seeds in the response of three pine species to wildfires

The post-fire regenerative ability of Pinus halepensis, Pinus nigra and Pinus sylvestris, three of the most important pine species present in the West Mediterranean basin, has been analyzed in the light of seed tolerance to different temperatures and times of exposure, and of seed position during the fire event (seeds inside cones versus free seeds). The combination of different fire intensities and degrees of seed protection allows us to draw different scenarios during the fire event: canopy scenarios (seeds inside cones), surface scenarios (seeds on the ground surface), and soil scenarios (seeds in the top soil layers). There were interspecific differences in the pattern of cone opening under the different heat treatments: cones of P. nigra and P. sylvestris showed similar percentages of opening, but considerably higher than those of P. halepensis. In the three species, seeds inside cones showed higher percentages of germination than those that were free, emphasizing the important role of cones in the protection of pine seeds from high temperatures. The percentage of germination decreased when both the temperature and the time of exposure increased, and there was also a significant species effect: P. halepensis showed higher germination rates than P. nigra, and both were higher than P. sylvestris. The overall scores of seed germination of these three pine species under the conditions tested suggests that their regeneration after fire should come either from the soil bank, or from the canopy bank, but rarely from the ground surface. As the existence of a permanent seed bank in Mediterranean pines is probably limited or nil, pine recruitment after fire appears to be mainly controlled by the existence of a canopy seed bank. The contribution of this canopy bank to the differences in postfire regeneration success of the three pine species is discussed in the light of their seeding phenology and the effects of fire severity on cone opening. The results obtained in this study contribute to explain the successful regeneration of P. halepensis, and the failure of P. nigra and P. sylvestris after fire.     

Reproductive ecology of Thymelaea velutina (Thymelaeaceae)- Factors contributing to the maintenance of heterocarpy

Thymelaea velutina (Thymelaeaceae) is a dioecious shrub that presents a unique type of heterocarpy which consists of the simultaneous production of dry and fleshy fruits. It is endemic to the Balearic Islands (Western Mediterranean) and is found both in dunes and mountain areas. The goal of this study was to identify which factors influence the production of both fruit types, examining the variation of their effects at a spatio-temporal scale (comparing two localities in different years). Specifically, we investigated (1) whether pollen limitation influences the type of fruit produced, and (2) the possible differences in seed size, mass, dispersal capacity, seed predation, germination patterns and seedling survival between fruit types. We also examined if the production of fleshy fruits was modified with the application of gibberellins to reproductive branches. Although fleshy fruits were consistently more abundant than dry ones at both populations, their proportion was significantly higher at the site with greater precipitation. The addition of either pollen or gibberellins did not affect the proportion of each fruit type. Seeds in fleshy fruits are consistently larger, heavier and more likely to be dispersed than seeds in dry fruits, but germinability, germination rate and seedling survival was similar among fruit types. Heterocarpy in this species is currently maintained as there is no apparent factor that exerts any strong selective pressure on either fruit type. The two fruit types might even have different `functions', one serving especially for dispersal and population expansion and the other for producing a seed bank that ensures an eventual germination.     

Seed weight and seed number affect subsequent fitness in outcrossing and selfing Primula species

Using the outcrossing Primula farinosa and its autogamous selfing relatives P. scotica , P. scandinavica and P. stricta , we compared the fitness of light and heavy seeds. Heavy seeds germinated in greater numbers and more quickly. In competition with seedlings grown from lighter seeds, heavy seeds produced larger rosettes. In P. farinosa such seedlings went on to produce more seeds, and in two populations heavier seeds, than plants from lighter seeds. After transplantation to natural populations, seedlings of P. farinosa derived from heavy seeds produced larger rosettes, more flowers and seeds than those from lighter seeds in certain populations so that seedlings born of heavy seeds were much fitter than seedlings from lighter seeds. Average seed weight varied in inverse proportion to seed number per capsule. The autogamous species produced on average about twice as many seeds per capsule as P. farinosa . In P. scotica and P. stricta this difference appears to be due in part to assured fertilization, but this high fecundity did not cause disadvantageously light seeds. As these species produced fewer capsules per scape, their overall seed production was on average no greater than for P. farinosa . P. farinosa traded-off fitness between capsules with large seed numbers, which donated more offspring to the next generation, and those with small seed numbers, whose heavy seeds would be more likely to reproduce themselves in the next generation. We conclude that low fecundity in outcrossing species might at times be advantageous.     

Seed production and germination in two rare and three common co‐occurring Acacia species from south‐east Australia

Abstract Seed set, size, viability and germination requirements were investigated for two rare (Acacia ausfeldii and A. willianisonii) and three common (A. pycnantha, A. genistifolia and A.paradoxa) co‐occurring congeners in box‐ironbark eucalypt forests near Bendigo, south‐east Australia to investigate correlates of rarity. Seed size was significantly smaller for the two rare species and germinants were less able to emerge from deeper sowing depths than were the larger seeded common congeners. All species had a strong heat‐stimulated germination response. While the rare A. ausfeldii showed strong germination only at the highest temperature treatment (100°C), the common and widespread A.pycnantha showed strong germination across a broad range of temperatures (60‐100°C), likely to be experienced by soil‐stored seeds during a fire. Seed viability, number of seeds per plant, and number of firm, aborted and eaten seeds per pod varied between species, but the pattern of variation was not related to rarity. Small seed size and a very specific temperature requirement for germination may help to explain rarity in A. ausfeldii, and to a lesser extent in A. willianisonii. Fires are often patchy and heating of the soil is likely to be highly spatially variable, so species with germination responses to a broad range of temperatures have an advantage over those that respond only to a narrow range. A narrower range of soil depths from which seeds can emerge will further reduce the proportion of the seed bank that might recruit following fire. Human impacts on species habitats, such as fragmentation, loss of topsoil through mining, timber harvesting, grazing and urbanization, and consequent reduction in fire intensity, are likely to have further contributed to rarity in these species. The role of pollination and other factors in relation to population size is the subject of further investigation.     

Diversity of germination strategies and seed dormancy in herbaceous species of campo rupestre grasslands

The effects of fire on the vegetation vary across continents. However, in Neotropical fire‐prone grasslands, the relationship between fire and seed germination is still poorly understood, while their regeneration, especially after strong anthropogenic disturbance, is challenging for their conservation. In the present study, we assessed diversity of germination strategies in 15 dominant herbaceous species from Neotropical altitudinal grasslands (locally known as campos rupestres). We exposed seeds to several fire‐related treatments. We also compared germination between regularly and post‐fire fruiting species. Finally, we investigated the diversity of dormancy classes aiming at better understanding the biogeography and phylogeny of seed dormancy. Germination strategies varied among families. Velloziaceae and Xyridaceae produced non‐dormant, fast‐germinating seeds. Cyperaceae and Poaceae showed an extremely low or null germination due to a high proportion of unviable or embryo‐less seeds. The seeds of campo rupestre grasslands are fire resistant, but there is no evidence that fire triggers germination in this fire‐prone ecosystem. Although heat and charred wood did not promote germination, smoke enhanced germination in one grass species and decreased the mean germination time and improved synchrony in Xyridaceae and Velloziaceae. Fire had a positive effect on post‐fire regeneration by stimulating fruit set in some Cyperaceae and Poaceae species. These species produced faster germinating seeds with higher germination percentage and synchrony compared to regularly fruiting Cyperaceae and Poaceae species. This strategy of dispersion and regeneration seems to be an alternative to the production of seeds with germination triggered by fire. Physiological dormancy is reported for the first time in several clades of Neotropical plants. Our data help advance the knowledge on the role of fire in the regeneration of Neotropical grasslands.     

Fruit Fate, Seed Germination and Growth of an Invasive Vine – an Experimental Test of ‘sit and Wait’ Strategy

Oriental bittersweet (Celastrus orbiculatus Thunb.) is a non-indigenous, invasive woody vine in North America that proliferates in disturbed open sites. Unlike most invasive species, C. orbiculatus exhibits a sit and wait strategy by establishing and persisting indefinitely in undisturbed, closed canopy forest and responding to canopy disturbance with rapid growth, often overtopping trees. We compared fruit fates of C. orbiculatus and native American holly (Ilex opaca). We also explored mechanisms for this sit and wait invasion strategy by testing the effect of C. orbiculatus fruit crop density on removal rates and by examining the influence of seed treatment and light intensity on seed germination and seedling growth. More C. orbiculatus than I. opaca fruits became damaged, and damage occurred earlier. More fruit fell from C. orbiculatus than I. opaca, but removal rates by frugivores did not differ (76.0 ± 4.2% vs 87.5 ± 3.7%, respectively). Density (number of fruits in a patch) of C. orbiculatus did not influence removal rates. Scarification (bird-ingestion) of C. orbiculatus seed delayed germination but seeds germinated in similar proportion to manually defleshed seeds (sown either singly or all seeds from a fruit). Germination of seeds within intact fruits was inhibited and delayed compared to other treatments. Seed treatment did not affect seedling growth. The proportion of seeds germinating and time until germination was similar among five light intensity levels, ranging from full sun to closed-canopy. Seedlings in >70% photosynthetically active radiation (PAR) had more leaves, heavier shoots, and longer, heavier roots than seedlings at lower PAR levels. Results show that most (>75%) C. orbiculatus seeds are dispersed, seedlings can establish in dense shade, and plants grow rapidly when exposed to high light conditions. Control strategies for this highly invasive species should likely focus on minimizing seed dispersal by vertebrates.     

Fire damage on seeds of Calliandra parviflora Benth. (Fabaceae), a facultative seeder in a Brazilian flooding savanna

Optimal sexual reproduction in relation to fire effects varies in Fabaceae species. Calliandra species have a large investment in reproduction. We investigated the consequences of fire during the fruiting period of Calliandra parviflora Benth., by checking fruit exposure to fire, pre-dispersal seed predator infestation, and the effect of fruit burning on germination. We conducted this study in a floodable savanna in central Brazil, where we collected burnt and unburnt fruits. We measured the fruit and seed mass, and counted the number of damaged and undamaged seeds and live larvae per fruit. We analyzed the seed germination percentage from burnt and unburnt fruits. The burnt fruits presented greater mass than the unburnt fruits, despite their seed mass being similar. The number of damaged seeds per fruit was only slightly higher in burnt compared to unburnt fruits (p = 0.047). The number of larvae on pre-dispersal seeds per fruit varied from 0 to 4 and did not differ between burnt and unburnt fruits. The germination percentage of unburnt fruit seeds (mean = 22 ± 17%), was significantly higher than that of burnt fruit (mean = 3.0 ± 2.0%, p < 0.001). Fire during fruiting or pre-dispersion decreases seed germination from 22 to 3%, but it does not hurt vegetative regeneration or resprout capacity of C. parviflora, which is a facultative seeder. Hence, we suggest that C. parviflora has potential for post-fire restoration in floodable open grassy savannas, in the ecotone between Cerrado and Pantanal, because this species may sprout quickly after first post-fire rains.     

Demographic consequences of delayed germination in two annual grasses from two locations of contrasting aridity

Delayed seed germination is considered to be a bet-hedging strategy, but experimental evidence of its adaptive role as an inherited trait is still lacking. In each of two co-occuring annual grass species, populations of Mediterranean and desert origin were studied during three consecutive years for population demography and seed germination in the reciprocally introduced experimental soil seed banks. The two environments strikingly differed in productivity (annual rainfall) and predictability (variation in amount and timing of annual rainfall). The two species exhibited highly similar pattern of seed size and dormancy across the two environments. In both species, a higher proportion of dormant seeds was observed at the desert location and for the seeds of desert origin, consistent with bet-hedging buffering against unpredictability of rainfall and high probability of drought in this environment. In addition, in both species seed mass was significantly less in plants of desert origin than in plants of Mediterranean origin. The two environments differed in demographic consequences of temporal variation in precipitation. In the Mediterranean population, even in the year of least precipitation, adults grew to maturity and seeds were produced. These seeds served to maintain population size. In contrast, in the desert population, in the year of least rainfall no seedlings survived to maturity and the soil seed bank was the only source of population persistence. Altogether, the results concur with predicted by adaptive bet hedging importance of delayed germination under marginal precipitation.     

Seed heat tolerance and germination of six legume species native to a fire-prone longleaf pine forest

Recognition of spatial heterogeneity of fire at fine scales is emerging, particularly in ecosystems characterized by frequent, low-intensity fire regimes. Differences in heat flux associated with variation in fuel and moisture conditions create microsites that affect survivorship and establishment of species. We studied the mechanisms by which fire affects seed germination using exposure of seeds to fire surrogates (moist and dry heat). Tolerance (survival) and germination responses of six perennial, herbaceous legume species common to the fire-prone longleaf pine–wiregrass ecosystem of the southeastern USA were examined the following heat treatments. Moist heat was more effective in stimulating germination than dry heat flux for most species examined. We also compared intrinsic seed properties (relative seed coat hardness, percent moisture, and seed mass) among species relative to their heat tolerance and heat-stimulated germination responses. Seed coat hardness was closely associated with the probability of dry and moist heat-stimulated germination. Variation among species in optimal germination conditions and degree of heat tolerance likely reflects selection for specific microsites among a potentially diverse suite of conditions associated with a low-intensity fire regime. Fire-stimulated germination, coupled with characteristics of seed dormancy and longevity in the soil, likely fosters favorable recruitment opportunities in restoration situations aimed at reintroducing a frequently prescribed burn regime to a relict longleaf pine site. In a restoration context in which externally available seed pool inputs are limited, this regenerative mechanism may provide a significant source of recruitment for vegetative recovery in a post-fire landscape.     

Antelope ingestion enhances germination of the marula (Sclerocarya birrea), an important African savannah tree

The marula (Sclerocarya birrea Hochst.) is an important forage and fruit tree in African savannahs. This study compared germination rate (days to germination) and success (percentage of stones that produced seedlings) among an intact control and four treatments, where fruits were (i) ingested by antelope (Cephalophinae and Neotragini), (ii) manually depulped, (iii) manually depulped and burnt and (iv) burnt intact. Measurements on three unrelated trees showed that whilst stone size differed significantly, germination success was comparable. Antelope regurgitated and expelled stones during rumination, within 16 h of ingestion. Seedling emergence commenced approximately 6 months after fruit drop when ambient temperature increased. Removal of fruit pulp increased germination rate and germination success, but moderate exposure to fire inhibited germination, especially following depulping. Germinated seeds were from significantly smaller stones than ungerminated seeds, suggesting that thicker‐walled endocarps inhibited seedling emergence. However, germination of the second seed in a stone was from larger stones in the germinated subset, possibly due to larger seed size. That antelope ingestion significantly enhanced germination over other treatments suggests that endozoochory is an important mode of seed dispersal in marulas. Appropriate fire management is therefore required in savannahs, as high‐intensity fires may limit germination and recruitment of marulas.     

Seed longevity and fire: germination responses of an exotic perennial herb in NW Patagonian grasslands (Argentina)

Fire affects grassland composition by selectively influencing recruitment. Some exotic species can increase their abundance as a consequence of fire-stimulated seed germination, but response may depend on seed age. Rumex acetosella L. (Polygonaceae, sheep's sorrel) is a cosmopolitan herb that has invaded NW Patagonia's grasslands. This species forms persistent soil seed banks and increases after disturbances, particularly fire. We studied how fire and seed longevity influence R. acetosella germination. In 2008, we conducted laboratory experiments where we exposed different-aged seeds (up to 19 years old) to heat, smoke, charcoal, ash and control treatments. Total percentage germination and mean germination time depended on both seed age and fire treatment. Germination of younger seeds decreased with increasing temperature. There was no general pattern in germination responses of different-aged seeds to smoke, charcoal and ash. While smoke improved the germination of fresh seeds, charcoal decreased germination. Germination of untreated seeds was negatively correlated with seed age, and mean germination time increased with seed age. In most treatments, fresh seeds had lower germination than 1-5-year-old seeds, indicating an after-ripening requirement. Smoke stimulates R. acetosella germination, causing successful recruitment during post-fire conditions. Fresh seeds are particularly responsive to fire factors, possibly because they have not experienced physical degradation and are more receptive to environmental stimuli. Knowing the colonisation potential from the soil seed bank of this species during post-fire conditions will allow us to predict their impact on native communities.     

Seed germination and seedling growth of two Pseudobombax species (Malvaceae) with contrasting habitats from Brazilian Cerrado

Pseudobombax tomentosum and P. longiflorum are common trees in the Cerrado region, but the former species is more common in forest edges while the later is present in open cerrado areas. This work aimed to investigate differences in seed germination and seedling growth in these species, from seed collected from Cerrado areas in Central Brazil. For this, a seed germination experiment was designed and included four replicates with 25 seeds per species; seeds were randomly distributed in the germination chamber. To evaluate initial seedling growth, seedlings height was measured up to 67 days after seedling emergence; besides, some of these seedlings were grown for biomass evaluation during nine months. Results showed that seeds of the two species had the same germinability (near 100%) and mean germination time (ca. 12 days). However, P. longiflorum showed a more spread seed germination through time, with higher values of coefficient of variation in germination time and uncertainty index; and lower values of synchronization than P. tomentosum. The two species showed basically the same growth pattern, but lower values for height of apical meristem, diameter of underground structures (mostly roots), dry mass of shoots, underground structure and total mass of seedlings in P. tomentosum were obtained, compared to P. longiflorum. Both species allocated more dry mass to underground structures in detriment of shoot. This probably allows resprouting behavior which prevents hydric stress and detrimental fire action typical of the open Cerrado areas.