Fire season and intensity affect shrub recruitment in temperate sclerophyllous woodlands

The season in which a fire occurs may regulate plant seedling recruitment because of: (1) the interaction of season and intensity of fire and the temperature requirements for seed release, germination and growth; (2) post-fire rainfall and temperature patterns affecting germination; (3) the interaction of post-fire germination conditions and competition from surrounding vegetation; and (4) the interaction of post-fire germination conditions and seed predators and/or seedling herbivores. This study examined the effects of different fire intensities and fire seasons on the emergence and survival of shrubs representing a range of fire response syndromes from a summer rainfall cool climate region. Replicated experimental burns were conducted in two seasons (spring and autumn) in 2 consecutive years and fuel loads were increased to examine the effects of fire intensity (low intensity and moderate intensity). Post-fire watering treatments partitioned the effects of seasonal temperature from soil moisture. Higher intensity fires resulted in enhanced seedling emergence for hard-seeded species but rarely influenced survival. Spring fires enhanced seedling emergence across all functional groups. Reduced autumn recruitment was related to seasonal temperature inhibiting germination rather than a lack of soil moisture or competition. In Mediterranean-type climate regions, seedling emergence has been related to post-fire rainfall and exposure of seeds to seed predators. We think a similar model may operate in temperate summer rainfall regions where cold-induced dormancy over winter exposes seeds to predators for a longer time and subsequently results in recruitment failure. Our results support the theory that the effect of fire season is more predictable where there are strong seasonal patterns in climate. In this study seasonal temperature rather than rainfall appears to be more influential.     

The effects of date of planting on field establishment of serotinous cape Proteaceae

Season of fire have marked effects on the germination and establishment of serotinous shrubs of the family Proteaceae in fynbos vegetation. To investigate reasons for this, we simulated the effects of different fire seasons by planting seeds into cleared fynbos and then followed their progress. Four species of Proteaceae were planted monthly at four sites over two and a half years. Exclosures were used to exclude rodent seed predators. Germination was confined largely to the three winter months (June–Aug.). Seeds planted from January–June had higher germination than those planted in the second half of the year. Higher levels of regeneration noted after fires in the first half of the year, were previously hypothesised to be results of predation. However, we obtained similar results despite the exclusion of seed predators. Monthly minimum temperature was strongly correlated with germination percentage but monthly rainfall was not. Loss of seed viability may be important, in determining post-fire seedling densities. Differential seedling mortality of earlier and late germinants appears to be unimportant in determining establishment levels. Our results nevertheless support the current practice of restricting management fires in fynbos to the summer-autumn period.     

Seed predation limits post-fire recruitment in the waratah (<Emphasis Type="Italic">Telopea speciosissima</Emphasis>)

Seed predation may reduce recruitment in populations that are limited by the availability of seeds rather than microsites. Fires increase the availability of both seeds and microsites, but in plants that lack a soil- or canopy-stored seed bank, post-fire recruitment is often delayed compared to the majority of species. Pyrogenic flowering species, such as Telopea speciosissima, release their non-dormant seeds more than 1 year after fire, by which time seed predation and the availability of microsites may differ from that experienced by plants recruiting soon after fire. I assessed the role of post-dispersal seed predation in limiting seedling establishment after fire in T. speciosissima, in southeastern Australia. Using a seed-planting experiment, I manipulated vertebrate access to seeds and the combined cover of litter and vegetation within experimental microsites in the 2 years of natural seed fall after a fire. Losses to vertebrate and invertebrate seed predators were rapid and substantial, with 50% of seeds consumed after 2 months in exposed locations and after 5 months when vertebrates were excluded. After 7 months, only 6% of seeds or seedlings survived, even where vertebrates were excluded. Removing litter and vegetation increased the likelihood of seed predation by vertebrates, but had little influence on losses due to invertebrates. Microsites with high-density vegetation and litter cover were more likely to have seed survival or germination than microsites with low-density cover. Recruitment in pyrogenic flowering species may depend upon the release of seeds into locations where dense cover may allow them to escape from vertebrate predators. Even here, conditions suitable for germination must occur soon after seed release for seeds to escape from invertebrate predators. Seed production will also affect recruitment after any one fire, while the ability of some juvenile and most adult plants to resprout after fire buffers populations against rapid declines when there is little successful recruitment.     

Effects of seed dispersal on Juniperus communis recruitment on a Mediterranean mountain

Abstract. The recruitment of the relict shrub Juniperus communis on a mountain in SE Spain was studied during the period 1994–1998. The main objective was to determine both the quantitative and qualitative effects of bird dispersal on seedling establishment. Seed removal by birds, seed rain, post‐dispersal seed predation, germination, and seedling emergence and survival were analysed in different microhabitats. Birds removed 53 ‐ 89% of the seeds produced by plants. Seed rain was spatially irregular as most seeds accumulated near stones used by birds as perches and below mother plants while a few seeds were dropped in wet meadows and open ground areas. Post‐dispersal seed predation by rodents affected < 10% of dispersed seeds but varied significantly among microhabitats. Only 3.6 ‐ 5.5% of dispersed seeds appeared viable, as many seeds had aborted or showed wasp damage. Seeds germinated in the second and third springs after sowing, reaching a germination percentage of 36%. Seedling emergence was concentrated in wet meadows. Seedling mortality was high (75–80%), but significantly lower in wet meadows, the only microhabitat where seedlings could escape from summer drought, the main mortality cause. Seed abortion, germination and seedling mortality proved to be the main regeneration constraints of J. communis on Mediterranean mountains. Birds exerted a strong demographic effect, although their qualitative effect was limited by abiotic factors which caused the pattern of seed rain to differ from the final pattern of recruitment between microhabitats.     

Seasonal cycle of seed dormancy controls the recruitment of Butia odorata (ARECACEAE) seedlings in savanna‐like palm tree formations in southern Brazil

Butia odorata (Barb. Rodr.) Noblick is a palm tree that grows in savanna‐like formations in subtropical regions of South America, and whose regeneration is threatened by agricultural management. Its diaspores are dormant after dispersal which takes place during the summer and early autumn. The aim of this study was to investigate seasonal and microhabitat effects on the germination and seedling recruitment of this palm species. Diaspores were sown in the field, in both open lands and forest patches. During 2 years, we measured seed germination, viability and moisture, seedling emergence and germination response to warm stratification of those seeds that failed to germinate in the field. Germination was concentrated during the summer, when soil temperatures were highest, whilst seedling emergence peaked in the autumn and early winter, when temperature and humidity conditions became less extreme. In open lands, there were two pulses of germination (first and second summer), whilst in forest patches, a single pulse (second summer) was detected. Although overall germination did not differ between microhabitats, the percentage of seedling emergence from seeds that remained buried until the end of the experiment was almost twice as large in the forest patches compared with open areas. The viability of seeds declined over time, particularly in open areas. Laboratory‐induced warm stratification was found to act on seed dormancy release in a cyclic way, being far more effective on seeds retrieved from the field in spring–summer months than in those retrieved in the winter. This cyclic pattern of dormancy in B. odorata seeds results in major seedling recruitment after the summer, under wetter and cooler conditions, thus reducing mortality risk. This process can be enhanced by the presence of surrounding vegetation, which both increases seedling emergence and/or prolongs seed viability.     

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.     

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

Field experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface‐sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment.     

Comparative seed biology and co-existence of two fynbos shrub species

Abstract. Fire-prone fynbos communities include numerous species which are difficult to distinguish in terms of conventional niche axes since they are morphologically similar and share the same broad life-history traits. These species compete for space after each fire and lottery theory predicts that those with dissimilar per capita regeneration would not co-exist in the long-term. We studied seed production, soil seed bank dynamics, germination and post-fire seedling establishment and mortality of two obligate reseeding, morphologically similar shrubs which co-occur in dune fynbos throughout the southwestern Cape. Passerina paleacea produced 30 x more plump seeds and maintained soil seed banks between 1.5 x (pre-dispersal) and 2.6 x (post-dispersal) times larger than Phylica ericoides. Seeds of both species showed a high degree of dormancy although germination was stimulated by direct fire-related cues for Phylica only. Approximately 4 x more Passerina seedlings than Phylica seedlings were counted at the end of the winter germination period after a mild autumn fire. However, seedling mortality in the first five months of the dry summer was more than twice as high for Passerina than for Phylica. We suggest that the long-term co-existence of these two species does not result from similar post-fire regeneration success but rather from population instabilities arising from differential regeneration in relation to fire regime. The results predict that hot fires followed by dry summers would favour Phylica over Passerina as the former has fire-stimulated germination and greater drought tolerance; cool fires followed by moist summers would favour Passerina.     

Fire season effects on the recruitment of non‐sprouting serotinous Proteaceae in the eastern (bimodal rainfall) fynbos biome,South Africa

Abstract Research in Mediterranean‐climate shrublands in both South Africa and Australia shows that recruitment of proteoid shrubs (non‐sprouting, serotinous Proteaceae) is best after warm‐season (summer and autumn) fires and worst after cool‐season (winter and spring) ones. This pattern has been attributed to post‐dispersal seed attrition as well as size of pre‐dispersal seed reserves. Here we investigate patterns of post‐fire recruitment for four proteoid species in the eastern part of South Africa's fynbos biome, which has a bimodal (spring and autumn) rainfall regime. Despite the lack of significant differences in recruitment between cool‐ and warm‐season burns, we find some evidence for favourable recruitment periods following fires in spring and autumn, immediately before, and coinciding with, the bimodal rainfall peaks. This suggests that enhanced recruitment is associated with conditions of high soil moisture immediately after the fire, and that rapid germination may minimize post‐dispersal seed attrition. In two of the species, we also find a shift from peak flowering in winter and spring in the Mediterranean‐climate part of the fynbos biome, to summer and autumn flowering in the eastern part. Because these two species are only weakly serotinous, warm‐season flowering would result in maximal seed banks in spring, which could explain the spring recruitment peak, but not the autumn one. We conclude that eastern recruitment patterns differ significantly from those observed in the western and central parts of the biome, and that fire management protocols for the east, which are currently based on data and experience from the winter‐rainfall fynbos biome, need to be adjusted accordingly. Fire managers in the eastern fynbos biome should be less constrained by requirements to burn within a narrow seasonal range, and should therefore be in a better position to apply the required management burns.     

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.     

Effects of seed burial and fire on seedling and sapling recruitment, survival and growth of African savanna woody plant species

Seed predation and seedling mortality can act as strong demographic “bottlenecks” to sapling recruitment in African savanna woodlands. Fire also limits tree recruitment from saplings by suppressing their growth. I conducted field experiments with 13 woody plant species to assess the effects of seed burial on seedling emergence rates and effects of fire on seedling and sapling survival and growth rates over a period of 8 years at a savanna plot in central Zambia, southern Africa. Seed removal rates by small rodents varied among years and buried seeds had significantly higher emergence rates than seeds exposed to predators in most but not all the species. Annual burning reduced sapling growth in some species but in other species saplings experienced successive shoot die back even in the absence of fire. The findings show that for some woody species, seed predation is an important constraint to seedling recruitment but not for others and annual fires are important hindrances to demography and growth for some species but not others. Thus, demographic “bottlenecks” occur at different life history stages in different savanna woody species and these have the potential to alter woody tree competitive relationships and ultimately savanna structure.     

Recruitment limitation along disturbance gradients in river flood plains

Abstract. Question: Along river floodplains lower distribution limits of plant species seem largely determined by their tolerance to rarely occurring floods in the growing season. Such distribution patterns remain fixed for many years suggesting additional effects of winter floods at lower positions. Our objective was to investigate the direct and indirect effects of winter floods on colonization of floodplains in a series of field experiments. Location: River Rhine, The Netherlands. Methods: We measured the direct effects of winter floods on seedling survival and seed removal and survival at low and high floodplain elevation. Indirect effects of winter flooding through changes in the soil were investigated by measuring seedling emergence on soil transplants that were exchanged between high and low floodplain elevation. To investigate indirect effects of floods on the germination environment through changes in the vegetation structure, we measured the effects of vegetation removal on recruitment of sown species. Results: Recruitment was seed limited at both floodplain elevations. An additional effect of vegetation removal on seedling emergence was also observed. Soil types from both zones did not differently affect seedling emergence. Seeds were not removed from the soil surface by a single winter flood. Moreover, seeds remained viable in the soil for at least two years, while the experimental plots were flooded several times during the experimental period. During one of those floods a thick sand layer was deposited at the low zone and subsequently no seedlings were observed anymore. Conclusions: Colonization of low floodplain zones in years between subsequent summer floods is prevented by seed limitation while the direct effects of winter floods are limited except for irregularly occurring sand depositions.     

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.     

Effects of soil disturbance,fire and litter accumulation on the establishment of Cistus clusii seedlings

Cistus clusii is an early successional shrub, the most drought-resistant species of Cistus which colonises perturbed areas in the southern Iberian Peninsula and regenerates entirely from seeds after a fire. Some of the factors controlling germination and seedling establishment were examined in a field experiment in which we tested the effect of soil disturbance, fire, and litter accumulation on seedling emergence. In a first experiment, soil disturbance and fire were applied to plots in the field with a factorial design in April 1992. In a second experiment, we measured the effect of adding C. clusii litter. The number of seedlings, annual plants and grasses, and the coverage of mosses were recorded in 1993 and 1994.Soil disturbance increased the number of C. clusii seedlings almost seven-fold but did not change the number of grasses, annual plants nor moss cover. Fire significantly increased the number of C. clusii seedlings only in combination with soil disturbance, and it did not affect the presence of annuals, grasses nor moss cover. Litter increased the number of C. clusii seedlings in the following two years.Overall, soil disturbance as a single factor had the most significant effect on seedling emergence, though plots disturbed and treated with fire had the highest number of seedlings, implying that germination of C. clusii seeds was enhanced by processes that alter the hard seed coat and break the physical dormancy imposed by the testa. In addition, recruitment of C. clusii was dependent on rainfall, as drought significantly reduced the number of seedlings appearing in winter.     

Flooding and grazing promote germination and seedling establishment in the perennial grass Paspalum dilatatum

Seed germination and seedling emergence are key processes for population recruitment. Flooding and grazing are disturbances forming gaps that may strongly influence recruitment patterns in space and time, but their combined effects and action mechanisms have rarely been addressed. In this study we analysed the effects of microhabitat conditions associated with winter flooding and spring‐summer defoliation on seed germination and seedling establishment of Paspalum dilatatum, a dominant perennial C₄ grass in native grasslands of the Flooding Pampa, Argentina. The dynamics of seedling emergence from natural seed banks and buried seeds was studied in a factorial experiment with flooding and defoliation treatments applied to soil monoliths (mesocosms) collected from natural grassland. Additional laboratory experiments were applied to investigate seed germination under different combinations of temperature, light quality and simulated flooding. Seed germination and seedling emergence of P. dilatatum were promoted by flooding and high intensity defoliation. Gaps generated by flooding were maintained by high intensity defoliation exercising a synergistic effect on survival seedlings. Flooding resulted in the breaking of seed dormancy and higher germination rates associated with alternating temperature and the activation of the phytochrome system. Our results indicate that microhabitat conditions associated with the disturbances forming gaps, such as flooding and heavy grazing, synergistically promote the recruitment process of this dominant grass species.     

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.     

Impact of fire on Picea mariana and Pinus banksiana seedlings in subarctic lichen woodlands

Abstract. The demography of Picea mariana (black spruce) and Pinus banksiana (jack pine) seedlings was monitored through five censuses over 13 months in four different seed bed types after fire of four severity levels in lichen woodland. Most seeds germinated just before early frost in late summer 1990 or immediately after snow thaw in early spring 1991; the germination rate subsequently decreased. For both species, germination rate decreased along a gradient of fire severity. The proportion of Pinus seeds that produced a seedling surviving 13 months after sowing was 4.3 % and 0.4 % respectively in the intact lichen mat and in the mineral soil seed bed type. For Picea these values are 3.2 % and 0.2 % respectively. The low germination rate in a severely burned seed bed type appeared to be associated with the formation of a water-repellent crust at the soil surface following the fire. Seedlings were contagiously distributed and were more frequent in flat and hollow microsites, where there is probably more water available than on bumps or among pebbles. Properties of experimentally burned seed bed types may differ from those under natural fires where regeneration by seed generally occurs following dispersal. However, the high germination rate observed in the intact lichen mat suggests that scattered lichen woodland patches may respond to increased seed input by a higher frequency of seedling establishment.     

Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m⁻²), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m⁻²). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence. Received: 24 October 1996 / Accepted: 4 February 1997     

Seedling emergence patterns and dormancy/germination physiology of <Emphasis Type="Italic">Primula modesta</Emphasis> in a subalpine region

On the basis of the germination/dormancy responses of seeds to temperature and light, and local seed rain, we attempted to interpret the seedling emergence patterns of Primula modesta Bisset et Moore (Primulaceae) in two different types of habitats in a subalpine zone of Mt. Asama: an oligotrophic flat moor and a grassland with relatively dense herbaceous vegetation cover. The seasonal pattern of seedling emergence was well explained by the dormancy/germination physiology revealed in laboratory germination tests. The seeds were demonstrated to have a strict light requirement even after experiencing moist chilling, which might facilitate the incorporation of the seeds into the soil seed bank. Despite sufficient seed production, the seedlings emerging were far less at the grassy site than the moor site, but the number of seedlings was significantly dependent on the seed rain within previous season and on the litter cover of the microsite in both sites. Therefore, the spatiotemporal patterns of seedling emergence in the habitats could be well explained by the spatiotemporal patterns of seed rain and safe-sites for germination.     

Multiple causes of seedling rarity in scrub plum, Prunus geniculata (Rosaceae), an endangered shrub of the Florida scrub

Conservation of an imperiled plant often requires an understanding of its reproductive ecology. Scrub plum (Prunus geniculata) is an endangered Florida shrub endemic to pyrogenic xeric uplands. Although plants are long-lived and may flower profusely, particularly after burning, fruit yield is sparse and seedlings are rare. We investigated potential causes of seedling rarity in scrub plum by studying its floral sex and breeding systems, fruit development, and germination ecology. We also developed a flow chart based on two time-since-fire scenarios to illustrate the cumulative impact of these factors on seedling recruitment. Breeding system experiments suggested partial gametophytic self-incompatibility in this functionally andromonoecious species, with inbreeding depression in self-compatible individuals. Predispersal seed predation, the major constraint on seedling recruitment, was lower in the first postburn season. In two field experiments, seed germination was <11% irrespective of treatment. These results indicate that seedling recruitment in scrub plum may be limited by multiple factors, including sexual reproductive failure, high predispersal seed predation, and low seed germination. The chance of a bisexual flower producing a seedling ranges from 5 in 1000 to 5 in 100000, depending on the time since fire. Restoration of scrub plum's historical fire regime may mitigate some of these factors.