Seed predation,disease and germination on landslides in Neotropical lower montane wet forest

Abstract. Seed mortality (caused by predators and pathogens) and germination were compared between Puerto Rico and Costa Rica on landslides in lower montane wet forest. Seeds of six common species on five Puerto Rican landslides and four common species on two Costa Rican landslides were used with a Cecropia species and a Gonzalagunia species included at both sites. In the Puerto Rican experiments Cecropia schreberiana was the only species to show significant seed predation (which was due to insects), pathogens grew from all species and fewer seeds were lost to predators than pathogens. Also in Puerto Rico mean germination across all species was 57 % before dispersal (filled seeds collected while still on the tree) and 71 % after, with Phytolacca rivinoides seeds germinating most abundantly, followed in descending order by Ocotea leucoxylon, Cecropia spec, Miconia racemosa, Palicourea riparia and Gonzalagunia spicata. In the Costa Rican experiments three species had significant predation: Cecropia polyphlebia and Urera caracasana (both due to insects) and Witheringia coccoloboides (due to mammals); pathogenic disease caused more seed loss than predation, and germination was high (61 % pre-dispersal, 69 % post-dispersal). Similarities between these island and mainland sites included (1) percentage of seeds lost to predation and percentage lost to pathogens (all in the 5–15 % range), (2) generalist pathogens which claimed more seeds than predators and (3) majority germination with a general increase after dispersal. Finally sites were dissimilar only in the number of species with significant predation loss and whether it was by insects or mammals, casting doubt on the traditional island/mainland dichotomy.     

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.     

POPULATION BIOLOGY OF BONAMIA GRANDIFLORA (CONVOLVULACEAE): EFFECTS OF FIRE ON PLANT AND SEED BANK DYNAMICS

The effects of disturbance on reproduction and plant and seed bank dynamics in the perennial herb Bonamia grandiflora were studied by comparing populations in recently burned, mechanically disturbed, and undisturbed habitats in central Florida over a 3-year period. Plant densities, seed production, and the occurrence of herbivory and predispersal seed predation varied considerably between sites and between years, with recently disturbed sites supporting the densest and most dynamic populations. Death of established plants was rare in all sites. In each site, the soil seed bank was several-fold larger than single season seed rains suggesting that B. grandiflora seeds are long-lived and accumulate in the soil. There was no evidence that postdispersal predation or pathogens have any significant influence on the seed bank dynamics. Fire resulted in large increases in stem densities due to both increased clonal stem production and new genet recruitment from seed. Burning also caused significant increases in the percentage of flowers producing seed and the numbers of capsules and seeds per plant. The seed rain was ten to thirty times greater in the burned site relative to adjacent unburned site during the 3 years after burning. However, additions to the seed bank from the postfire seed rain were balanced by equivalent losses due primarily to seed mortality during fire, and to a much lesser extent due to germination and new genet establishment. As a result, the subsequent densities of seeds stored in the soil in these two sites were similar, indicating that fire results in a significant turnover in the seed bank population but no immediate change in its size. These effects on seed bank dynamics, in addition to new genet recruitment, suggest that periodic fires may play an important role in the maintenance of genetic variability as well as the size of these populations.     

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.     

Intensity of pre‐dispersal seed predation in the invasive legume Leucaena leucocephala is limited by the duration of pod retention

Abstract The intensity of seed predation the invasive tropical legume Leucaena leucocephala by the bruchid Acanthoscelides macropthalmus was investigated in south‐eastern Queensland, Australia. The number of seeds damaged by A. macropthalmus as a proportion of total seeds available was found to increase the longer the pods remained on the tree. Seed predation ranged from a mean of 10.75% of seeds on pods that remained on the plant for 1 month and increased to 53.54% for pods that remained of the plant for 4 months. The low bruchid populations at high pod densities results in ‘predator satiation’. However, pods dehisce over time and the proportion of pods available over time to the bruchid correspondingly declines. By the time bruchid densities build up, most pods have dehisced and the seeds consequently escape predation. As a result the number of seeds lost to bruchid damage increases only marginally over time. Despite the levels of seed predation observed over the course of the study, the number of seeds in the soil seedbank almost doubled over time increasing from 8.5 seeds m^?3 to 15.5 seeds m^?3 over a 4‐month period. Levels of seed predation and addition of seeds to the soil seedbank were not correlated. The taxonomic (subspecies) status and apparency of host plants as measured by plant and patch traits (average plant height, density of podding plants and patch size) did not influence levels of seed predation. Pre‐dispersal seed predation studies need to take into account the pod/seed retention behaviour of the plant. The ability of the bruchid to regulate the invasiveness of Leucaena through influencing its demography is likely to be diminished if the insect populations cannot increase rapidly enough to use the seeds before pod dehiscence.     

茂兰喀斯特森林中华蚊母树群落土壤种子库动态初探

 茂兰喀斯特森林中华蚊母树群落900m2样地内共有种子植物59种,其中19种主要植物已繁殖产生种子。群落每年以种子雨形式向种子库输入种子351.1粒·m-2,其中对群落更新有用的成熟有效种子150.8粒·m-2。土壤种子库中活力种子丰富,共有41种2510.5～2646.8粒·m-2。种子库中活力种子的种类和数量均随时间的变化而变化,不同时间种子库的差异主要在于现存植物活力种子的变化。土壤种子库每年输出活力种子171.9粒·m-2,其中萌发输出21.9粒·m-2,动物采食输出133.5粒·m-2,病菌危害引起的输出为3.2粒·m-2,种子自然衰老引起的输出为13.3粒·m-2。每年输出超过输入21.1粒·m-2。演替前期植物种子没有输入,只有缓慢输出,但没有萌发输出。     

Seed dynamics during forest succession in Costa Rica

Soil seed banks and current seed inputs each play a role in tropical succession. We compared the abundance and floristic composition of seeds from these two sources at a Costa Rican site by germinating seeds from the soil, measuring seed inputs for 3 yr, and monitoring the earliest colonists in a forest clearing.There were an estimated 6800 viable seeds/m² in the soil of 3.3-yr-old vegetation, 9500 seeds/m² in 11-yr-old vegetation, and 7000 seeds/m² in a 75-yr-old forest. An estimated 10100 seeds/m² fell on the soil surface of the young successional vegetation during 3 yr and 3700 seeds/m² fell during that same time in the forest.Locally produced seeds accounted for about 75% of the seed input to the soil surface early in succession. Seeds dispersed out of young successional vegetation increased the quantity and species richness of the seed input and storage in an adjacent forest. Much of the species richness of the young successional vegetation resulted from seeds dispersed there from other communities by animals.Deforestation stimulated germination of most seeds in the surface soil of the old forest, including seeds of the dominant canopy tree. The recruitment of seedlings from the soil seed bank numerically overwhelmed that from post-disturbance seed rain and sprouts.We evaluated patterns of soil seed storage during succession and predicted the ability of vegetation of differing ages to respond to disturbance. Immediately after disturbance the number of seeds in the soil plummeted due to mortality, low inputs, and germination. As the vegetation regrew, the soil seed bank increased to a peak after 4 to 7 yr, then gradually decreased to its pre-disturbance size. High-frequency pulses of disturbance should result in reduced species richness, dominance by species with long-lived seeds, and fast recovery by seedling recruitment from the soil seed bank.Journal series number 6459 from the Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611, USA.Reprint requests to J. J. E. at Florida.     

Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level

Seed predation may cause important seed losses in plant populations, but its impact on the dynamics of populations will depend on the degree of seed or microsite limitations for recruitment. Seed losses will only affect recruitment if it is seed limited. The nature of recruitment limitation (seeds or microsites) is usually ascribed to whole plant populations but it may vary within populations among microhabitats and habitats. Thus, the potential impact of seed predation will also vary within the population, being highest where recruitment is seed limited. The impact to the whole population will depend on the spatial concordance between the intensity of seed predation and that of seed limitation. Recruitment limitations (with seed addition experiments), seed predation (with seed removal experiments), and the dynamics of seed availability in the soil (with soil samples taken both after seed dispersal and before the following dispersal event) of the shrub Corema album (Empetraceae) were investigated in dunes in NW Spain, at microhabitats ‘open ground’, ‘underneath C. album ♀’, and ‘underneath C. album ♂’ at two habitats, sparse and dense scrub. The nature of recruitment limitation (seeds vs. microsites) varied within the population. It was seed limited in the microhabitat ‘open ground’ and microsite limited under shrub cover. The spatial patterns of seedling recruitment were unrelated to seed availability but strongly affected by germination requirements. The spatial discordance between seed availability and recruitment implies a crucial constraint for processes affecting seed availability (seed predation but also e.g., dispersal) to impact recruitment. They will not affect its spatial pattern but only its quantity as long as they act in those sites selected by seeds to germinate. Seed predation was highest underneath mother plants and lowest in open ground. Thus, its potential impact is low, as it is centred where recruitment is not seed limited. This study shows that the analysis of seed predation in relation to recruitment limitations at smaller spatial scales within the population provides more insight to understand its impact.     

Spatial dynamics of <Emphasis Type="Italic">Ilex aquifolium</Emphasis> populations seed dispersal and seed bank: understanding the first steps of regeneration

The objective of this study was to analyse quantitatively the spatial distribution of holly (Ilex aquifolium L.) seed rain and seed bank, and to detect the relationships between these consecutive processes. We measured seed dispersal by birds and fallen fruits, and also density and viability of seed bank in two Ilex populations in central Spain. Analysis was made distinguishing the following microhabitats: holly woodland, edge of holly woodland, open grassland 10 m and 100 m from the woodland, fleshy fruit shrubs, dry fruit shrubs, and adjacent non-holly woodland. Spatial distribution of dispersed and in-soil seeds was measured by the clumping index. Seed rain and seed bank under holly woodlands were significantly higher than in the other microhabitats. Forest edges and fleshy fruit shrubs were the next microhabitats with the highest seed rain and seed bank density. Interannual and interlocality variations were not significant. The relative importance of the different dispersal methods varied between microhabitats, with a similar support of bird dispersed seeds and fallen fruits within the woodland and a greater influence of cattle dispersal in open areas. Seed spatial aggregation was significant in both dispersed seeds and soil seeds from holly woodlands and the edge of the forest. Aggregation under shrubs, grasslands and the adjacent forests evidenced a general random distribution of holly seeds (only in some cases clumping index was significant). Quantitative differences between seed rain and seed bank are important. Post-dispersal seed predation did not modify seed rain distribution, which was mirrored in the seed bank pattern. These two phases of holly regeneration had a heavy spatial influence, determined by the landscape structure and activity of the dispersal agents, that reflects a differential recruitment potential. Comparisons between both populations suggest that in the southern locality (Robregordo) holly has a weaker capacity to colonize open areas, and a stronger recruitment limitation due to propagule availability.     

Seed dormancy and longevity in <Emphasis Type="Italic">Stipa tenacissima</Emphasis> L. (Poaceae)

In the present paper we studied the life history traits related to seed germination of Stipa tenacissima, a key species in semiarid environments of western Mediterranean areas. S. tenacissima is a perennial tussock grass, which has traditionally been considered to expand mainly by vegetative propagation with little or no sexual reproduction. We analysed seed longevity as well as the type of seed dormancy and the role of the seed covers from seeds collected from different populations in SE Spain. We also studied the variation in seed germinability among populations, individuals, and years and the ability of seeds of S. tenacissima to form soil seed banks. There was significant variation in seed germination among individuals, populations and years. Lemma and palea were the main factor controlling these differences since their removal promoted higher and faster germination and eliminated the differences in germination parameters among populations. However, the control of dormancy by lemma and palea was independent of their weight, suggesting that their chemical nature plays a more important role than does size in controlling seed germination. Mechanical scarification treatments (via abrasion with sand) did not affect seed germination. The decay in seed germinability two years after seed collection and the low density of viable seeds in soils one year after seed dispersal indicated that S. tenacissima forms transient soil seed banks.     

The impact of flooding and drought on seeds of Cnidium dubium, Gratiola officinalis, and Juncus atratus, three endangered perennial river corridor plants of Central European lowlands

The effects of flooding and drought on the post-flooding survival and germinability of seeds in the soil were investigated for three endangered river corridor angiosperms Cnidium dubium (Schkuhr) Thell., Gratiola officinalis L. and Juncus atratus Krocker. Freshly matured seeds were placed in small nylon bags and were buried in pots filled with soil collected from a wetland along the Lower Havel River. The pots were subjected to different winter/spring flooding treatments (30, 60, 120, and 180 days) and to drought for up to 3 years. Every year in spring, after drawdown, the seeds were exhumed, counted and tested for germination. Seed mortality of flooded seeds was low, except for C. dubium. Flooding did not affect germination of seeds of J. atratus. Flooding of seeds of C. dubium and G. officinalis had inconsistent, but no beneficial effects on germination of either. The year of harvest and the interannual variability of weather are found to be the main factors affecting germinability of C. dubium and G. officinalis in the course of the three consecutive years.The experiment showed that the capacity of the flooded seed bank to maintain germination ability and viability for extended periods of time could be directly related to the flooding duration of natural habitats of the three species. The three species are resilient to variable flooding. Seeds appeared desiccation tolerant, and single flooding events do not deplete their seed bank.     

土壤种子库的研究进展及若干研究热点

 土壤种子库是指存在于表层土壤（包括凋落物）中的有生命的种子。土壤种子库的研究已是植物生态学研究不可缺少的一部分,现已成为植物种群生态学中比较活跃的领域。土壤种子库时期是植物种群生活史的一个重要阶段,有人称之为潜种群阶段。土壤种子库对一年生植物来说尤其重要。土壤种子库简单地可分为瞬时土壤种子库和长久土壤种子库,即使给予理想的萌发条件如季节、温度、湿度等,土壤种子库中也仍有部分种子保持休眠状态,休眠的种子组成了土壤长久种子库的成分。时空异质性是土壤种子库的基本特性之一,不仅不同植被类型的土壤种子库具有不同的组成、大小和多样性,而且微环境也影响土壤种子库的分布格局。由于萌发、捕食和衰老等原因,土壤种子库具有季节动态,一般在旧种子萌发之后,新种子散布之前达到最低点。在高等植物占据的大多数生境中,以休眠繁殖体形式存在的个体远远超过地上植株的数量;土壤种子库、幼苗库和成年植被相互联系相互影响。由于各种原因如群落类型的差异、群落的演替阶段、取样的时间等,地上植被和土壤种子库之间关系大体上可分为两种情况,即相似性和差异性;研究土壤种子库的方法通常有萌发法和物理分离法。土壤种子库能部分反应群落的历史,对退化生态系统的恢复起着重要的作用。目前土壤种子库的主要研究热点问题可分为以下几个方面：1）土壤种子库的研究方法,2）土壤种子库的分类问题,3）土壤种子库分布的时空格局,4）地上植被和土壤种子库的关系,5）土壤种子库的动态等。     

FACTORS INFLUENCING THE REPRODUCTIVE SUCCESS OF IPOMOEA PES-CAPRAE (CONVOLVULACEAE) AROUND THE GULF OF MEXICO

The reproductive biology of Ipomoea pes-caprae, a pantropical beach morning glory, was studied at five sites around the Gulf of Mexico. The primary pollinators were Xylocopa species (carpenter bees) which dispersed pollen up to 90 m. Exclusion experiments demonstrated that ants feeding on extrafloral nectaries increased seed set but did not protect seeds from predation by the bruchid beetle Megacerus. The water-dispersed seed and long-range dispersal of pollen may function to counter the sporophytic incompatability mechanism of the plant; populations exhibit a large neighborhood size. Key reproductive factors in the life cycle of I. pes-caprae are the long-range pollen flow and mass germination of water-dispersed seeds.     

Natural regeneration of Spanish black pine [Pinus nigra Arn. ssp. salzmannii (Dunal) Franco] at contrasting altitudes in a Mediterranean mountain area

Information on plant seed dispersal, natural loss dynamics of seeds and germination are critical for understanding natural regeneration mechanisms. The aim of this study was to assess the effect of different forest stand densities on seedfall, seed predation, and seedling germination of two populations of the endangered Spanish black Pine forests located at lower (Central population) and higher elevation near the limit of the species?? range (peripheral population) in the Cuenca Mountains of Central Spain. The seed predation and germination experiment also included a nested site preparation treatment. Seed fall varied significantly between 2006 and 2005 or 2007 in both populations. During the only mast year of 2006, higher seedfall was observed at lower elevation and in higher density stands. Predation rates were influenced by the seed crop since predators consumed more than 75?% of seeds in years with lower production and less than 15?% in a mast year. Seed germination is influenced by forest habitat, stand density and soil scalping. For common habitat types, and in a high seed production year, better seed germination rates were observed in medium and dense stands (25?C30 and 35?C40?m²?ha^?1, respectively, in terms of basal area). No statistical difference in seed germination rate was found for Spanish black pine forest at its ecological distribution limit between lower and higher densities (15?C20 and 35?C40?m²?ha^?1, in terms of basal area). In both sites, closed stands with soil scalping exhibited higher germination rates.     

Seed rain and soil seed bank of Picea asperata Mast. in subalpine coniferous forest of western Sichuan,China

Abstract

Dragon spruce (Picea asperata Mast.) is widely planted on clear-cuts from natural subalpine coniferous forests in western Sichuan. To assess the natural regeneration potential of this species in spruce plantations of different ages, field studies on the seed rain, seed bank, and seedling recruitment were conducted in 20-year, 30-year, and 60-year plantations, and in a retained natural forest ca. 150 years old for comparison. Moreover, a series of temperature and light regimes were also designed in March 2003 to test germination/dormancy responses of the P. asperata seeds to different conditions. In the plantations considered, both the densities of seed rain and soil seed bank increased with increasing stand age, whereas they were both low beneath the natural forest partially due to low adult density of P. asperata adult trees. P. asperata has a transient seed bank, and ca. 60% of seeds were found in the litter layer. Seed decay and seed predation were the two most important factors affecting soil seed bank dynamics, which caused a number of seed losses from the soil. Only a small fraction germinated and produced seedlings, and the indices of the losses from the seed bank via germination in the 30-year plantation, 60-year plantation and natural forest were 4.75, 5.10, and 2.80%, respectively. High seedling mortality was observed after seedling emergence, and most of the germinated seedlings died out within one growing season. The P. asperata seeds showed a high percent germination and no significant differences under most light and temperature regimes except for full sunlight or temperature below 5°C. In conclusion, despite a substantial number of seed produced, the high depletion of soil seed, the low seedling output, and high seedling mortality may obscure the natural regeneration potential of this tree species.     

The relationship between seed dormancy,seed size and weediness,in Crepis tectorum (Asteraceae)

Summary I examined the germination characteristics of weed and outcrop populations of Crepis tectorum to test the hypothesis that the presumably more ephemeral weed habitat favors the highest levels of seed dormancy. The winter annual habit characterizing most plants of this species was reflected in a rapid germination of seeds sown in late summer. A slightly higher fraction of surface-sown seeds of weed plants delayed germination. Buried seeds of weed plants also survived better than seeds produced by plants in most outcrop populations, supporting the idea that weediness favors seed dormancy and a persistent seed bank. However, the differences in seed dormancy between the two ecotypes were small and not entirely consistent. Furthermore, high levels of seed dormancy were induced during burial in the outcrop group, suggesting that there is a potential for a dormant seed population in this habitat as well. Demographic data from one of the outcrop populations verified the presence of a large between-year seed bank. Possible environmental factors favoring seed dormancy in outcrop populations are discussed. The unusually large seeds of weedy Crepis contrasts with the relatively small difference in seed dormancy between the two ecotypes.     

Seed dynamics of resprouting shrubs in grassy woodlands: Seed rain,predators and seed loss constrain recruitment potential

Abstract Measuring the fate of seeds between seed production and seedling establishment is critical in understanding mechanisms of recruitment limitation of plants. We examined seed fates to better understand the recruitment dynamics of four resprouting shrubs from two families (Fabaceae and Epacridaceae) in temperate grassy woodlands. We tested whether: (i) pre‐dispersal seed predation affected seed rain; (ii) post‐dispersal seed predation limited seed bank accumulation; (iii) the size of the seed bank was related to seed size; and (iv) viable seeds accumulated in the soil after seed rain. There was a distinct difference in seed production per plant between plant families with the legumes producing significantly more seeds per individual than the epacrids. Seed viability ranged from 43% to 81% and all viable had seed or fruit coat dormancy broken by heat or scarification. Pre‐dispersal predation by Lepidopteran larvae removed a large proportion of seed from the legume seed rain but not the epacrids. Four species of ants (Notoncus ectatomoides, Pheidole sp., Rhytidoponera tasmaniensis and Iridomyrmex purpureus) were major post‐dispersal seed removers. Overall, a greater percentage of Hardenbergia (38%) and Pultenaea (59%) seeds were removed than the fleshy fruits of Lissanthe (14%) or Melichrus (0%). Seed bank sizes were small (<15 seeds m^?2) relative to the seed rain and no significant accumulation of seed in the soil was detected. Lack of accumulation was attributed to seed predation as seed decay was considered unlikely and no seed germination was observed in our study sites. Our study suggests that seed predation is a key factor contributing to seed‐limited recruitment in grassy woodland shrubs by reducing the number of seeds stored in the soil.     

How important is seed predation to recruitment in stable populations of long-lived perennials?

Summary The many evolutionary modifications to seed biology in response to seed predation do not necessarily imply that seed predators have an important impact on population recruitment. This is because competition between individual plants for rare safe sites can cause an oversupply of seeds so far as a population is concerned. The importance of seed losses to population recruitment at any point in time is related to the abundance of safe sites (Fig. 1): it is zero when safe sites are absent, negligible when safe sites are rare, and greatest when safe sites are numerous enough for recruitment to be limited by seed supply. Here I interpret the impact of severe seed losses on population recruitment in four species of long-lived perennials (Eucalyptus baxteri, Leptospermum juniperinum, Casuarina pusilla and L. myrsinoides) by considering these losses in terms of the overall seed dynamics of the populations. I focus on seed supply and seedling survival, as a measure of the current abundance of safe sites, and the maintenance of seed banks, as a measure of the ability of populations to exploit any future changes in safe site abundance. Insect seed predators destroyed about 95% of total seeds in each case. However, these losses do not necessarily have an important impact on population recruitment, because: (i) in most years recruitment appears to be limited by a rarity of safe sites, and not by seed supply (which was as high as 43 germinable seeds/m²/yr); and (ii) the losses did not prevent the establishment of large seed banks (ranging from >30 to >1000 viable seeds/m²) potentially capable of exploiting temporary conditions favourable for recruitment. In contrast to the situation with many annual plants, patterns of recruitment in stable populations of long-lived perennials are often extremely complex, and the significance of seed losses therefore difficult to determine.     

Assessment of the Biological Control Impact of Seed Predators on the Invasive Shrub Acacia nilotica (Prickly Acacia) in Australia

An impact assessment study was undertaken to determine seed predation rates by two bruchid beetles, Bruchidus sahlbergi Schilsky and Caryedon serratus Olivier, on the invasive shrub Acacia nilotica (L.) Willd. ex Del. The former bruchid was released as a biological control agent for A. nilotica, whereas the latter is naturalized in Australia. We attempted to determine the dynamics and magnitude of bruchid predation, following a number of differing reports of their effectiveness. To investigate the importance of seed availability on bruchid numbers, we compared seed death in high- and low-seed-density habitats, both before and after pods had dropped from tree branches. Bruchid predation was initially low (<2%) in pods on tree branches in both habitats, but increased abruptly to 12% in riparian and 32% in nonriparian exclosures by the third collection date. Bruchid numbers then crashed to below 2% in both habitats, when pods dropped to the ground. B. sahlbergi predation later increased markedly (up to 65%) again at high-pod-density sites. No further increase in predation took place beyond this level. Seed predation by C. serratus was found to be minimal (<5% of seeds infested) throughout the monitoring period. We suggest that seed predation by the bruchids, prior to pod drop and removal by cattle, is insufficient to cause major impacts on A. nilotica populations. Bruchid seed damage is likely to be significant only in situations with low cattle numbers, where cattle cannot remove the majority of fallen seed pods or where cattle are excluded. Exclusion of cattle from some areas within A. nilotica-invaded landscapes to potentially increase bruchid effectiveness is proposed. The demographic impact of optimal A. nilotica seed losses is discussed.     

Factors affecting seed germination and seedling establishment of a long-lived desert shrub (<Emphasis Type="Italic">Coleogyne ramosissima</Emphasis>: Rosaceae)

Long-lived desert shrubs exhibit infrequent, episodic recruitment from seed. In spite of this long time scale, selection on life history attributes that affect seedling recruitment should be strong. We studied factors affecting germination phenology and seedling establishment for Coleogyne ramosissima, a dominant shrub species in the ecotone between warm and cold deserts in western North America. We also examined ecotypic differentiation in establishment strategy in response to selection regimes in two contrasting habitats. We followed patterns of dormancy loss, germination, emergence, and survival in reciprocal field experiments at warm winter Mojave Desert and cold winter Colorado Plateau study sites. Seed germination took place in late winter, under winter rain conditions at the warm desert site and under snow at the cold desert site. Distinctive germination phenologies for the two seed populations at contrasting field sites followed predictions based on laboratory germination experiments. There was no seed bank carryover across years. Seedling survival at the end of three growing seasons was remarkably high (mean survival 54%). Most seedling mortality was due to sprout predation by rodents early the first spring in unprotected caches. Emergence and establishment at each site were significantly higher for seeds from the local population, supporting the idea of ecotypic differentiation in establishment strategy. Establishment success was an order of magnitude greater overall at the Colorado Plateau site, which represents the leading edge of an upward elevational shift in distribution for this species under the current climatic regime. The Mojave Desert site is on the trailing edge of this shift, and recruitment there is apparently a much less frequent occurrence.