Showing papers on "Seed dispersal published in 2004"

The Genetic, Developmental, and Molecular Bases of Fruit Size and Shape Variation in Tomato

Abstract: Fruit, corresponding to the plant's ovary (or ovaries), protect seed development and serve as the vehicle for seed dispersal to different habitats for species propagation. Fruit also provide humans with a source of nutrition, culinary diversity, and often great pleasure. Humans consume fruit from a

Hybridization as a mechanism of invasion in oaks

Abstract: Summary We review here our own research and related work on hybridization between two widespread and largely sympatric European oak species (Quercus petraea and Q. robur). There is a near total lack of local differentiation in chloroplast DNA markers between them. A model is proposed to account for this lack of differentiation: invasion by one species of the range occupied by the other through pollen swamping. In support of this model, ecological, palaeoecological and population genetic studies indicate that one species (Q. robur) disperses its seeds better than the other one, that pollen flow is much more efficient than seed flow in oaks and that hybridization and introgression are asymmetric, hence reinforcing the ecological dynamics by facilitating the dispersal of Q. petraea in regions already colonized by Q. robur. ‘Resurrection’ of Q. petraea following this wave of hybridization appears to be rapid. More generally, available evidence indicates that hybridization could constitute an important mechanism of dispersal in both natural and human-induced plant invasions.

Conifers as invasive aliens: a global survey and predictive framework

Abstract: We summarize information on naturalized and invasive conifers (class Pinopsida) worldwide (data from 40 countries, some with remote states/territories), and contrast these findings with patterns for other gymnosperms (classes Cycadopsida, Gnetopsida and Ginkgoopsida) and for woody angiosperms. Eighty conifer taxa (79 species and one hybrid; 13% of species) are known to be naturalized, and 36 species (6%) are ‘invasive’. This categorization is based on objective and conservative criteria relating to consistency of reproduction, distance of spread from founders, and degree of reliance on propagules from the founder population for persistence in areas well outside the natural range of species. Twenty-eight of the known invasive conifers belong to one family (Pinaceae) and 21 of these are in one genus (Pinus). The Cupressaceae (including Taxodiaceae) has six known invasive species (4%) in four genera, but the other four conifer families have none. There are also no known invasive species in classes Cycadopsida, Gnetopsida or Ginkgoopsida. No angiosperm family comprising predominantly trees and shrubs has proportionally as many invasive species as the Pinaceae. Besides the marked taxonomic bias in favour of Pinaceae, and Pinus in particular, invasiveness in conifers is associated with a syndrome of life-history traits: small seed mass (< 50 mg), short juvenile period (< 10 year), and short intervals between large seed crops. Cryptomeria japonica, Larix decidua, Picea sitchensis, Pinus contorta, Pinus strobus, and Pseudotsuga menziesii exemplify this syndrome. Many rare and endangered conifer species exhibit opposite characters. These results are consistent with earlier predictions made using a discriminant function derived from attributes of invasive and noninvasive Pinus species. Informative exceptions are species with small seeds (< 4 mg, e.g. Chamaecyparis spp., Pinus banksiana, Tsuga spp. — mostly limited to wet/mineral substrates) or otherwise ‘non-invasive’ characters (e.g. large seeds, fleshy fruits, e.g. Araucaria araucana, Pinus pinea, Taxus baccata that are dependent on vertebrates for seed dispersal). Most conifers do not require coevolved mutualists for pollination and seed dispersal. Also, many species can persist in small populations but have the genetic and reproductive capacity to colonize and increase population size rapidly. The underlying mechanisms mediating conifer invasions are thus easier to discern than is the case for most angiosperms. Further information is needed to determine the extent to which propagule pressure (widespread dissemination, abundant plantings, long history of cultivation) can compensate for low ‘inherent invasiveness’.

Seed mass and mast seeding enhance dispersal by a neotropical scatter-hoarding rodent

Abstract: Many tree species that depend on scatter-hoarding animals for seed dispersal produce massive crops of large seeds at irregular intervals. Mast seeding and large seed size in these species have been explained as adaptations to increase animal dispersal and reduce predation. We studied how seed size and seed abundance simultaneously influenced seed dispersal and predation by scatter-hoarding rodents in the large-seeded rain forest tree Carapa procera (Meliaceae) in French Guiana. We individually tracked the fates of 3000 seeds, using remote video monitoring and thread-marking. Seed size was manipulated by broadly varying intraspecific seed mass, whereas effects of seed abundance were examined by tracking seeds in three seed-rich years and two seed-poor years. The hypotheses, that seed mass and seed abundance both enhance dispersal success and that seed abundance reinforces the effect of seed mass, were supported by the results. Most seeds were removed by the scatter-hoarding rodent red acouchy (Myoprocta acouchy) and subsequently were buried in scattered, single-seeded caches up to distances >100 m. Seeds that were not removed failed to establish seedlings. Seed removal was slower, pre-removal seed predation was greater, and seed dispersal was less far in seed-rich years than in seed-poor years, suggesting poorer dispersal under seed abundance. However, this was more than counterbalanced by a disproportionally greater survival of cached seeds in seed-rich years. The per capita probability of seed survival and seedling establishment was at least 4½ times greater under seed abundance. Large seeds were removed faster, were more likely to be scatter-hoarded, and were dispersed farther away than smaller ones, resulting in a higher probability of seedling establishment for larger seeds. Size discrimination was greater under seed abundance, albeit only during seed removal. Overall, large seeds shed in rich years had the highest probability of seedling establishment. Hence, both larger seed size and greater seed abundance stimulate rodents to act more as dispersers and less as predators of seeds. We conclude that scatter-hoarding rodents can select for both large seed crops and large seeds, which may reinforce mast seeding.

When Seed Dispersal Matters

Abstract: A profusion of fruit forms implies that seed dispersal plays a central role in plant ecology, yet the chance that an individual seed will ultimately produce a reproductive adult is low to infinitesimal. Extremely high variance in survival implies that variations in fruit production or transitions from seed to seedling will contribute little to population growth. The key issue is that variance in survival of plant life-history stages, and therefore the importance of dispersal, differs greatly among and within plant communities. In stable communities of a few species of long-lived plants, variances in seed and seedling survival are immense, so seed-to-seedling transitions have little influence on overall population dynamics. However, when seedlings in different circumstances have very different chances of survival—in ecological succession, for example, or when dispersed seeds escape density-dependent mortality near parent trees—the biased survival of dispersed seeds or seedlings in some places rath...

Diplochory: are two seed dispersers better than one?

Abstract: Diplochory is seed dispersal by a sequence of two or more steps or phases, each involving a different dispersal agent. Here, we describe five forms of diplochory and derive general characteristics of each phase of seed dispersal. The first and second phases of diplochory offer different benefits to plants. Phase one dispersal often results in escape from density-dependent seed and seedling mortality near the parent plant and can result in the colonization of habitat patches far from the parent. Phase two dispersal often moves seeds to discrete and predictable microsites, where the probability of seedling establishment is disproportionately high (i.e. directed dispersal). Diplochory appears to have evolved because combining two means of seed dispersal can increase the benefits of seed dispersal whilst reducing the likelihood of seed mortality.

How fast and far might tree species migrate in the eastern United States due to climate change

Abstract: Aim We describe and use a model, SHIFT, to estimate potential migration due to climate change over the next 100 years. Location Eastern United States. Methods Five species, currently confined to the eastern half of the United States and not extending into Canada, were used to assess migration potential: Diospyros virginiana (persimmon), Liquidambar styraciflua (sweetgum), Oxydendrum arboreum (sourwood), Pinus taeda (loblolly pine), and Quercus falcata var. falcata (southern red oak). SHIFT is a matrix simulation model using simple inverse power functions to provide a distance decay of seed dispersal and is driven primarily by the abundance of the species near the boundary, the forest density within and beyond the boundary, and the distance between cells. For each cell outside the current boundary, the model creates an estimate of the probability that each unoccupied cell will become colonized over a period of 100 years. SHIFT is a ‘fat-tailed’ migration model that allows rare very long distance dispersal events and colonization could occur up to 500 km beyond the current distribution boundary. Model outputs were analysed using transects through sections showing relatively low and high colonization probabilities as a result of low and high densities of target trees (high source strength) as well as high densities of forest (high sink strength). We also assess migration potential for species by concentric rings around the current boundary.

Determinants of long-distance seed dispersal by wind in grasslands

Abstract: Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species.

Seed dispersal by white-tailed deer: implications for long-distance dispersal, invasion, and migration of plants in eastern North America

Abstract: For many plant species in eastern North America, short observed seed dispersal distances (ranging up to a few tens of meters) fail to explain rapid rates of invasion and migration. This discrepancy points to a substantial gap in our knowledge of the mechanisms by which seeds are dispersed long distances. We investigated the potential for white-tailed deer (Odocoileus virginianus Zimm.), the dominant large herbivore in much of eastern North America, to disperse seeds via endozoochory. This is the first comprehensive study of seed dispersal by white-tailed deer, despite a vast body of research on other aspects of their ecology. More than 70 plant species germinated from deer feces collected over a 1-year period in central New York State, USA. Viable seeds included native and alien herbs, shrubs, and trees, including several invasive introduced species, from the full range of habitat types in the local flora. A mean of >30 seeds germinated per fecal pellet group, and seeds were dispersed during all months of the year. A wide variety of presumed dispersal modes were represented (endo- and exozoochory, wind, ballistic, ant, and unassisted). The majority were species with small-seeded fruits having no obvious adaptations for dispersal, underscoring the difficulty of inferring dispersal ability from diaspore morphology. Due to their broad diet, wide-ranging movements, and relatively long gut retention times, white-tailed deer have tremendous potential for effecting long-distance seed dispersal via ingestion and defecation. We conclude that white-tailed deer represent a significant and previously unappreciated vector of seed dispersal across the North American landscape, probably contributing an important long-distance component to the seed shadows of hundreds of plant species, and providing a mechanism to help explain rapid rates of plant migration.

Consistency of plant species and trait responses to grazing along a productivity gradient: a multi-site analysis

Abstract: Summary 1 Studies of the responses of communities to land use or climate change at a functional rather than species level have used species traits as response or effect variables to enable generalization between different communities and biogeographical regions. Data from 10 published experiments on semi-natural and agricultural grasslands in Scotland, and from published information on plant traits, were used to determine whether (a) species and trait attributes behave consistently with respect to changing grazing intensity at sites of different productivity and (b) whether species and attribute responses to grazing were affected by site productivity. 2 Only 9 out of 22 species common enough to show a consistent response to grazing did so. For example, Deschampsia flexuosa and Molinia caerulea declined when grazing intensity increased, whilst Anthoxanthum odouratum and Cerastium fontanum increased. A similar proportion (12/29) of traits behaved consistently between studies. 3 Increased grazing intensity was accompanied by an increase in species with a ruderal strategy, an annual life history, seasonal regeneration by seed, flowering and seed dispersal early in the season, rosette habit, higher requirement for light and a lower minimum height. 4 The response of five species was modified by site productivity. In four cases, the effect was to modulate the rate of response to grazing change. In contrast, Nardus stricta increased with grazing at low productivity sites, but decreased at high productivity sites. Many more traits (22), including attributes related to life history, life form, and vegetative and sexual reproduction, showed a response to grazing modulated by productivity. Eight of these changed in a complex manner, akin to that of N. stricta. 5 The response of some species and traits to grazing appears predictable. However, for some species and many traits either the rate or the direction of response is controlled by the productivity of the vegetation.

Dispersal phenology of hydrochorous plants in relation to discharge, seed release time and buoyancy of seeds: the flood pulse concept supported

Abstract: 1 Restored floodplains and backwaters lacking a viable propagule bank, may need flood pulses to facilitate inward dispersal of diaspores. Temporal patterns of hydrochorous plant dispersal are, however, not well known. 2 Diversity and abundance of diaspores dispersed in a water body over 12 months were quantified using a 200 mum net in order to: (i) test for a relationship between discharge and the number of species and diaspores dispersed; (ii) examine the effect of seed buoyancy and seed release period on the length of the dispersal period; and (iii) test whether diaspores of species that disperse during a similar period of the year are characterized by similar dispersal and dormancy traits. 3 A total 359 188 individuals of 174 vascular species developed from 144 samples, with most (90%) from vegetative diaspores and only 10% from seeds. Mean number of species and diaspores varied between months in parallel with discharge levels. Stepwise multiple regression analysis showed that both seed buoyancy and seed release influenced dispersal periods. 4 In general, species that dispersed most diaspores in spring and summer had non-dormant seeds, a shorter seed release period and a shorter seed dispersal period than species whose dormant seeds dispersed in autumn and winter. Vegetative diaspores were dispersed on average over 8 months, indicating their importance to long-distance dispersal. Several species dispersed both generative and vegetative diaspores, often in different seasons. 5 Our results may assist the planning of regenerative processes in riverine wetlands at landscape scales, as dispersal phenology, and discharge rates must be taken into consideration. Vegetative diaspores may be more important than seeds, although the latter may extend the species dispersal period into other seasons. Temporal heterogeneity in diaspore dispersal influences the identity of diaspores reaching restored habitats.

Evolution of genome size in pines (Pinus) and its life-history correlates: Supertree analyses

Abstract: Genome size has been suggested to be a fundamental biological attribute in determining life-history traits in many groups of organisms. We examined the relationships between pine genome sizes and pine phylogeny, environmental factors (latitude, elevation, annual rainfall), and biological traits (latitudinal and elevational ranges, seed mass, minimum generation time, interval between large seed crops, seed dispersal mode, relative growth rate, measures of potential and actual invasiveness, and level of rarity). Genome sizes were determined for 60 pine taxa and then combined with published values to make a dataset encompassing 85 species, or 70% of species in the genus. Supertrees were constructed using 20 published source phylogenies. Ancestral genome size was estimated as 32 pg. Genome size has apparently remained stable or increased over evolutionary time in subgenus Strobus, while it has decreased in most subsections in subgenus Pinus. We analyzed relationships between genome size and life-history variables using cross-species correlations and phylogenetically independent contrasts derived from supertree constructions. The generally assumed positive relation between genome size and minimum generation time could not be confirmed in phylogenetically controlled analyses. We found that the strongest correlation was between genome size and seed mass. Because the growth quantities specific leaf area and leaf area ratio (and to a lesser extent relative growth rate) are strongly negatively related to seed mass, they were also negatively correlated with genome size. Northern latitudinal limit was negatively correlated with genome size. Invasiveness, particularly of wind-dispersed species, was negatively associated with both genome size and seed mass. Seed mass and its relationships with seed number, dispersal mode, and growth rate contribute greatly to the differences in life-history strategies of pines. Many life-history patterns are therefore indirectly, but consistently, associated with genome size.

Aggregated Seed Dispersal by Spider Monkeys Limits Recruitment to Clumped Patterns in Virola calophylla

Abstract: The initial spatial pattern of seed deposition influences plant population and community structure, particularly when that pattern persists through recruitment. In a vertebratedispersed rain forest tree, Virola calophylla, we found that spatially aggregated seed deposition strongly influenced the spatial structure of later stages. Seed dispersion was clumped, and seed densities were highest underneath V. calophylla females and the sleeping sites of spider monkeys (Ateles paniscus), the key dispersal agent. Although these site types had the lowest per capita seed-to-seedling survival, they had the highest seedling/sapling densities. Conversely, seed and seedling/sapling densities were lowest, and seed survival was highest, at sites of diurnal seed dispersal by spider monkeys. Negative density-dependent and positive distance-dependent seed survival thinned seed clumps. Nonetheless, the clumped dispersion at sleeping and parental sites persisted to the seedling/sapling stage because differences in seed deposition were large enough to offset differences in seed survival among these site types.

Dispersal potential in plant communities depends on environmental conditions

Abstract: 1 Local plant communities can only function within a metacommunity context if they are connected by appropriate dispersal vectors, accommodating the transport of propagules between sites. The capacity for long-distance dispersal may be a key factor in the survival of local populations, especially in fragmented landscapes, and hence may have a large impact on local species composition. Dispersal vectors with a large efficiency for long-distance dispersal included in this study are: water, wind, large mammals and birds. 2 We tested the hypothesis that variation in dispersal traits across plant communities is related to the position of the communities along major environmental gradients. This hypothesis was tested for (i) separate long-distance dispersal vectors and (ii) multiple dispersal vectors (the number of potential long-distance dispersal vectors per species). 3 To quantify linkages between dispersal traits and environmental gradients, we coupled a data base containing dispersal attributes with another data base, containing 40 000 local vegetation descriptions aggregated into 123 plant communities. For each dispersal vector, the proportions of species that have access to this vector per community (weighted trait scores) were projected along three major environmental gradients: soil moisture, nutrient availability and light availability. 4 The potential importance of individual dispersal vectors showed clear differences along the three environmental gradients, with the greatest differences along the light availability gradient. The differences in dispersal traits probably reflect environmental constraints on the availability or efficiency of individual dispersal vectors. 5 The ability to be dispersed by multiple dispersal vectors is a common phenomenon in most plant communities (an average of 2.15 vectors per species). The mean number of potential long-distance dispersal vectors per species increases with light availability. This probably implies that plant communities differ in their response to both habitat fragmentation and habitat restoration. 6 Despite differences in trait spectra among communities, all dispersal syndromes are represented in nearly all communities. An important consequence of this complementarity in dispersal traits is that species within the same community may experience different connectivity. 7 The results emphasize the need for dispersal models based upon multiple dispersal vectors that explicitly include parameters for habitat characteristics.

Phenology of neotropical pepper plants (Piperaceae) and their association with their main dispersers, two short‐tailed fruit bats, Carollia perspicillata and C. castanea (Phyllostomidae)

Abstract: To relate differences in phenological strategies of a group of closely related plants to biotic (pollinators, dispersers) and abiotic (water, light) factors, we studied leafing, flowering, and fruiting phenology of 12 species of Piper (Piperaceae) in a neotropical lowland forest in Panama for 28 months. We asked how Piper may partition time and vertebrate frugivores to minimize possible competition for dispersal agents. Based on habitat preferences and physiological characteristics we discriminate between forest Piper species (eight species) and gap Piper species (four species). Forest Piper species flowered synchronously mostly at the end of the dry season. Gap Piper species had broader or multiple flowering peaks distributed throughout the year with a trend towards the wet season. Both groups of Piper species showed continuous fruit production. Fruiting peaks of forest Piper species were short and staggered. Gap Piper species had extended fruiting seasons with multiple or broad peaks. Both groups of Piper species also differed in their time of ripening and disperser spectrum. Forest Piper species ripened in late afternoon and had a narrow spectrum consisting mainly of two species of frugivorous bats: Carollia perspicillata and C. castanea (Phyllostomidae). Fruits of gap Piper species, in contrast, ripened early in the morning and were eaten by a broader range of diurnal and nocturnal visitors, including bats, birds, and ants. We conclude that the differences in flowering phenology of forest and gap Piper species are primarily caused by abiotic factors, particularly the availability of water and light, whereas differences in fruiting patterns are mostly influenced by biotic factors. The staggered fruiting pattern of forest Piper species may reflect competition for a limited spectrum of dispersers. The long and overlapping fruiting periods of gap Piper species are associated with a larger spectrum of dispersers and may be a strategy to overcome the difficulty of seed dispersal into spatially unpredictable germination sites with suitable light conditions.

Ant body size predicts dispersal distance of ant-adapted seeds: implications of small-ant invasions

Abstract: The services provided within a community can change as the species composition of that community changes. For example, ant–seed dispersal mutualisms can be disrupted in habitats dominated by invasive ants. We propose that this disruption is related to changes in mean ant body size, given that invasive ants are smaller than most native seed-dispersing ants. We demonstrate that the mean and maximum distances that ants transport seeds adapted for ant dispersal increase with worker body size, and that this relationship is an accelerating power function. This pattern is consistent among three ant subfamilies that include most seed-dispersing ants as well as most invasive ant species, is generalizable across ant species and communities, and is independent of diaspore mass. Using a case study, we demonstrate that both the mean body size of seed-collecting ants and seed dispersal distances are decreased in sites invaded by Solenopsis invicta, the imported red fire ant. Furthermore, we demonstrate that the mean si...

A simple mechanistic model of seed dispersal, predation and plant establishment: Janzen‐Connell and beyond

Abstract: Summary 1 Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump-shaped (Janzen-Connell (J-C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns. 2 We introduce a simple mechanistic model showing that distance and density-dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J-C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms. 3 The model shows that the J-C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host-specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds. 4 The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators. 5 The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance-dependent predation promotes either short or long dispersal distances, or both (dimorphism).

Flower visitors and pollination in the Oriental (Indomalayan) Region

Abstract: Current knowledge of flower visitors and pollination in the Oriental Region is summarised. Much less is known about pollination than seed dispersal and the coverage of habitats and taxa in the region is very uneven. The available evidence suggests that pollination in lowland forests is dominated by highly social bees (mainly Trigona and Apis species), with beetles probably the next most important group, followed by other bees and flies. In comparison with the better-studied Neotropics, large solitary bees, moths, Lepidoptera and vertebrates are relatively less important. These differences are greatest in the canopy of the lowland dipterocarp forests of Southeast Asia, where they probably reflect the unique temporal pattern of floral resource availability resulting from 'general flowering' at supra-annual intervals. Apis bees (but not Trigona species) are also important in most montane, subtropical and non-forest habitats. Apart from the figs (Ficus spp.), there are few well-documented examples of plant species visited by a single potential pollinator and most plant-pollinator relationships in the region appear to be relatively generalised. The small sizes of most pollinators and the absence of direct human exploitation probably make pollination mutualisms less vulnerable to failure as a result of human impacts than dispersal mutualisms, but more subtle impacts, as a result of altered gene flows, are likely to be widespread. On current evidence, pollination systems in the Oriental Region do not require any specific conservation action, but this review reinforces arguments for making the preservation (or restoration) of habitat connectivity the major focus of Oriental conservation.

Long‐distance seed dispersal in a metapopulation of Banksia hookeriana inferred from a population allocation analysis of amplified fragment length polymorphism data

Abstract: There is currently a poor understanding of the nature and extent of long-distance seed dispersal, largely due to the inherent difficulty of detection. New statistical approaches and molecular markers offer the potential to accurately address this issue. A log-likelihood population allocation test (AFLPOP) was applied to a plant metapopulation to characterize interpopulation seed dispersal. Banksia hookeriana is a fire-killed shrub, restricted to sandy dune crests in fire-prone shrublands of the Eneabba sandplain, southwest Australia. Population genetic variation was assessed for 221 individuals sampled from 21 adjacent dune-crest populations of B. hookeriana using amplified fragment length polymorphism. Genetic diversity was high, with 175 of 183 (96%) amplified fragment length polymorphism markers polymorphic. Of the total genetic diversity, 8% was partitioned among populations by amova and FST. There was no relationship between genetic diversity within populations and population demographic parameters such as population size and sample size. A population allocation test on these data unambiguously assigned 177 of 221 (80.1%) individuals to a single population. Of these, 171 (77.4% of total) were assigned to the population from which they were sampled and 6 (2.7% of total) were assigned to a known population other than the one from which they were sampled. A further 9 (4.1% of total) were assigned to outside the sampled metapopulation area, and 35 individuals (15.8%) could not be assigned unambiguously to any particular population. These results suggest that both the extent [15 of 221 (6.8%) individuals originating from a population other than the one in which they occur] and distance (1.6 to > 2.5 km), of seed dispersal between dune-crest populations is greater than expected from previous studies. The extent of long-distance interpopulation seed dispersal observed provides a basis for explaining the survival of populations of the fire-killed B. hookeriana in a landscape experiencing frequent fire, where local extinctions and recolonizations may be a regular occurrence.

Effects of dung and seed size on secondary dispersal, seed predation, and seedling establishment of rain forest trees

Abstract: Seeds dispersed by tropical, arboreal mammals are usually deposited singly and without dung or in clumps of fecal material. After dispersal through defecation by mammals, most seeds are secondarily dispersed by dung beetles or consumed by rodents. These post-dispersal, plant-animal interactions are likely to interact themselves, as seeds buried by dung beetles are less likely to be found by rodents than unburied seeds. In a series of three experiments with seeds of 15 species in central Amazonia (Brazil), we determined (1) how presence and amount of dung associated with seeds influences long-term seed fate and seedling establishment, (2) how deeply dung beetles bury seeds and how burial depth affects seedling establishment, and (3) how seed size affects the interaction between seeds, dung beetles, and rodents. Our overall goal was to understand how post-dispersal plant-animal interactions determine the link between primary seed dispersal and seedling establishment. On average, 43% of seeds surrounded by dung were buried by dung beetles, compared to 0% of seeds not surrounded by dung ( n=2,156). Seeds in dung, however, tended to be more prone than bare seeds to predation by rodents. Of seeds in dung, probability of burial was negatively related to seed size and positively related to amount of dung. Burial of seeds decreased the probability of seed predation by rodents three-fold, and increased the probability of seedling establishment two-fold. Mean burial depth was 4 cm (0.5-20 cm) and was not related to seed size, contrary to previous studies. Probability of seedling establishment was negatively correlated with burial depth and not related to seed size at 5 or 10 cm depths. These results illustrate a complex web of interactions among dung beetles, rodents, and dispersed seeds. These interactions affect the probability of seedling establishment and are themselves strongly tied to how seeds are deposited by primary dispersers. More generally, our results emphasize the importance of looking beyond a single type of plant-animal interaction (e.g., seed dispersal or seed predation) to incorporate potential effects of interacting interactions.

Large herbivores as mobile links between isolated nature reserves through adhesive seed dispersal

Abstract: Question: Do large herbivores contribute to the dispersal of plant seeds between isolated habitats by epizoochory? Location: Nature reserves in Flanders, Belgium. Methods: Epizoochory was studied by brushing plant seeds from the fur of 201 domesticated large herbivores (Galloway cattle, donkeys and horses), grazing in 27 Flemish nature reserves. Several herbivores were examined after transport between different nature reserves as part of the seasonal grazing system in Flanders, allowing detection of seed dispersal both within and between reserves. The seedling emergence method was used to identify the dispersed plant species. Results: In total, 6385 epizoochorous seeds from 75 species germinated, yet the real seed quantity was underestimated by the seedling emergence method. A wide variety of seed morphology, seed weights and plant heights was represented among the dispersed species, many of which had a transient seed bank. There was a gradual turnover in epizoochorous species composition in the course of the vegetation season, and seed dispersal occurred both within and between different nature reserves. Conclusions: Domesticated large herbivores, as models for wild mammals in the present and the past, are important dispersers of many plant species. Through seasonal grazing, the herbivores function as ‘mobile link organisms’, connecting isolated nature reserves through seed dispersal, possibly influencing vegetation development and long-term survival of plant populations. As such, large herbivores are important instruments in ecological restoration, especially in fragmented ecosystems.

Seed dispersal and recruitment limitation across spatial scales in temperate forest fragments

Abstract: Despite increasing evidence of seed limitation in forest ecosystems, data remain sparse on spatial patterns of seed rain at large (>1 ha) spatial scales. We monitored seed rain (28.5 m2) throughout five northern hardwood forest fragments (27 ha sampled across 14-km2 area) in southeastern Michigan over two years. Four fragments were nearest neighbors (300–700 m), yet varied in species composition, providing the opportunity to detect landscape-scale seed exchange. Of the 37 species of woody plants present in the seed rain (98 032 mature seeds), only three (Betula papyrifera, Ostrya virginiana, and Ulmus americana) had widespread seed dispersions within all fragments containing resident sources (seed in >70% of traps in each fragment). Seed dispersions, measured as the percentage of traps within a fragment receiving seed, differed among species using different dispersal vectors with animal-dispersed species arriving in a lower percentage of seed traps than wind-dispersed seeds. At a given source density, seed dispersions increased with decreasing seed mass. Light-seeded, fecund species such as Betula or Tsuga required lower source densities to saturate fragments with seed compared to heavy-seeded species (Acer, Fraxinus, Tilia). Heavy-seeded wind- and animal-dispersed species also displayed the strongest evidence of seed limitation, with seedling presence significantly associated with presence of seed for Carpinus caroliniana, Fagus grandifolia, Prunus avium, and Tilia americana. Of 17 species, landscape-scale seed exchange was detected for only four disturbance-associated species (Acer negundo, Betula papyrifera, Celastrus scandens, Eleaganus umbellata). No exchange was detected for Acer rubrum, Betula alleghaniensis, or Tsuga canadensis, despite broad seed dispersions (>50%) in fragments with resident sources, suggesting the potential for seed limitation for these species at larger spatial scales. Seed encounter probabilities suggest that potential seed competitors often fail to simultaneously colonize microsites. We suggest that all dominant species in northern hardwood forests can be seed limited at some spatial scale and that results are consistent with “winning by forfeit” scenarios of diversity maintenance in forest ecosystems.

Ecological correlates of seed desiccation tolerance in tropical African dryland trees.

Abstract: In the tropics, species with recalcitrant or desiccation-sensitive, Type III seeds are largely restricted to regions with comparatively high rainfall, because desiccation-induced seed death will be minimal in these environments. However, species with recalcitrant seeds do occur in drylands, although little is known about ecological adaptations to minimize seed death in these environments. Here we present data for the seed desiccation tolerance of 10 African dryland species and examine the relationships between seed size, rainfall at the time of seed shed, and desiccation tolerance for these and a further 70 species from the scientific literature. The combined data set encompasses species from 33 families. Three species (Syzygium cumini, Trichilia emetica, and Vitellaria paradoxa) had desiccation-sensitive seeds, and the remaining seven species investigated were desiccation-tolerant. The desiccation-sensitive species had large (>0.5 g) seeds, germinated rapidly, and had comparatively small investments in seed physical defenses. Furthermore, seed was shed in months of high rainfall (>60 mm). In comparison, for species with desiccation-tolerant seeds, seed mass varied across five orders of magnitude, and seed was shed in wet and dry months. Although infrequent in dryland environments (approximately 11% of the species examined here), species with desiccation-sensitive seeds do occur; large size, rapid germination, and the timing of dispersal all reduce the likelihood of seed drying. Furthermore, desiccation-sensitivity may be advantageous for large-seeded species by increasing the efficiency of resource use in seed provisioning.

An evaluation of alternative dispersal functions for trees

Abstract: Summary 1 We compared three commonly used empirical seed/seedling dispersal functions for trees (lognormal, 2Dt, and two-parameter Weibull) by analysis of published studies where the location of the source is known, as well as by inverse modelling within an old growth hardwood forest in southern Quebec Almost all the species were wind-dispersed 2 For the discrete source studies, the lognormal was clearly superior, while for the inverse modelling the performance of the three dispersal functions was somewhat more even We speculate that collisions with boles spuriously enhanced the likelihood of the 2Dt and the Weibull with inverse modelling, as both these functions assume that the greatest seed/seedling density will occur at the base of the maternal parent bole 3 We conclude that the lognormal function is to be preferred because, as well as providing a framework for mechanistic interpretation, it tends to provide a closer approximation to observed dispersal curves 4 We also argue that mean distances travelled by seed crops are far more extensive than indicated by previous studies that used the Weibull function

Fire adaptations in the Canary Islands pine (Pinus canariensis)

Abstract: A wide set of phenotypic characteristics related to life history were studied in mature stands of Pinus canariensis throughout its natural range of distribution in the Canary archipelago. Natural forests ranging from those located in xeric areas through to the sub-tropical cloud forests and high mountain stands were classified into eight ecological regions according to their main climatic features. The recent history of forest fires (covering the last 30 years) was taken into account using a categorical factor with three levels. The phenotypic variables studied included those related to seed dispersal (cone size, number of seed scales, seed and wing size and percentage of serotinous trees) and stem growth both on the breast height section (bark thickness, radial growth at various ages and sapwood and heartwood sizes) and on the entire stem (height growth related to age). The average percentage of serotinous trees present in the ecological regions studied varied from 3 to 35%. Average bark thickness in adult trees ranged from 22 to 49 mm and was found to be unrelated to age or diameter. Growth both in height and diameter was found to decline after an average of 25 years, although clear trends in relation to this could not be established across the ecological regions. A high correlation was found to exist between annual rainfall, fire frequency, serotiny and bark thickness at a regional level. Sapwood area per hectare proved to be a valuable indirect site-quality index for the objectives of this paper. Favourable sites (characterised by a high sapwood area per hectare) displayed the highest levels of both bark thickness and serotiny. These particular areas are those, which have suffered more frequent and intense fires over the last decades. The evolutionary implications of this trend and of other general traits of the species, such as vegetative resprouting, are discussed here in relation to the role of understorey vegetation in fire regimes, competition and volcanic history of the islands.

Gene flow in forest trees: how far do genes really travel? 1

Abstract: Gene flow is one of the most important factors shaping the genetic structure of populations. In recent decades, a number of studies have addressed issues of contemporary gene flow in forest trees, including pollen and seed dispersal, and gene immigration into natural and breeding populations (primarily seed orchards). Gene flow might be considered either beneficial or deleterious from the point of view of a conservation geneticist or a tree breeder. Extensive gene dispersal within local populations promotes panmixis and reduces family structuring in natural regeneration, thus reducing the potential for inbreeding. However, gene flow may reduce fitness of offspring if genes come from populations maladapted to the habitat of offspring establishment. Furthermore, substantial gene flow limits divergence among populations that might otherwise occur because of genetic drift and directional selection, and may enhance genetic diversity within populations. The robustness and discriminatory power of parentage analysis have been significantly improved in recent years due to advances in molecular marker technology and analytical techniques. However, knowledge of gene flow in forest trees is still unsatisfactory due to continued shortcomings of available markers, inherent limitations of statistical models, and the anecdotal nature of many gene flow studies, which are typically limited in scope. Another limitation is that pollen gene flow is usually estimated by sampling seeds from a number of mother trees. Restricting sampling to seeds seems adequate when evaluating gene flow in seed collections to be used for artificial reforestation. However, in naturally regenerating populations, the most important parameter is the proportion of immigrant alleles in established seedlings. If natural selection favours local genotypes, this should be reflected in the difference between potential gene flow measured at the seed stage and realized gene flow measured in established seedlings. Realized gene flow is a complex phenomenon that depends on a large number of deterministic and stochastic variables. Profound understanding of effective gene flow observed at the landscape level may require joint efforts in population genetics, ecology, advanced multivariate statistics, and spatial simulation modelling.