Probability Of Seedling Establishment Research Articles

Perfect poopers; passerine birds facilitate sexual reproduction in clonal keystone plants of the boreal forest through directed endozoochory towards dead wood

Berry-producing Vaccinium shrubs are keystone species in boreal forest ecosystems and their berries provide an essential food source for many vertebrates. In Fennoscandia, both the abundance and cover of Vaccinium have decreased in forests, with intensive forestry practice as a main driver. Vaccinium seedling recruitment is constricted to recruitment windows of opportunity and appears to be infrequent, mainly due to seed dispersal limitation. Passerine birds are key players in seed dispersal, which can be directed towards perching points such as coarse woody debris.In this study, we investigated the potential for directed endozoochory by passerine birds towards cut stumps and if such stumps were viable recruitment windows for Vaccinium species, in an intensively managed boreal forest landscape of central Norway. We surveyed stump and paired forest floor microsites for passerine scat (i.e. potential seed rain) and Vaccinium seedlings. We collected passerine scat and conducted germination experiments to assess if they contained viable seed. We tested the microsite effect (i.e. forest floor/stump) on scat deposition and seedling establishment and then used modified ensemble classifiers to identify important environmental factors affecting scat deposition and seedling establishment patterns.We found that passerine scat was disproportionally deposited on stumps, and that the vast majority of scat contained viable Vaccinium seed. Stumps were also suitable recruitment windows for Vaccinium species, as a higher probability of seedling establishment occurred at stumps compared with the forest floor. However, scat deposition and seedling establishment at stumps were rather contextual and determined by environmental variables. The probability of scat deposition increased with higher complexity of the vertical forest structure and lower canopy cover, whereas seedling establishment required bryophyte cover and larger stumps, or alternatively smaller stumps with competition-free spaces.Our results highlight a pathway to successful sexual reproduction for Vaccinium species in managed forests: passerine birds direct endozoochorous seed dispersal towards tree stumps, which can offer suitable conditions for seedling establishment. However, the spatiotemporal variability both in forest stand structure and in stump conditions have a strong influence on the success of sexual recruitment via this pathway.

Harvest and dispersal syndromes influence the probability of tree seedling establishment in the eastern United States

AbstractAimThe goal of this work was to investigate the influence of disturbance, in particular tree harvesting, on seedling establishment rates both within and beyond the range extent of individual species. In addition, we considered how disturbances differentially affect seedling establishment depending on the dispersal syndromes and shade tolerance of species.LocationThis project covers the eastern forests of the USA.Time periodData for this project extend from 1996 to 2019.Major taxa studiedTree species of the eastern USA.MethodsWe used the US Forest Service's Forest Inventory and Analysis (FIA) data for tree species of the eastern USA to investigate the influence of disturbance, the dispersal mechanism and shade tolerance of species on the establishment of seedlings. We ran generalized linear models for seedling presence/absence and the number of seedlings as the response variables. Disturbance categories were no disturbance, natural disturbance, harvest, and both harvest and natural disturbance.ResultsThe probability of seedling establishment increased in response to harvest within FIA plots but not in response to natural disturbances. Harvest increased the probability of seedling presence the most for shade‐intolerant species; however, intermediately shade‐tolerant and ‐intolerant species also increased in harvested plots. Additionally, bird‐dispersed species had a higher probability of colonizing new plots outside their range, in comparison to mammal‐dispersed or wind‐dispersed tree species.Main conclusionsOur results suggest that stand competition is an important limiting factor for seedling establishment in the dense, secondary forests of the eastern USA. Harvesting creates sizeable gaps that promote seedling establishment and species turnover. Harvesting especially facilitates the establishment of tree species with bird dispersal outside of their range. Thus, timber harvesting can be a useful management tool when the goal is to assist range shifts for certain species. We recommend that future research should include harvested plots when looking for evidence of recent tree migration.

The effect of seed-dispersal timing on seedling recruitment is modulated by environmental conditions that vary across altitude in a threatened palm.

The timing of seed dispersal determines the environmental conditions that plants face during early life stages. In seasonal environments, selection is expected to favour dispersal timing that is matched to environmental conditions suitable for successful recruitment. Our aim here was to test whether the timing of seed dispersal influences seedling establishment success in two populations of Euterpe edulis that are located at contrasting altitudes, have different seed-dispersal phenologies and are subjected to distinct climatic conditions. We sowed E. edulis seeds in contrasting altitudes on different dates, and monitored seed germination, emergence and seedling establishment at each altitude over 4 years. At the high-altitude site, five seed-dispersal cohorts were established during the natural dispersal period. At the low-altitude site, three seed-dispersal cohorts were established during natural dispersal, and two were established either before or after natural dispersal. At the high-altitude site, seed-dispersal timing did not affect seed germination, seedling emergence or seedling establishment success. In contrast, at the low-altitude site, late seed dispersal near the end of the wet season resulted in a lower probability of seedling establishment, possibly due to the exposure of seeds, germinants and seedlings to unfavourable drought conditions. In addition, at the low-altitude site, the natural seed-dispersal period was poorly matched to favourable environmental conditions for seedling establishment. The greater effect of seed-dispersal timing on seedling establishment at the low-altitude site is probably related to a more seasonal and drought-prone environment that favours a restricted period of seed dispersal. The magnitude of the effect of dispersal timing on seedling establishment success was modulated by environmental conditions that vary across altitude. Furthermore, reproductive phenology appears to be subject to more intense selection at the lower limit of the altitudinal range, due to a more restrictive window of opportunity for successful seedling establishment.

Philomatry in plants: why do so many species have limited seed dispersal?

Many have noted limited seed dispersal of plants in diverse environments and attempted evolutionary explanations for it. Although philopatric ("love of fatherland") is used by zoologists to describe organisms that remain near their place of origin, philomatric ("love of motherland") is proposed as more appropriate for plants because seeds develop on the maternal parent, fecundity and dispersal are maternally influenced characteristics, and the term dovetails with the mother-site hypothesis (MSH) for the evolution of restricted dispersal. Proximate reasons for philomatry include intrinsic drivers such as morphological features of diaspores and where on the maternal parent they are produced. Extrinsic drivers include local environmental conditions, surrounding vegetation, and ineffective dispersal agents. The MSH proposes that selection should favor philomatry in a population adapted to a particular habitat because offspring will likewise be adapted to that same habitat. Several studies show philomatry can mitigate distance-dependent costs of dispersing into surrounding inhospitable areas. Undispersed diaspores can eliminate energetic costs of accessory structures or biochemicals needed by dispersible diaspores, but it is unclear whether these costs are significant to the evolution of philomatry. Disadvantages of limited dispersal are inability to escape deteriorating habitat conditions, inability to colonize new habitats, and inbreeding among offspring. Heterocarpic species offset these disadvantages by producing dispersed plus undispersed diaspores. A conceptual framework is presented relating dispersal distance to the probability of seedling establishment. Future research should recognize dispersal as a covarying syndrome of multiple life history traits and focus on ecological selection agents that favor philomatry.

Horizontal seed dispersal by dung beetles reduced seed and seedling clumping, but did not increase short-term seedling establishment.

Dung beetles are secondary seed dispersers, incidentally moving many of the seeds defecated by mammals vertically (seed burial) and/or horizontally as they process and relocate dung. Although several studies have quantified this ecological function of dung beetles, very few have followed seed fate until seedling establishment, and most of these have focused on the effects of seed burial. We know very little about the effects of horizontal seed movement by dung beetles, though it is generally assumed that it will affect plant recruitment positively through diminishing seed clumping. The objective of our study was to assess the effects of dung beetle activity on the spatial distribution of seeds and seedlings, and on the probability of seedling establishment. In a tropical rainforest in Mexico we carried out two complementary field experiments for each of two tree species (Bursera simaruba and Poulsenia armata), using seeds experimentally imbedded in pig dung and recording their fate and spatial location over time. For both species, dung beetle activity reduced the spatial clumping of seeds and seedlings; however, it did not increase the probability of seedling establishment. We discuss the context- and species-specificity of the combined effects of horizontal and vertical dispersal of seeds by dung beetles, and the need to quantify long-term seedling fates to more accurately determine the effects of seed movement by dung beetles on plant recruitment.

Re-caching behaviour of rodents improves seed dispersal effectiveness: Evidence from seedling establishment

Long-distance seed dispersal is disproportionately important for recruitment, range expansion and gene flow in plant populations. The re-caching of seeds by rodents is considered a mechanism of long-distance seed dispersal. The effect of re-caching on dispersal distance is well studied. However, how re-caching by rodents affects seedling establishment remains unclear. To assess the role of re-caching by rodents on seedling establishment, we tracked a large number of seeds and investigated the seed dispersal and seedling establishment of two large-seeded tree species (Scleropyrum wallichianum and Garcinia xanthochymus) in a tropical forest. We found that rodents scatter hoarded 48.9–71.3% of the released seeds, and a total of 23.8–36.6% of the initial caches of both species experienced re-caching up to 2 or 3 times. Re-caching was correlated with farther dispersal and a higher probability of seedling establishment for both species. In addition, re-caching contributed to medium and long-distance seed dispersal, with 17.2% and 18.5% of seedlings located >50 m from parent trees for G. xanthochymus and S. wallichianum, respectively, and 3.4% of seedlings of G. xanthochymus located >100 m from parent trees. Our results suggested that re-caching by rodents can promote seed dispersal effectiveness and contribute to effective long-distance seed dispersal.

The importance of fire refugia in the recolonization of a fire-sensitive conifer in northern Patagonia

Seed dispersal and seedling establishment are essential for plant recolonization after disturbances, especially for plants that rely exclusively on sexual reproduction such as post-fire colonizer trees. Fire refugia may play a key role not only allowing trees to survive fire, but also functioning as seed sources after it. The estimation of seed dispersal and seedling establishment are essential for assessing plant recolonization ability, understanding landscape dynamics and determining which areas may not be able to recover due to lack of seed arrival. Here we study the post-fire recolonization ability of Austrocedrus chilensis (Cordilleran Cypress) from fire refugia in burned areas of northwest Patagonia, Argentina. We mapped all female trees, saplings and seedlings within and around fire refugia, recorded the reproductive capacity of female trees and characterized the microsite conditions for establishment. We used an inverse modelling approach and Approximate Bayesian Computation to estimate the seed dispersal kernel and the probability of seedling establishment. We found that the average dispersal distance of an A. chilensis seed was 88.52 m. The dispersal kernel was fat-tailed, meaning that A. chilensis has the capacity of producing accelerating expansions. Large woody debris, litter, and the protection of shrubs were the most important factors associated with the presence of recruits. We highlight the importance of fire refugia as seed sources for the recolonization of burned areas and thus the relevance of protecting these places to allow the persistence of fire-sensitive species.

Consequences of disperser behaviour for seedling establishment of a mistletoe species

AbstractThe dispersal process in plants links adults and their offspring. For frugivore‐dispersed plants, animal behaviour can have a strong effect on plant fitness. Many mistletoes are totally dependent on animals that deposit seeds on suitable hosts and particular branch diameters. We characterised the seed dispersal and seedling establishment of the mistletoe Tristerix corymbosus, which at our study site, is exclusively dispersed by the marsupial Dromiciops gliroides. Mistletoes’ fruits have a viscous pulp that remains in the seed even after dispersal. This substance adheres the seed to the host branch. We estimated host branch availability in the forest and seed deposition (faeces) by the marsupial in the study area. Specifically, the branch suitability factors we assessed were host identity, branch status (alive or dead), branch diameter, height, and canopy cover. Lodged faeces were individually marked and the number of seed deposited within these droppings was counted, and we recorded the number of seedlings with true leaves that had established after 1 year to estimate the probability of seedling establishment. Branch diameter and canopy cover had a significant positive effect on seed deposition probability. Seedling establishment probability decreased with the number of seeds deposited per faeces and with canopy cover. In general, the marsupial deposited mistletoe seeds in microsites that increase the chance of seedling establishment. Thus, the movement behaviour of the marsupial has a positive effect on the regeneration process of this mistletoe species.

Fire legacies impact conifer regeneration across environmental gradients in the U.S. northern Rockies

An increase in the incidence of large wildfires worldwide has prompted concerns about the resilience of forest ecosystems, particularly in the western U.S., where recent changes are linked with climate warming and 20th-century land management practices. To study forest resilience to recent wildfires, we examined relationships among fire legacies, landscape features, ecological conditions, and patterns of post-fire conifer regeneration. We quantified regeneration across 182 sites in 21 recent large fires in dry mixed-conifer forests of the U.S. northern Rockies. We used logistic and negative binomial regression to predict the probability of establishment and abundance of conifers 5–13 years post-fire. Seedling densities varied widely across all sites (0–127,500 seedlings ha−1) and were best explained by variability in distance to live seed sources (β = −0.014, p = 0.002) and pre-fire tree basal area (β = 0.072, p = 0.008). Beyond 95 m from the nearest live seed source, the probability of seedling establishment was low. Across all the fires we studied, 75 % of the burned area with high tree mortality was within this 95-m threshold, suggesting the presence of live seed trees to facilitate natural regeneration. Combined with the mix of species present within the burn mosaic, dry mixed-conifer forests will be resilient to large fires across our study region, provided that seedlings survive, fire do not become more frequent, high-severity patches do not get significantly larger, and post-fire climate conditions remain suitable for seedling establishment and survival.

Multiscale Controls on Natural Regeneration Dynamics after Partial Overstory Removal in Douglas-Fir Forests in Western Oregon, USA

We examined natural regeneration following operational-scale variable density retention treatments in 40 – 60-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests at seven sites for a decade following treatment. Treatments included residual overstory densities of 300, 200, and 100 trees/ha, with leave islands and gaps of three sizes (0.1, 0.2, and 0.4 ha) and an untreated control (600 trees/ha). Natural regeneration was influenced by factors from multiple spatial scales including broad-scale differences in overstory composition among sites, mesoscale variability in topographic position, and fine-scale variability in overstory and understory competition. High local basal area (BA) decreased the probability of seedling establishment, though some seedlings established even under high BA, particularly shade-tolerant western hemlock. In contrast, recruitment of saplings (> 1.37 m height) required lower residual overstory density (i.e., 100 trees/ha), especially for shade-intolerant Douglas-fir. Understory vegetation had little effect on saplings but was negatively related to seedling densities, particularly when overstory density was low. Variable density prescriptions can take advantage of the importance of fine-scale variability to promote regeneration of desired species mixtures, though other factors such as site overstory species composition and variation in topographic position will also influence regeneration dynamics. Including heavy overstory removal or gap creation could facilitate rapid recruitment of saplings, especially for shade-intolerant Douglas-fir.

Conifer reinvasion of montane meadows following experimental tree removal and prescribed burning

Tree invasion of mountain grasslands and meadows, pervasive throughout western North America, has become a management concern. Restoration of these systems requires tree removal and possibly prescribed burning; however, subsequent reinvasion by trees may compromise these efforts. In this study, we assess patterns of tree seedling establishment 8years after tree removal (with and without burning) from 1-ha experimental plots in conifer-invaded meadows in the Oregon Cascades. We quantify variation in the timing, spatial distribution, and density of establishment of species with differing seral roles; compare effects of burning; and explore relationships with distance to and characteristics of adjacent, residual forests. Seedlings established continuously after tree removal, dominated by late-seral Abies grandis (cumulative plot densities of 116–460/ha). Early-seral Pinus contorta and Pseudotsuga menziesii were much less abundant (0–25 and 1–52/ha, respectively). Frequency (percentage of 5×5m subplots) and density of seedlings did not differ between treatments (burned vs. unburned), nor did seedling growth rates (inferred from height-age relationships). Seedling spatial distributions and relationships with distance to adjacent forest varied both within and among plots. On average, however, seedlings were concentrated along edges. In the 5-m outer band of subplots, frequency averaged 37% and density, 682/ha, compared to 18% and 140/ha in the remaining cores of the plots. Density of Abies was significantly greater along more shaded southern edges (north-facing) than along more exposed northern edges (south-facing), but it declined steeply with distance from edge, especially for southern edges. Plot-level density of Abies seedlings was also correlated with basal area of Abies in the adjacent forest (but not with tree density or summed height). Strong relationships of seedlings with distance, exposure, and characteristics of adjacent forests suggest that conifer reinvasion can be minimized by targeting tree removal to maximize distances to residual trees. However, absence of a treatment effect suggests that burning—critical for reducing woody residues—does not increase the probability of seedling establishment, particularly in systems in which late-seral species are the principal colonists.

Cache sites preferred by small rodents facilitate cache survival in a subtropical primary forest, central China

Context Scatter-hoarding rodents tend to hoard plant seeds in preferred places that facilitate seed survival, resulting in high food rewards for hoarders and a high probability of seedling establishment for plants. Aims To test the hypothesis that caches placed in rodent-preferred microhabitats, nearest-neighbour distances between caches (cache density) or cache depths have an increased chance of survival. Methods Rodent-made caches and observer-established caches of Himalayan hazelnut (Corylus ferox) seeds were tracked in a subtropical primary forest in central China. The survival of caches within rodent-preferred microhabitats, nearest-neighbour distances and cache depths were identified. Key results Rodents preferred to hoard Himalayan hazelnut seeds in microhabitats under and at the edge of shrubs, with the nearest-neighbour distance of <4.0 m, and at 2.1–4.0-cm depth. The survival times of both rodent-made caches and observer-established caches were longer within rodent-preferred microhabitats than in other habitats and increased with an increasing nearest-neighbour distance and cache depth, up to what appeared to be an optimal nearest-neighbour distance and depth. Conclusions Conditions of rodent-preferred cache microhabitats, nearest-neighbour distances and cache depths can facilitate cache survival. Implications Rodents often move plant seeds away from parent trees and bury them in shallow soil, which benefits seed dispersal and establishment of plants. The present study demonstrates that rodent-preferred cache conditions may translate into higher seed survival and a higher tendency of seeds to germinate and establish. Thus, careful management and conservation of rodent-preferred hoarding habitats may benefit seed dispersal and survival.

Seedling establishment and survival on decaying logs in boreal mixedwood stands following a mast year1This article is one of a selection of papers from the International Symposium on Dynamics and Ecological Services of Deadwood in Forest Ecosystems.

In the boreal forest, establishment of tree regeneration is tightly linked to both mast years and the availability of adequate germination beds for seedlings. We took advantage of a mast year (2006) in the eastern boreal mixedwood to compare seedling establishment in 2007 and seedling survival 2 and 4 years later on sections of fallen logs and equivalent areas of adjacent forest floor. Several factors that could explain establishment of seedlings on logs were measured, including wood resistance, density, moisture content, and C/N ratio. Our results show that small-seeded species, such as white birch ( Betula papyrifera Marsh.) and white spruce ( Picea glauca (Moench) Voss), establish preferentially on logs whereas balsam fir ( Abies balsamea (L.) Mill.), a relatively large-seeded species, establishes more often on the forest floor. Using logistic regressions, we confirmed that the probability of seedling establishment on logs declines with wood resistance, while the survival probability is inversely proportional to stand deciduous basal area. Survival rate was similar for seedlings established on the forest floor and on logs. However, none of the white birch seedlings established on the forest floor in 2007 were alive by 2011. Even following an exceptional mast year, log occurrence in eastern mixedwood stands would not suffice to obtain adequate white spruce stocking levels.

The interacting effects of clumped seed dispersal and distance‐ and density‐dependent mortality on seedling recruitment patterns

Summary1. Seed dispersal and natural enemies both influence spatial patterns of seedlings, which in turn influence future abiotic and biotic interactions, with consequences for plant populations, distributions and diversity. Clumped seed deposition is common, especially for vertebrate‐dispersed seeds, and has the potential to significantly affect interactions with density‐responsive enemies, yet has received relatively little attention.2. We used spatially explicit simulation models to examine how different patterns of seed dispersal and natural enemy attack structure seedling spatial patterns. We simulated clumped seed dispersal by combining a two‐dimensional Student’s T dispersal kernel for expected seed rain with a negative binomial distribution for seed deposition. We based our models for seed mortality on published data reflecting differing life histories of insect seed predators and soil‐borne pathogens. We varied dispersal distance, degree of clumping, type of enemy, enemy dispersal distance and fecundity among simulations.3. Under insect seed predation, seeds escaped predation by dispersing longer distances than insects, resulting in ‘Janzen–Connell’ patterns in which seedling recruitment peaks at intermediate distances. When insects dispersed longer distances than seeds, higher seed densities near the tree satiated insects, resulting in ‘McCanny’ patterns in which seed deposition, survivorship and seedling establishment all decrease with distance from the parent tree. Total seedling establishment was lowest when insects and seeds dispersed similar distances.4. Under pathogen attack, Janzen–Connell patterns predominated except when seedling survival was virtually zero or one everywhere, or in the case where pathogen dispersal distances exceeded seed dispersal distances, producing ‘Hubbell’ patterns in which seed deposition and seedling establishment decrease with distance, though survivorship increases.5. Clumped seed deposition increased the probability of seedling establishment under both insect seed predation and pathogen attack as it led to local satiation of insect seed predators and made it harder for pathogen distributions to track seeds.6. Synthesis. Our modelling study suggests that the relative dispersal distances of seeds and natural enemies are crucial to determining establishment rates and spatial patterns of seedlings. Better characterization of the movement and natural histories of natural enemies is critical to improving our understanding of seedling distributions and plant–enemy interactions.

Mating Behavior Drives Seed Dispersal by the Long‐wattled Umbrellabird Cephalopterus penduliger

AbstractFrugivores exhibit considerable variation in the seed dispersal services they provide. Understanding what drives these differences is a key goal for ecologists because of the central role seed dispersal plays in shaping ecological and genetic diversity in plant populations. The lek‐mating system of the Long‐wattled Umbrellabird (Cephalopterus penduliger) provides a powerful lens to examine how mating behavior may impact seed dispersal outcomes. As in all lek‐breeding species, male Umbrellabirds congregate in traditional sites (leks) to display, whereas females are solitary and visit leks only rarely. This study demonstrates how differences in mating behavior between the sexes drive distinctive seed movement and deposition patterns by male vs. female Umbrellabirds. Using radio tracking and gut retention trials, we documented divergent movement patterns between the sexes that are directly attributable to mating behavior differences. These movement differences led males to disperse seeds long distances from source trees and to deposit the majority of seeds they ingested within the lek; females dispersed seeds shorter distances and more evenly across the landscape. We empirically confirmed that the density of dispersed seeds was higher in leks than in control areas outside the lek, yet found no evidence that this higher density of seeds in leks reduced probability of seedling establishment. This research not only provides a mechanistic explanation for long dispersal distances and high levels of genetic diversity previously reported for seeds in Umbrellabird leks, but also highlights the importance of explicitly considering behavior in studies of animal‐mediated seed dispersal.

Internal dispersal of seeds by waterfowl: effect of seed size on gut passage time and germination patterns

Long distance dispersal may have important consequences for gene flow and community structure. The dispersal of many plants depends on transport by vertebrate seed dispersers. The shapes of seed shadows produced by vertebrates depend both on movement patterns of the dispersers and on the dynamics and effects of passage through the disperser's gut (i.e. the retention time, survival and germination of ingested seeds). A combination of experiments with captive waterbirds and aquatic plant seeds was used to analyse the following: (a) the effects of inter- and intra-specific variation in seed size and duck species on seed retention time in the gut and (b) the relationship between retention time and the percent germination and germination rates of seeds. Among the three Scirpus species used, those with smaller seeds showed higher survival after ingestion by birds and longer retention times inside their guts than those with larger seeds. For Potamogeton pectinatus, only seeds from the smaller size class (<8 mg) survived ingestion. Retention time affected the percent germination and germination rate of Scirpus seeds but in a manner that varied for the different plant and bird species studied. We recorded both linear and non-linear effects of retention time on percent germination. In addition, germination rate was positively correlated with retention time in Scirpus litoralis but negatively correlated in Scirpus lacustris. Small seed size can favour dispersal over larger distances. However, the effects of retention time on percent germination can modify the seed shadows produced by birds due to higher percent germination of seeds retained for short or intermediate periods. The changes in dispersal quality associated with dispersal distance (which is expected to be positively related to retention time) will affect the probability of seedling establishment over longer distances and, thus, the spatial characteristics of the effective seed shadow.

Fenologia reprodutiva de Prepusa montana Mart. (Gentianaceae) em uma área de campo rupestre da Chapada Diamantina, BA, Brasil

The phenological, flowering and fruiting strategies of Prepusa montana Mart. (Gentianaceae) in an area of campo rupestre vegetation in the Chapada Diamantina were investigated in terms of the influence of local rainfall, relative humidity, temperature, and photoperiod. The numbers of buds, flowers, and mature fruits of this species were monitored during monthly visits to the Mucuge Municipal Park between 6/2006 and 8/2007. Environmental variables (average accumulated rainfall, average temperature and relative humidity) were monitored at the site, while the regional photoperiod was calculated from geographical data. Prepusa montana demonstrated annual flowering of intermediate duration during the dry season. The flowering phenophase did not demonstrate any correlation with rainfall or relative humidity, but was found to be negatively correlated with temperature and photoperiod. Fruit maturation initiated during the dry season and was most intense at the start of the rainy season. Seed dispersal demonstrated a negative correlation with relative humidity. As this species occurs along water courses, flowering appears to be independent of any water stress during the dry period. The negative correlation between fruit opening and relative humidity is associated with the desiccation processes necessary for diaspore maturation and dispersal. The dissemination of P. montana seeds is apparently facilitated by the wind during the dry season and then by rainfall at the start of the rainy season (while rainfall would also increase the probability of seedling establishment).

ON THE RELATIVE CONTRIBUTIONS OF WIND VS. ANIMALS TO SEED DISPERSAL OF FOUR SIERRA NEVADA PINES

Selective pressures that influence the form of seed dispersal syndromes are poorly understood. Morphology of plant propagules is often used to infer the means of dispersal, but morphology can be misleading. Several species of pines, for example, have winged seeds adapted for wind dispersal but owe much of their establishment to scatter-hoarding animals. Here the relative importance of wind vs. animal dispersal is assessed for four species of pines of the eastern Sierra Nevada that have winged seeds but differed in seed size: lodgepole pine (Pinus contorta murrayana, 8 mg); ponderosa pine (Pinus ponderosa ponderosa, 56 mg); Jeffrey pine (Pinus jeffreyi, 160 mg); and sugar pine (Pinus lambertiana, 231 mg). Pre-dispersal seed mortality eliminated much of the ponderosa pine seed crop (66%), but had much less effect on Jeffrey pine (32% of seeds destroyed), lodgepole pine (29%), and sugar pine (7%). When cones opened most filled seeds were dispersed by wind. Animals removed > 99% of wind-dispersed Jeffrey and sugar pine seeds from the ground within 60 days, but animals gathered only 93% of lodgepole pine seeds and 38% of ponderosa pine seeds during the same period. Animals gathered and scatter hoarded radioactively labeled ponderosa, Jeffrey, and sugar pine seeds, making a total of 2103 caches over three years of study. Only three lodgepole pine caches were found. Caches typically contained 1-4 seeds buried 5-20 mm deep, depths suitable for seedling emergence. Although Jeffrey and sugar pine seeds are initially wind dispersed, nearly all seedlings arise from animal caches. Lodgepole pine is almost exclusively wind dispersed, with animals acting as seed predators. Animals treated ponderosa pine in an intermediate fashion. Two-phased dispersal of large, winged pine seeds appears adaptive; initial wind dispersal helps to minimize pre-dispersal seed mortality whereas scatter hoarding by animals places seeds in sites with a higher probability of seedling establishment.

Interaction Between Seed Dormancy-release Mechanism, Environment and Seed Bank Strategy for a Widely Distributed Perennial Legume, Parkinsonia aculeata (Caesalpinaceae)

Parkinsonia aculeata (Caesalpinaceae) is a perennial legume with seeds that have hard-seeded (physical) dormancy and are potentially very long-lived. Seed dormancy is a characteristic that can both help maximize the probability of seedling establishment and spread the risk of recruitment failure across years (bet-hedging). In this study, dormancy-release patterns are described across the diverse environments in which this species occurs in order to test whether wet heat (incubation under wet, warm-to-hot, conditions) alone can explain those patterns, and in order to determine the likely ecological role of physical dormancy across this species distribution. A seed burial trial was conducted across the full environmental distribution of P. aculeata in Australia (arid to wet-dry tropics, uplands to wetlands, soil surface to 10 cm deep). Wet heat explained the pattern of dormancy release across all environments. Most seeds stored in the laboratory remained dormant throughout the trial (at least 84 %). Dormancy release was quickest for seeds buried during the wet season at relatively high rainfall, upland sites (only 3 % of seeds remained dormant after 35 d). The longest-lived seeds were in wetlands (9 % remained dormant after almost 4 years) and on the soil surface (57 % after 2 years). There was no consistent correlation between increased aridity and rate of dormancy release. The results suggest that physical dormancy in P. aculeata is a mechanism for maximizing seedling establishment rather than a bet-hedging strategy. However, seed persistence can occur in environmental refuges where dormancy-release cues are weak and conditions for germination and establishment are poor (e.g. under dense vegetation or in more arid micro-environments) or unsuitable (e.g. when seeds are inundated or on the soil surface). Risks of recruitment failure in suboptimal environments could therefore be reduced by inter-year fluctuations in microclimate or seed movement.

ANNUAL AND SPATIAL VARIATION IN SEEDFALL AND SEEDLING RECRUITMENT IN A NEOTROPICAL FOREST

An economy of scale may lead to selection to increase interannual variation in seed production when the per seed probability of seedling establishment increases with seed production. Variable annual seedfall will, however, reduce this probability when postdispersal seed fate is negatively density dependent on the local density of seeds, and seed dispersal and density dependence act identically across years. Intuitively, more variable annual seedfall causes the representative seed to experience a greater density of conspecific seeds and suffer greater density-dependent effects. This handicap must be overcome for the per seed probability of recruitment to be greater in years with greater seed production. We quantified spatial and annual variation in seedfall and seedling recruitment, evaluated density dependence and economies of scale during the seed-to-seedling transition, and investigated the synergistic consequences of density dependence and variable annual seedfall for seedling recruitment on Barro Colorado Island (BCI), Panama. Weekly censuses of 200 0.5-m2 seed traps documented seedfall for 15 years and 108 plant species. Annual censuses of 600 1-m2 seedling plots documented recruitment for nine years and 32 species. The density of seedling recruits tended to increase with the density of seeds; however, the per seed probability of recruitment invariably decreased with seedfall density. Negative density dependence characterized the seed-to-seedling transition. Observed levels of spatial and interannual variation in seedfall density would reduce long-term recruitment by up to 28% if negatively density-dependent survival acted identically across years; however, the strength of negative density dependence varied significantly among years for 12 of 32 species. Negative density dependence occurred in all years for these species but was significantly weaker during the one or two years of greatest seedfall than during the remaining years of lower seedfall. The per seed probability of recruitment increased significantly with annual seedfall for eight of these species. These eight species realized postdispersal economies of scale despite the reduction in long-term recruitment expected from the synergism between variable annual seed production and negatively density-dependent seed fate.