Seed dispersal in Erythronium grandiflorum (Liliaceae)

Intrinsic and extrinsic drivers of intraspecific variation in seed dispersal are diverse and pervasive

There is growing realization that intraspecific variation in seed dispersal can have important ecological and evolutionary consequences. However, we do not have a good understanding of the drivers or causes of intraspecific variation in dispersal, how strong an effect these drivers have, and how widespread they are across dispersal modes. As a first step to developing a better understanding, we present a broad, but not exhaustive, review of what is known about the drivers of intraspecific variation in seed dispersal, and what remains uncertain. We start by decomposing 'drivers of intraspecific variation in seed dispersal' into intrinsic drivers (i.e. variation in traits of individual plants) and extrinsic drivers (i.e. variation in ecological context). For intrinsic traits, we further decompose intraspecific variation into variation among individuals and variation of trait values within individuals. We then review our understanding of the major intrinsic and extrinsic drivers of intraspecific variation in seed dispersal, with an emphasis on variation among individuals. Crop size is the best-supported and best-understood intrinsic driver of variation across dispersal modes; overall, more seeds are dispersed as more seeds are produced, even in cases where per seed dispersal rates decline. Fruit/seed size is the second most widely studied intrinsic driver, and is also relevant to a broad range of seed dispersal modes. Remaining intrinsic drivers are poorly understood, and range from effects that are probably widespread, such as plant height, to drivers that are most likely sporadic, such as fruit or seed colour polymorphism. Primary extrinsic drivers of variation in seed dispersal include local environmental conditions and habitat structure. Finally, we present a selection of outstanding questions as a starting point to advance our understanding of individual variation in seed dispersal.

Post-floral Erection of Stalks Provides Insight into the Evolution of Fruit Orientation and Its Effects on Seed Dispersal

That stalks reorient after flowering to face upwards is a common phenomenon in many flowering plants, indicating the potential importance of fruit orientation on seed dispersal. But this idea has not been subject to an empirical test. We examined this hypothesis by analysing the evolutionary correlation between fruit orientation and other characters and by investigating the effects of fruit orientation on seed dispersal. We found that 1) in a sub-alpine plant community, upward fruit orientation strongly correlates with fruits that act as seed containers, which are often of dry type and are dispersed by non-animal vectors; 2) as exemplified by the Campanulaceae s. str., fruit orientation strongly correlates with dehiscence position. Upwardly-oriented capsules dehisce at the apex, whereas pendent ones dehisce at the base, in both cases ensuring that seeds are released from an upright position; 3) in manipulation experiments on Silene chungtienensis, upward fruits (the natural state) exhibit much greater dispersal distances and more dispersive pattern than pendent ones, and have a more even distribution of dispersal direction than horizontal ones. Our results suggest that fruit orientation may have important function in seed dispersal, which may be the reason why the phenomenon that stalk erection after flowering occurs widely.

Seed dispersal by rodents in a lowland forest in central Panama

We studied the removal of seeds of three species of large-seeded tree (Astrocaryum standleyanum, Attalea butyracea and Dipteryx oleifera) from three different heights within six study plots in a lowland forest in central Panama. Fresh fruits with intact seeds fitted with industrial sewing bobbins were placed within semi-permeable exclosures. Removed seeds were tracked to deposition sites, and seed fate was determined. Removals were likely perpetrated by two small rodents, the strictly terrestrial Proechimys semispinosus and the scansorial Sciurus granatensis, because they were the most abundant small rodents in the study site during the study period and were of sufficient size to remove large seeds. Rodent abundance and fruit availability were estimated by conducting censuses. Nine microhabitat variables were measured at each deposition site to determine if these two rodents were preferentially depositing seeds in sites with certain characteristics or were randomly depositing seeds. During the study, rodents handled 98 seeds, 85 of which were not predated upon and could potentially germinate. Removal rates were not influenced by rodent abundance or fruit availability. Seeds were most frequently moved <3 m and deposited with the fruit eaten and the seed intact. However, some seeds did experience relatively long-distance dispersal (>10 m). Rodents preferentially deposited seeds in locations with large logs (>10 cm diameter), dense herbaceous cover, and an intact canopy. The number of large logs was different from random locations. Despite not being able to determine long-term fate (greater than c. 1 y), we show that these small rodents are not primarily seed predators and may in fact be important mutualists by dispersing seeds relatively long distances to favourable germination sites.

Reproductive uncertainty and the relative competitiveness of simultaneous hermaphroditism versus dioecy

Hermaphroditism is typically associated with a sedentary existence, whereas dioecy is associated with mobility. This pattern is reflected within flowering plants, as dioecious species commonly possess traits that promote high dispersal. We investigated these associations with three population dynamics models (an individual-based simulation and two mathematical models, one deterministic and the other stochastic) that allowed us to examine competition for space between a hermaphroditic and dioecious species from different perspectives. The competing species are identical in every way but their sexual system. Separation of the sexes increases the variances of pollen import and seed dispersal for the dioecious species. These variances propagate through subsequent reproductive processes and ultimately reduce mean recruitment as a result of nonlinear averaging (Jensen's inequality). A dioecious species could overcome this disadvantage simply by producing more gametes than hermaphrodites; however, in line with the association with mobility, selection on dioecious species should also favor traits that reduce reproductive uncertainty, such as extensive dispersal.

Autecology of Erythronium grandiflorum in western Canada

Habitat requirements of the yellow glacier lily, Erythronium grandiflorum Pursh, were studied at 38 sites in southern British Columbia and southwestern Alberta. At each site we recorded densities of E. grandiflorum growth stages from seedling to flowering, environmental characteristics of the site, and percent cover of associated plant species. We carried out detrended correspondence analysis (DCA) of the sites based on cover of associated species, and examined rank correlations between site environmental variables and site ordination scores. Variation among sites was related primarily to elevation and the presence of deciduous vs. coniferous trees. All growth stages of E. grandiflorum were less abundant in coniferous forests than in open areas or sites with deciduous trees, suggesting that evergreen canopies restrict the species on many sites where it could otherwise grow. Although E. grandiflorum populations were most commonly found in subalpine meadows, they flowered more abundantly in low-elevation populations. Recruitment was frequent, with seedlings occurring at many sites; we also showed that detached bulb segments can give rise to new ramets. The present widespread distribution of E. grandiflorum may derive from a post-glacial period with extensive meadow habitat that was favourable for rapid spread.Key words: canopy cover, elevation, yellow glacier lily, post-glacial migration, subalpine meadows.

Fine-scale genetic structure and clinal variation in silene acaulis despite high gene flow

We investigated whether the distribution of genes reflects the patchy distribution of individuals of Silene acaulis on Pennsylvania Mountain in central Colorado. Five polymorphic protein loci were analysed using both F-statistics and spatial autocorrelation. Low thetaPOP (FST) indicated little genetic differentiation between populations approximately 1 km apart. This indicates high gene flow within our study site, perhaps as a result of long-distance pollen dispersal. Despite little differentiation between populations, there was clinal variation at the 6-Pgd-1 locus and significant within-population genetic structure (indicated by both thetaPATCH and spatial autocorrelation). We infer that this fine-scale genetic structure is the result of limited seed dispersal combined with genetic drift. The level of genetic structure varied markedly among populations, with the greatest genetic structure (highest Moran's I and thetaPATCH values) in two low-altitude, small, low-density populations. Intensive sampling such as used in this study may reveal similar patterns of fine-scale genetic differentiation in other patchily distributed plant species, particularly those with limited seed dispersal.

Morphological and genetic variation in disjunct populations of the avalanche lilyErythronium montanum

The genetic structure and morphological variability of species are influenced by both life-history traits and historical factors. We studied morphological and genetic variability in 12 populations of the avalanche lily, Erythronium montanum, representing the four disjunct regions of its geographic range in Oregon, Washington, and British Columbia. Floral traits exhibited significant differences among regions and among populations within regions, but these differences showed no clear geographic pattern, and for all traits there was substantial overlap among populations. In contrast, analysis of isozyme variation indicated a marked north–south gradient in genetic variability. The northernmost populations (from the B.C. Coast Range) had an average of 13% polymorphic loci and 1.13 alleles per locus, whereas southern populations (from the Cascade Mountains) averaged 48% polymorphic loci and 1.67 alleles per locus. The northern populations possessed no unique alleles but contained a subset of the alleles found in regions further south. We conclude that disjunct populations of E. montanum in B.C. probably arose through long-distance dispersal from more southern populations following deglaciation. Keywords: Erythronium montanum, genetic variation, glacial refugia, isozymes, long-distance dispersal, phytogeography.