Michael L. Cain

Long-distance seed dispersal in plant populations

Long-distance seed dispersal influences many key aspects of the biology of plants, including spread of invasive species, metapopulation dynamics, and diversity and dynamics in plant communities. However, because long-distance seed dispersal is inherently hard to measure, there are few data sets that characterize the tails of seed dispersal curves. This paper is structured around two lines of argument. First, we argue that long-distance seed dispersal is of critical importance and, hence, that we must collect better data from the tails of seed dispersal curves. To make the case for the importance of long-distance seed dispersal, we review existing data and models of long-distance seed dispersal, focusing on situations in which seeds that travel long distances have a critical impact (colonization of islands, Holocene migrations, response to global change, metapopulation biology). Second, we argue that genetic methods provide a broadly applicable way to monitor long-distance seed dispersal; to place this argument in context, we review genetic estimates of plant migration rates. At present, several promising genetic approaches for estimating long-distance seed dispersal are under active development, including assignment methods, likelihood methods, genealogical methods, and genealogical/demographic methods. We close the paper by discussing important but as yet largely unexplored areas for future research.

ARE LONG-DISTANCE DISPERSAL EVENTS IN PLANTS USUALLY CAUSED BY NONSTANDARD MEANS OF DISPERSAL?

It has been argued that nonstandard mechanisms of dispersal are often responsible for long-distance dispersal in plants. For example, plant seeds that appear to be adapted for wind dispersal may occasionally be dispersed long distances by birds, or vice versa. In this paper, we explore whether existing data on dispersal distances, colonization rates, and migration rates support the idea that dispersal processes suggested by the morphology of the dispersal unit are responsible for long distance dispersal. We conclude that the relationship between morphologically defined dispersal syndrome and long-distance dispersal is poor. This relationship is poor because the relationship between the morphology of dispersal units and the multiple processes that move seeds are often complex. We argue that understanding gleaned from the often anecdotal literature on nonstandard and standard means of long distance dispersal is the foundation for both statistical and mechanistic models of long-distance dispersal. Such models hold exciting promise for the development of a quantitative ecology of long-distance dispersal.

SEED DISPERSAL AND THE HOLOCENE MIGRATION OF WOODLAND HERBS

The distribution of many woodland herbs extends 1000-2000 km in a north- south direction, yet the majority of these species grow clonally, have little recruitment by seed, and possess no obvious mechanism for long-distance seed dispersal. Although aware that woodland herbs disperse poorly, ecologists have tacitly assumed that, given long periods of time, even small dispersal distances would allow woodland herbs to colonize the vast geographic region they now occupy. We examined this assumption for the understory herb Asarum canadense. To estimate long-term rates of spread by seed, we calibrated seed- dispersal diffusion models with life history data and with data on seed carries by ants. We supplemented our field observations and modeling results forA. canadensewith a literature survey on the dispersal capabilities of other plant species. Ants transported A. canadense seeds up to 35 m, the largest distance ants are known to move the seeds of any woodland herb. Empirically calibrated diffusion models indicated that over the last 16 000 yr A. canadense should only have traveled 10-11 km from its glacial refugia. In actuality, A. canadense moved hundreds of kilometers during this time. Models that examined the tail of A. canadenses seed-dispersal curve indicated that oc- casional dispersal events had to have a high frequency (

Sampling spatial and temporal variation in soil nitrogen availability

0.001 on a per seed basis) and a large magnitude (dispersal distance .1 km) for A. canadense to have traveled over 200 km in 16 000 yr. The literature survey showed that most woodland herbs and many other forest, desert, coastal, and open-habitat plant species have limited seed-dispersal capabil- ities, similar to those in A. canadense. We conclude that woodland herbs, as well as many other plants, disperse so slowly that there is no documented mechanism by which most of these species could have reached their present geographical range since the last glacial maximum. This suggests that occasional events leading to long-distance dispersal dominate the Holocene colonization of northern temperate forest by woodland herbs, and this, in turn, has implications for issues ranging from the importance of genetic analyses to the structure of metapopulation models.

Consequences of Foraging in Clonal Plant Species

Abstract There are few studies in natural ecosystems on how spatial maps of soil attributes change within a growing season. In part, this is due to methodological difficulties associated with sampling the same spatial locations repeatedly over time. We describe the use of ion exchange membrane spikes, a relatively nondestructive way to measure how soil resources at a given point in space fluctuate over time. We used this method to examine spatial patterns of soil ammonium (NH+4) and nitrate (NO−3) availability in a mid-successional coastal dune for four periods of time during the growing season. For a single point in time, we also measured soil NH+4 and NO−3 concentrations from soil cores collected from the mid-successional dune and from an early and a late successional dune. Soil nitrogen concentrations were low and highly variable in dunes of all ages. Mean NH+4 and NO−3 concentrations increased with the age of the dune, whereas coefficients of variation for NH+4 and NO−3 concentrations decreased with the age of the dune. Soil NO−3 concentration showed strong spatial structure, but soil NH+4 concentration was not spatially structured. Plant-available NH+4 and NO−3 showed relatively little spatial structure: only NO−3 availability in the second sampling period had significant patch structure. Spatial maps of NH+4 and NO−3 availability changed greatly over time, and there were few significant correlations among soil nitrogen availability at different points in time. NO−3 availability in the second sampling period was highly correlated (r = 0.90) with the initial soil NO−3 concentrations, providing some evidence that patches of plant-available NO−3 may reappear at the same spatial locations at irregular points in time.

CONSPECIFIC SPERM PRECEDENCE IS AN EFFECTIVE BARRIER TO HYBRIDIZATION BETWEEN CLOSELY RELATED SPECIES

Some clonal plant species decrease rhizome or stolon internode lengths and/or increase the frequency of branching when they grow in favorable environments. This foraging response is thought to be beneficial since it should allow ramets to concentrate in areas of favorable habitat. However, there have been few critical tests of the effectiveness with which ramets are placed in favorable habitat as a result of the foraging response. In this paper, I use empirically calibrated stochastic simulation and diffusion models to compare the growth of clones in favorable and unfavorable habitat. I ask whether observed changes in rhizome lengths and clonal branching patterns are likely to decrease significantly the distance clones move, and thus, to enable mets to remain for longer periods of time in favorable habitat. For the empirical data used in this study, results from the models indicate that the effectiveness of the foraging response is likely to be variable. In some cases, such as results from models based on the response of Glechoma hederacea to nutrients, there is no significant difference in the distance clones move in favorable and unfavorable habitat. Thus, even through rhizome lengths may be significantly shorter in favorable patches, this does not guarantee that clones disperse significantly less far and thereby remain longer in favorable habitat. The effectiveness of the foraging response depends strongly on the distribution of clonal growth angles, the pattern of clonal branching, and the variance in rhizome or stolon internode length. These results, particularly the importance of growth angles and the relatively limited effectiveness of the foraging response, differ from those in previously published models of foraging in clonal plant species. I conclude with a discussion of reasons that may underlie these differences.

CLONAL GROWTH AND RAMET PERFORMANCE IN THE WOODLAND HERB, ASARUM CANADENSE

Conspecific sperm precedence is widespread in animals, appears to evolve rapidly, and is thought to have the potential to prevent hybridization between closely related species. However, to date no study has tested the isolating potential of such a barrier in mixed populations of two taxa under conditions in which other potential barriers to gene flow are controlled for or are prevented from operating. We tested the isolating potential of conspecific sperm precedence in the ground crickets Allonemobius fasciatus and A. socius in population cage experiments in which the frequency of the two species was varied. Despite the observation of abundant interspecific matings, the proportions of hybrid progeny were low and differed statistically from the proportions expected in the absence of conspecific sperm precedence. The results demonstrate that conspecific sperm precedence can severely limit gene flow between closely related species, even when one species is less abundant than the other.

Analyzing Discrete Movement Data as a Correlated Random Walk

1 We report the results from a 7-year study on the patterns of reproduction and vegetative spread of Asarum canadense, conducted in replicate plots located in the understorey of early and late successional forest. 2 We recorded the spatial location, size, demographic performance and clonal fragment identity of all A. canadense ramets in our plots, as well as the position and fate of seedling recuits. We also collected detailed morphological data on patterns of clonal growth. 3 Recently germinated plants and the much larger, clonally produced daughter ramets had equal survivorship. Size also had little impact on rhizome production. Within sets of genetically identical ramets, flowering ramets were larger than non-flowering ramets. When there was an effect of habitat, ramets performed best in late successional forest. 4 Neither sexual nor asexual reproduction decreased the future performance of a ramet. Having connections to other ramets had little effect on survivorship or rhizome production, but often was associated with an increased probability of flowering. Clones with more ramets outperformed smaller clones and spread the risk of genet mortality. 5 Rhizome lengths were shorter in the high light, early successional forest, but no other clonal growth parameters differed between habitats. Empirically calibrated simulation and diffusion models indicated that the between-habitat differences in rhizome length were not great enough to affect the long-term expansion of clones. 6 Diffusion coefficients for the long-term spread by clonal growth or seed dispersal were compared for a number of species. Diffusion model results indicated that plants can disperse as far by clonal growth as they do by seed, although the values for A. canadense were 14 and 8895 cm2 year -1, respectively. 7 Asarum canadense appears to be under unusually strong selection for seedling survival. As has been observed for some other clonal plant species, few costs were found for either sexual or asexual reproduction.

Models of clonal growth in Solidago altissima

Correlated random walk models provide a framework for making quantitative predictions about an organisms rate of spread. In this paper we present an algorithm that calculates the variance in squared displacement for a correlated random walk model. This can be used to make statistical comparisons of observed displacements to the model and to approximate the expected net displacement of the model. We illustrate the techniques with examples of insect movement and the vegetative spread of clonal plants. See full-text article at JSTOR

REINFORCING SELECTION IS EFFECTIVE UNDER A RELATIVELY BROAD SET OF CONDITIONS IN A MOSAIC HYBRID ZONE

In this paper, field data on the old-field perennial Solidago altissima were used to test a variety of clonal growth models, including random-walk models, a new way to analyse the vegetative spread of clonal plants.