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.

Long-distance seed dispersal in plant populations.

Influence of tree species on understory vegetation diversity and mechanisms involved—A critical review for temperate and boreal forests

An ecological comparison between ancient and other forest plant species of Europe, and the implications for forest conservation

1. Histories of changing land use and vegetation of a 380-ha forested area in central Massachusetts (Prospect Hill tract of the Harvard Forest) were reconstructed to investigate (i) the environmental controls over land ownership patterns, agricultural practice and logging activity, and (ii) the vegetation response to these land-use factors. 2. Forest clearance and agricultural expansion parallel trends for central New England: increasing rates of deforestation through the late eighteenth century led to a peak in 1820-80 when more than 80% of the land was open. Reforestation on abandoned fields commenced in 1850 and increased progressively through the early twentieth century

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Land-Use History (1730-1990) and Vegetation Dynamics in Central New England, USA

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. canadense's seed-dispersal curve indicated that oc- casional dispersal events had to have a high frequency ($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.

Seed dispersal and the holocene migration of woodland herbs

(1) The vascular plant species composition of 190 woodland sites in central New England was analysed to determine the effects of overstorey composition and age on the understorey flora. (2) Multiway contingency table analysis suggested there were significant differences in the understorey flora of primary (old-growth) versus secondary (old-field) woods. Differences in the flora of conifer versus broadleaved woods were less pronounced. (3) A variety of factors, e.g. microclimatic conditions and the low seed production and poor colonizing ability of many species are probably responsible for the distinctive flora of central New England's primary woodlands.

D. R. Foster

Papers

Overstorey composition and age as determinants of the understorey flora of woods of central New England