Fecundity and Dispersal in Plant Populations: Implications for Structure and Diversity

Abstract

Abstract.-Demographic models of tree populations assume that seed availability does not depend on the populations themselves. We develop models to assess the consequences of fecundity and dispersal for population structure and diversity. Results show that population structure and reproductive success are importantly affected by seed production and dispersal for realistic parameterization of time scales describing thinning, disturbance, maturation, and longevity. Maturation age affects mean and variance in seed rain. Populations with well-dispersed seed have a structure that is most sensitive to maturation age when disturbance is frequent. With restricted dispersal, delayed maturation means increased variability in seed rain, maximized when half of all patches support reproductive individuals. Density-dependent thinning compensates for the initial variability conferred by limited dispersal but not enough to permit the neglect of fecundity and dispersal at the disturbance frequencies and thinning rates typical in many forests. Longevity matters most when it is short and disturbance rare. To assess the effects of dispersal on reproductive success, we partition the contributions of seed-rain mean and variance. Fecundity and population structure affect both the mean and the variance in seed rain, albeit in different ways. Dispersal affects only the variance. The partitioned contribution of mean and variance are used to consider two cases: how dispersal consequences for reproductive success depend on life-history schedules and disturbance regime, and boundary growth rates of a globally dispersed population invading a resident population with restricted dispersal. In both cases, restricted dispersal has important consequences on the scales observed in many real forests. Most models of forest tree dynamics assume a globally dispersed seed pool that is disconnected from the populations that should produce that seed. This assumption leads to two opposing (offsetting?) consequences for species diversity: artificially high diversity due to continuous seed supply and artificially low diversity due to lack of sites where good competitors with restricted dispersal should be absent.Recruitment represents a potentially significant, yet poorly described, aspect of tree population dynamics. Fecundity schedules may affect the success of a population in competition and when the environment varies in time (e.g., because of weather or disturbance). Dispersal governs spatial variance in seed rain. That variability can determine whether offspring locate suitable sites and the importance of density-and frequency-dependent interactions (e.g., pathogens, herbivores, competition with parent, siblings, other species). These relationships affect, in turn, species diversity and rates at which species ranges respond to changing environments (Watts 1973;Baker 1974;Payette and Gagnon 1985). Fecundity and dispersal are difficult to parameterize in closed stands, because seed production can vary with age, size, stand characteristics, and across tim...

Long-Term Studies of Vegetation Dynamics

“…In xeric habitats with stand-destroying fires, the competition-colonization mechanism may be dominant (54). Compared to late-successional species, earlysuccessional species tend to have long-dispersal, low-density wood, which leads to rapid growth in height, short longevity, early maturation, short-lived leaves, low specific leaf area (ratio of leaf mass to surface area), high rates of mortality under resource deprivation, and low total leaf area per unit mass (50,(55)(56)(57) (Fig. 1).…”

Plant recruitment bottlenecks in temperate forest fragments: seed limitation and insect herbivory

“…Our study contributes additional evidence to the increasingly dominant view that seed and seedling stages are critically important for temperate and tropical forest dynamics (Clark and Ji 1995;Clark et al 1998Clark et al , 1999Hubbell et al 1999;Curran and Leighton 2000;Curran and Webb 2000;Harms et al 2000;McEuen and Curran 2004). Seed distributions of most temperate trees and shrubs can be quite restricted, with many microsites within forests failing to receive seed (Clark et al 1998;McEuen and Curran 2004).…”

Effects of dispersal, shrubs, and density-dependent mortality on seed and seedling distributions in temperate forests

“…Ecologists have used theory to show that numerous processes can allow species to coexist (Janzen 1970;Connell 1971;Grubb 1977;Shmida and Ellner 1984;Pacala and Tilman 1994;Clark and Ji 1995;Hurtt and Pacala 1995). Empirical studies confirm that many of these processes operate in natural communities (e.g., Burton and Bazzaz 1991;Ribbens et al 1994;Takashi 1997;Bazzaz 1999a, 1999b;Carlton and Bazzaz 1998;Clark et al 1998;Harms et al 2000;Packer and Clay 2000;Hille Ris Lambers et al 2002).…”

“…Few temperate tree species saturate the forest floor with seed, even when abundant (Ribbens et al 1994;Clark et al 1998Clark et al , 1999a. This may limit interspecific competition and promote species diversity (Shmida and Ellner 1984;Clark and Ji 1995;Hurtt and Pacala 1995). Many temperate forest species experience strong density-dependent mortality at early life history stages (Streng et al 1989;Jones et al 1994;Packer and Clay 2000;Hille Ris Lambers et al 2002), which can also promote species diversity (Janzen 1970;Connell 1971;Harms et al 2000).…”