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2004. Contagious seed dispersal beneath heterospecific fruiting trees and its consequences. Á/ Oikos 107: 303 Á/308.An hypothesized advantage of seed dispersal is avoidance of high per capita mortality (i.e. density-dependent mortality) associated with dense populations of seeds and seedlings beneath parent trees. This hypothesis, inherent in nearly all seed dispersal studies, assumes that density effects are species-specific. Yet because many tree species exhibit overlapping fruiting phenologies and share dispersers, seeds may be deposited preferentially under synchronously fruiting heterospecific trees, another location where they may be particularly vulnerable to mortality, in this case by generalist seed predators. We demonstrate that frugivores disperse higher densities of Cornus florida seeds under fruiting (female) Ilex opaca trees than under non-fruiting (male) Ilex trees in temperate hardwood forest settings in South Carolina, USA. To determine if density of Cornus and/or Ilex seeds influences survivorship of dispersed Cornus seeds, we followed the fates of experimentally dispersed Cornus seeds in neighborhoods of differing, manipulated background densities of Cornus and Ilex seeds. We found that the probability of predation on dispersed Cornus seeds was a function of both Cornus and Ilex background seed densities. Higher densities of Ilex seeds negatively affected Cornus seed survivorship, and this was particularly evident as background densities of dispersed Cornus seeds increased. These results illustrate the importance of viewing seed dispersal and predation in a community context, as the pattern and intensity of density-dependent mortality may not be solely a function of conspecific densities.

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Switchgrass (Panicum virgatum L.) is a C4 perennial warm season grass indigenous to the North American tallgrass prairie. A number of its natural and agronomic traits, including adaptation to a wide geographical distribution, low nutrient requirements and production costs, high water use efficiency, high biomass potential, ease of harvesting, and potential for carbon storage, make it an attractive dedicated biomass crop for biofuel production. We believe that genetic improvements using biotechnology will be important to realize the potential of the biomass and biofuel-related uses of switchgrass. Tissue culture techniques aimed at rapid propagation of switchgrass and genetic transformation protocols have been developed. Rapid progress in genome sequencing and bioinformatics has provided efficient strategies to identify, tag, clone and manipulate many economically-important genes, including those related to higher biomass, saccharification efficiency, and lignin biosynthesis. Application of the best genetic tools should render improved switchgrass that will be more economically and environmentally sustainable as a lignocellulosic bioenergy feedstock.

Abstract. There currently exists a large push for the use, improvement, and expansion via landscape modification of dedicated biofuel crops (feedstocks) in the United States and in many parts of the world. Ecological concerns have been voiced because many biofuel feedstocks exhibit characteristics associated with invasiveness, and due to potential negative consequences of agronomic genes in native wild populations. Seed purity concerns for biofuel feedstock cultivars whose seeds would be harvested in agronomic fields also exist from the agribusiness sector. The common thread underlying these concerns, which have regulatory implications, is gene flow; thus detailed knowledge of gene flow in biofuel crop plants is important in the formulation of environmental risk management plans. Here, we synthesize the current state of knowledge of gene flow in an exemplary biofuel crop, switchgrass (Panicum virgatum L.), which is native to eastern North America and is currently experiencing conventional and technological advances in biomass yields and ethanol production. Surprisingly little is known regarding aspects of switchgrass pollen flow and seed dispersal, and whether native populations of conspecific or congeneric relatives will readily cross with current agronomic switchgrass cultivars. We pose that filling these important gaps will be required to confront the sustainability challenges of widespread planting of biofuel feedstocks.

This chapter illustrates using simulations how the assumed Janzen-Connell relationship between distance from parent and seed density can break down. Emphasis is given on spatial patterns of seed dispersal, the resultant spatial structure of seeds and the potential consequences for the population and the community. A special form of dispersal limitation is detailed, i.e. contagious seed dispersal, which is defined as the patchy deposition of seeds such that some sites receive many seeds and others receive few to none. By investigating contagious seed dispersal, the effects of spatial variability in seed dispersal curves on subsequent demographic processes are examined. The chapter explains where such dispersal is likely to take place and suggests potential outcomes for seed survival based on the resulting spatial deposition patterns. It illustrates how contagious seed dispersal relates to and modifies the original Janzen-Connell model and its community-level outcomes.

Although land preservation and promotion of successful regeneration are important conservation actions, their ability to increase population growth rates of slow-growing, long-lived trees is limited. We investigated the demography of Taxus floridana Nutt., a rare understory conifer, in three populations in different ravine forests spanning its entire geographic range along the Apalachicola River Bluffs in northern Florida (U.S.A.). We examined spatial and temporal patterns in demographic parameters and projected population growth rates by using four years of data on the recruitment and survival of seedlings and established stems, and on diameter growth from cross-sections of dead stems. All populations experienced a roughly 10-fold increase in seedling recruitment in 1996 compared with other years. The fates of seedlings and stems between 8 and 16 mm differed among populations. The fates of stems in two other size classes (the 2-to 4-mm class and the 4-to 8-mm class) differed among both populations and years. Individual stems in all populations exhibited similarly slow growth rates. Stochastic matrix models projected declines in all populations. Stochastic matrix analysis revealed the high elasticity of a measure of stochastic population growth rate to perturbations in the stasis of large reproductive stems for all populations. Additional analyses also indicated that occasional episodes of high recruitment do not greatly affect population growth rates. Conservation efforts directed at long-lived, slow-growing rare plants like Taxus floridana should both protect established reproductive individuals and further enhance survival of individuals in other life-history stages, such as juveniles, that often do not appear to contribute greatly to population growth rates.Resumen: Aunque la preservación de tierras y la promoción de regeneración exitosa son acciones de conservación importantes, su capacidad para incrementar las tasas de crecimiento poblacional deárboles longevos de crecimiento lento es limitada. Investigamos la demografía de Taxus floridana Nutt, una conífera de sotobosque rara, en tres poblaciones en diferentes bosques de cañada abarcando todo su rango de distribución a lo largo del Río Apalachicola en el norte de Florida, (E.U.A.). Examinamos los patrones espaciales y temporales de parámetros demográficos y las tasas de crecimiento poblacional proyectadas utilizando cuatro años de datos sobre reclutamiento y supervivencia de plántulas y tallos establecidos, y en el diámetro de crecimiento de secciones transversales de tallos muertos. Todas las poblaciones presentaron un incremento general de 10 veces en el reclutamiento de plántulas en 1996 en comparación con otros años. El destino de plántulas y tallos de 8 a 16 mm difirió entre las poblaciones. El destino de tallos en otras dos clases de tamaño (2-a 4-mm y 4-a 8-mm) fue diferente entre ambas poblaciones y entre años. Los tallos individuales en todas las ‡Current address: Savannah River Ecology Laboratory, Kwit et al. Demography of Taxus floridana 433 pob...