We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems.

Plant litter : its dynamics and effects on plant community structure

What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

We develop a model that outlines the movements and fates of seeds after they leave the parent plant, and then we examine the relative influences of abiotic and biotic factors on those movements and fates. Phase I dispersal is movement of a seed from the parent to a surface, while Phase II dispersal includes subsequent horizontal or vertical movements. Although less studied, Phase II dispersal is more likely to account for the patterning of plants in communities and ecosystems and is the focus of this review. Abiotic factors influence Phase II dispersal-the distance and type of movement depend on seed morphology, surface attributes, and the nature of the physical forces. Biotic factors (animals) move seeds to new sites passively either on body surfaces or by ingestion, or actively by consuming fruits or hoarding seeds. Animals also influence the movements of seeds through digging and burrowing activities. Arrival at microsites suitable for germination and establishment is critical and is affected not only by abiotic and biotic factors but also by seed morphology and ger- mination responses. We emphasize that seed banks are much more dynamic than they are usually portrayed. Although often poorly quantified, seed mor-

A Day in the Life of a Seed: Movements and Fates of Seeds and Their Implications for Natural and Managed Systems

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

We conducted a field experiment in a successional grassland to investigate the short-term effects of nitrogen enrichment and plant litter on plant species richness and on the establishment of a native grass (Andropogon gerardi) that was experimentally introduced to the study site as seed. Additions of nitrogen fertilizer to experimental plots over two growing seasons increased plant production as indicated by increases in both living plant biomass and litter biomass. Increased productivity reduced species richness by effectively preventing the seedling establishment of the subordinate forb species and reduced the recruitment of Andropogon gerardi by inhibiting both germination and survival. Litter removals carried out at each of two levels of nitrogen enrichment (no nitrogen, nitrogen added) showed that litter significantly reduced species richness by the same amount in fertilized and unfertilized plots, suggesting that living biomass and litter were purely additive, rather than interactive in their effects. In contrast, the results of litter additions suggested that the declines in richness associated with fertilization could largely be due to the suppressive effects of increased litter biomass alone. As a whole, the results indicate that litter and living biomass are largely substitutable in their inhibitory effects on species richness in highly productive successional grasslands due to their independent and equivalent capacities to attenuate light to very low levels. This study highlights the combined roles of competition and plant litter in influencing the diversity of grasslands through effects on seedling establishment.

/pdf/species-richness-in-a-successional-grassland-effects-of-9wbwnsymox.pdf

Species richness in a successional grassland: effects of nitrogen enrichment and plant litter

In natural populations of musk thistle (Carduus nutans L.) germination occurs over several months in the fall and spring. To better understand the physical environmental mechanisms that influence this pattern of germination, a greenhouse experiment was established consisting of ten treatments in which soil surface topography and the amount and consistency of litter cover was varied. Significant differences occurred among the treatments for germination percentage, levels of mortality and individual growth rates. Optimum levels ofgermination, survival and growth occurred in treatments that provided microhabitats with reduced evaporation. Treatments with cracks or moderately irregular surface topographies provided the best conditions. A light covering of litter benefited establishment and growth but thick layers of litter prevented seeds from reaching the soil surface and reduced germination and establishment. These results were consistent with what is known about the germination ecology of musk thistle in natural populations. IN MANY PLANT species the highest levels of mortality occur between the seed dispersal and the seedling establishment phases of the life cycle (Harper and White, 1974; Harper, 1977). Unfortunately, this is often the most poorly understood portion of the life cycle. Usual monitoring procedures are capable of quantifying seed production and seedling establishment but are generally incapable of describing interactions between seeds and the soil surface environment. Yet many biologists (e.g., Grubb, 1977; Grime, 1979) feel that these interactions are critical to the reproduction of populations and the composition of plant communities. Successful germination involves complex interactions between the seed's physical position, its physiological state, and its microenvironment. Some species have genetically determined, innate dormancy requirements (Harper, 1977) that prevent germination until after specific environmental keys have been experienced (e.g., Betula pubescens, Black and I Received for publication 13 February 1986; revision accepted 19 August 1986. We wish to express our gratitude to Karen J. Hamrick and Jenna R. Hamrick for their technical assistance. Steve Stewart provided valuable assistance during the data analyses. The comments of two anonymous reviewers greatly improved this paper. This work was supported by funds supplied by University of Kansas General Research Fund Grant #3675 and Environmental Protection Agency Grant R-805455030. 2 Current address: Departments of Botany and Genetics, University of Georgia, Athens, GA 30602. 3Current address: Department of Botany and Plant Sciences, University of Califomia, Riverside, CA 92521. Wareing, 1955; Avenafatua, Marshall and Jain, 1970; Naylor and Jana 1976; Ambrosia artemisifolia, Willemsen 1975a, b; Avena barbata, Marshall and Jain, 1970). Other species have less rigid dormancy (enforced, Harper, 1977) and can germinate whenever minimum environmental conditions are met (usually heat, light, moisture, and proper aeration). Adverse conditions such as drought, cold, or deep burial in soil or litter prevent or retard germination in these species (Werner 1975; Harper, 1977). Several investigators (Harper and Sager, 1953; Harper, Williams and Sager, 1965; Sheldon, 1974) have demonstrated that subtle features of the soil surface topography and the position of the seed in relation to this topography can have dramatic effects on germination and seedling establishment. Studies of the population ecology of musk thistle (Carduus nutans L.) indicate that germination occurs over periods of several months in both fall and spring (Lee and Hamrick, 1983). Subsequent survival, growth and fecundity of monitored individuals was influenced by the their time of germination. Generally, individuals germinating in the early fall had lower mortality, attained larger vegetative sizes and produced more seeds than those that germinated later in the fall or in the spring. Later studies (J. L. Hamrick, unpubl. data) indicate, however, that germination early in the fall can be followed by a period of high mortality if a period of drought immediately follows germination. It is clear, therefore, that the dynamics of musk thistle populations may be

Janet M. Lee

Papers

Effect of soil surface topography and litter cover on the germination, survival, and growth of musk thistle (Carduus nutans)