Interest in developing this multiauthored book grew from our work with seeds and seed-bank ecology. While seed production and seed-bank dynamics are critical stages, what happens to seedlings is also fundamental to explaining field observations of vegetation dynamics and recruitment. Although several recent books discuss seedlings, indicating their importance to plant regeneration (Fenner, 2000) and to seed ecology (Fenner & Thompson, 2005), only one, Swaine (1996), focuses on seedling ecology; it, however, deals exclusively with tropical forest seedlings and is now more than 10 years old. A fourth volume, Forget et al. (2005), is primarily about seed predation and dispersal. Seedling Ecology and Evolution will complement these works and provide a more all-encompassing discussion. Moreover, it bridges the life-cycle gap following seeds (e.g. Baskin & Baskin, 1998) and seed banks (e.g. Leck et al., 1989). Additional information about regeneration strategies may be found in Harper (1977), Grubb (1977, 1998), and Grime (2001).

We acknowledge the importance of understanding seedling biology in agriculture and horticulture; however, seedlings are well studied in these settings, whereas in natural systems, seedlings are less studied, and the literature is more diffuse. This book explores seedling adaptations and constraints to regeneration in natural and disturbed systems, where a better understanding of seedlings would stimulate study and development of theory regarding this dynamic and often neglected part of the plant life cycle.

After seeds, seedlings typically suffer the highest mortality rate of any life history stage and, therefore, are important in the selection and evolution of species.

Introduction

The last decade has seen rapid growth in research dedicated to seedling ecology (Kitajima, 2007). This reflects an increased recognition of the importance of variation in seedling survival in determining patterns of adult abundance and distribution. Many seedling studies have grown out of larger programs investigating community dynamics in natural systems. Nonetheless, the results of these studies are often directly relevant to the management and conservation of human-altered ecosystems where changing environmental conditions and altered biotic interactions can directly affect seedling recruitment success. In extreme cases, human-mediated disturbances may be sufficient to invoke community-wide recruitment failure, leading to stalled succession or to shifts in vegetation type. Disturbance effects, however, can also have more subtle effects on recruitment success, resulting in changes in forest composition that may take decades or more to become apparent (Dirzo & Miranda, 1990).

Seedlings are particularly vulnerable to disturbance. Most species have no equivalent of the dormancy that facilitates seed survival through periods with adverse environmental conditions. Instead, alterations in light or soil moisture availability often impact seedlings first because their small leaf area and shallow rooting depth limit their ability to integrate resource capture over space. Similarly, only a small fraction of tree species have seed reserves that persist for more than a few months after germination, leaving seedlings vulnerable to damage and to temporal fluctuations in resource supply (Kitajima, 1996a; Harms & Dalling, 1997). Seedlings are also especially susceptible to natural enemies and fire.

Introduction: Phytohormones, molecular biology, and the “real world” of early seedling ecology

The seedling stage is arguably the busiest phase in a plant's lifetime. A miniscule individual, possessing only a rudimentary cotyledon and radicle, must quickly make its way in the world before a host of environmental challenges and larger competitors bully it out of existence. The maternal reserves from the seed must be mobilized to provide the fuel for producing the first pair of photosynthetically competent leaves and proliferating roots that will assimilate nutrients and firmly secure the seedling to its new home in the soil. These critical activities are initiated within minutes of germination, requiring a flurry of rapid signal transduction, protein synthesis, and tissue differentiation. To grasp how the seedling accomplishes all this demands an examination of physiological mechanisms that govern life after germination.

Studies at the molecular level are making great headway in understanding the complex and fascinating physiological events that mark the early infancy of plants. Phytohormones (a broad term that encompasses a range of organic and polypeptide compounds) are increasingly recognized as the primary integrators of environmental signals that enable the nascent seedling to evaluate and adjust to existing conditions. Biologists working across the spectrum of organizational levels – from cells to trees, from genes to populations – are now in an exciting position to elucidate how phytohormones modulate phenotype, and seedlings from a variety of taxa are natural model systems for study and synthesis (Table 7.1).

Introduction

Seedlings are particularly susceptible to harsh conditions. Indeed, the seedling stage is considered to be the most vulnerable stage in the life of the plant (Stebbins, 1971; Fenner, 1987; Fenner & Thompson 2005) because even small reductions in biomass may lead to the death of the plant (Dirzo, 1985; Fenner & Thompson, 2005). Selection has favored strategies that reduce the high risk of the seedling stage primarily in two ways: first, maternal deployment of optimal amount of reserves to ensure maximum likelihood of seedling survival (Smith & Fretwell, 1974; Westoby et al., 1992; Leishman & Westoby, 1994a; Leishman et al., 2000), and second, timing of germination to avoid emergence during periods of high environmental stress as well as during transient favorable conditions too short to ensure postemergence survivorship (Grime, 1979; Baskin & Baskin, 1989, 1998; Fenner & Thompson, 2005). However precise the mechanism to adjust germination to low-risk conditions may be, many environments present inherently high risks for seedlings because the stress is chronic or favorable conditions are intermittent and uncertain (see Table 3.1). Because the ability of the seedling to accumulate or replace biomass decreases as the environment becomes less favorable, seedlings in stressful environments are at higher risk of mortality. Furthermore, when postemergence mortality is highly probable, avoidance of stress per se is not a viable strategy and seedlings are selected to tolerate stressful conditions.

Introduction

In this chapter, we will describe the intricate links between seedling ecology and life history traits such as seed mass, time to maturity, adult size, and reproductive life span. We will pay particular attention to seed mass, as this is the trait most closely linked to seedling ecology. Seed mass affects the initial size of the seedlings, the amount of reserves seedlings have for establishment, the sites to which seeds are dispersed, and the time seeds spend in the soil before germinating.

Much of our understanding of seed and seedling ecology has been based on the idea that plants face a trade-off between producing a few large seeds, each with high rates of survival as seedlings, versus producing many small seeds, each with lower rates of survival as seedlings. We, therefore, begin by reviewing the evidence for this trade-off. Our review shows that a full understanding of seed and seedling ecology requires consideration of life history variables such as plant height, reproductive life span, and the length of the juvenile period. Then we present a new framework for understanding seed and seedling traits as part of an overall life history strategy. Next we outline relationships between seed and seedling traits and other aspects of plant ecological strategy, such as seed dispersal syndrome, the capability to form soil seed banks, tissue density, and adult plant traits.

Introduction

In plant population ecology, recruitment refers to the process by which new individuals found a population or are added to an existing population. Although recruitment may refer to clonal offspring, by far the most common means of recruitment is by seedlings. Seedling recruitment includes three basic processes: seed germination, seedling survivorship, and seedling growth. Seedlings represent the interface between the seed and the developing plant, and as a transitional life cycle stage, it has been difficult to define unambiguously when a seedling stops being a seedling (Kitajima & Fenner, 2000). Population studies often define seedlings somewhat arbitrarily, implying also that population processes such as seedling recruitment may be assessed arbitrarily. Irrespective of the definition, it is clear that the seedling stage represents the most sensitive part of the plant life cycle (Harper, 1977; Silvertown & Charlesworth, 2001). Seedlings are usually small and vulnerable to various abiotic and biotic agents. They often have only a tiny supply of resources to consume before they must exploit external resources in competition with other plants. Thus, seedlings are commonly subject to the highest mortality rates of any stage in the plant life cycle.

Seedling recruitment varies widely across species and communities. A stable population is maintained if one reproductive individual is replaced, on average, by one successfully recruited offspring. Therefore, we expect that there is a relationship between the life span of the plants and the temporal pattern of seedling recruitment.

Introduction

Seedling establishment is one of the key processes that determines the structure and diversity of natural communities. There are many factors that contribute to seedling establishment as explored in Pickett et al. (1987) and this volume. To date, little attention has been paid to the effects of mycorrhizal fungi on seedling establishment. However, there are several compelling reasons to consider these symbiotic fungi. First, the vast majority of all land plant species form symbiotic associations with mycorrhizal fungi and seedlings of most species become colonized by these soil fungi immediately after germination and root formation (Newman, 1988; Read & Birch, 1988; Wang & Qiu, 2006). Second, seedlings usually receive mineral nutrients from mycorrhizal fungi and often show enhanced growth when colonized. In addition, several studies report that mycorrhizal fungi can protect seedlings against drought and the harmful effects of pathogenic soil fungi and heavy metals. Third, many mycorrhizal fungi are not host specific and can colonize a wide range of plant species (Molina et al., 1992; Smith & Read, 1997; Opik et al., 2006). Due to this lack of specificity, seedlings can quickly become integrated into hyphal networks that are usually already present and maintained by the surrounding vegetation. Hence, in this way, seedlings have immediate access to a cheap nutrient absorption machine in which they do not need to invest resources (Newman, 1988).

Introduction

This chapter focuses on phylogenetically diverse groups of plants that do not have typical life history strategies as seedlings, juveniles, and mature individuals. Plants that live on other plants are classified as epiphytes, and they include both vascular and nonvascular species (Benzing, 1990). Tropical orchids and bromeliads comprise the vast majority of epiphytic flowering plants. However, orchids are global in their distribution and many are terrestrial (Dixon et al., 2003). Carnivorous plants are also globally distributed, occurring in many types of ecosystems (Lloyd, 1976), as are parasitic plants (Press & Graves, 1995). The ability of seedlings to establish in habitats with extreme limiting resources is one of the few factors that links the diverse plants covered in this chapter. Epiphytes, for example, must initially become established on structures (i.e. branches) where resources are scarce. Once established, epiphytes may have to deal with combinations of stresses, including aridity, few available nutrients, and either high or low light conditions. Carnivorous plant species and many terrestrial orchids occur in habitats where nutrients or light are limiting. Many parasitic plants (e.g. mistletoes) also occur in resource-limited environments.

One of our approaches to organizing this chapter is to determine if seedlings differ in their physical or ecological characteristics in a manner similar to mature plants. The literature has few examples of investigations focusing specifically on seedlings. As an example, in the seminal book on Bromeliaceae, Benzing (2000) did not consider seedlings as a separate heading.

Introduction

Biological invasions constitute an environmental problem of growing global concern. The explosive growth of exotic, invading species is second only to habitat loss as a factor threatening endangered plants and animals worldwide (Cronk & Fuller, 1995; Hobbs & Mooney, 1998). Because invasive species did not evolve within their current ecological contexts, they often process resources and energy differently than natives. Thus, successful invasions often result in changes to ecosystem dynamics and interspecific interactions. Invasive species have been shown to alter disturbance regimes (Cronk & Fuller, 1995; Hobbs & Huenneke, 1992) and resource cycling (Mooney & Drake, 1989). These changes result in reduced biological diversity (Baskin, 1998; D'Antonio & Vitousek, 1992; Sala et al., 2000; Meiners et al., 2001) and contribute to reductions in ecosystem function and productivity. Biological invasions are also estimated to incur costs of nearly $137 billion per year in the United States alone (Pimentel et al., 2000).

Because invasions lead to so many undesirable ecological and economic changes, predicting and preventing them have become research priorities. Ecologists have used historical data on existing invasions to ask which species are likely to adversely affect newly recipient ecosystems (e.g. Mack, 1996; Sutherland, 2004) and which communities are likely to be especially susceptible to alterations by such invasives (e.g. Elton, 1958; Drake et al., 1989).

Introduction

Based on over two centuries of biogeographical research starting with Schimper (1898) and supported by intensive ecological studies during the past decades, a synthesis of large-scale plant distribution patterns and plant functional types may now be possible (Box, 1996; Westoby & Wright, 2006). Plant functional types are nonphylogenetic species groups that show close similarities in their response to ecological factors (Duckworth et al., 2000). Functional types are derived from morphological, ecophysiological, and life history traits, often cutting across taxonomic groupings. One advantage of this approach is that some traits can be readily measured in the field and may act as surrogates for others, which require time-consuming laboratory measurements or experiments. Plant functional types are the ones suggested by Grime (1979, 2001), the three main traits determining plant performance as proposed by Westoby (1998), hierarchical classifications (Lavorel et al., 1997), or the seedling strategy types described by Keddy et al. (1998); the latter were experimentally tested by Carlyle and Fraser (2006). These conceptual frameworks and some older systems like the life-forms by Raunkiaer (1934) or the scheme devised by Hallé and Oldeman (1975), based on the reiteration of plant modules, may now be integrated into the biogeographical types of the world vegetation (Whittaker, 1970; Holdridge et al., 1971; Breckle, 2002), as done for seed dormancy and germination by Baskin and Baskin (2001).

Introduction

Consider the following: tidal freshwater marshes along the East Coast of North America in springtime; the deserts near Death Valley, in Africa, and elsewhere following a substantial rainfall; the intermittent wetlands in the arid Australian landscape; and the wheat fields of Europe, North America, and New Zealand. Each landscape is awash with the greens of newly emerged seedlings, each species responding to its particular set of germination cues, each informed by its peculiar evolutionary history. Anyone interested in seed banks and seed germination ecology and physiology, as well as those who garden, are intimately familiar with seedlings. Seedlings are also well known to those who produce seeds for use in agriculture and horticulture and who are concerned with vigor and other seedling attributes (Geneve, 2005; Stephenson & Mari, 2005; Farooq et al., 2006). In this chapter, we explore the diverse and fascinating array of seedlings and seedling natural history. Topics include the seedling stage, morphological and physiological diversity, vivipary, seedling equivalents, seedling longevity and dispersal, and environmental filters and safe sites.

Nomenclature generally follows that of the author and family names (Mabberly, 1997).

The seedling stage and fate of seedlings

Contrary to what seems intuitive, the seedling stage is not always easily defined. The success of seedlings is, furthermore, influenced by many environmental factors that determine survival, establishment, and, ultimately, community composition.

The seedling

Seedling is used for a very young individual (Burger, 1972), but problems occur in determining the beginning and end of the stage.

Seedlings as part of a plant's life cycle

The seedling, the young spermatophyte plant following germination, is but one stage in the continuum of a seed plant's life cycle. For ecological purposes, discussion on the life cycle (illustrated in Fig. 1.1) focuses on the processes involved in replacing the adult and/or colonizing new habitats. A reproductive adult plant produces seeds that, once dispersed, become part of the seed bank (Parker et al., 1989; Simpson et al., 1989). Then, following germination, a seedling faces unpredictable environments and is limited by its particular genetic constraints. However, if successful, it survives to adulthood and reproduction.

Seedlings are highly vulnerable, subject to varied abiotic and biotic factors that affect growth and establishment. Their adversities, although variable in severity – depending on habitat and seedling form – include drought, flooding, herbivory, and lack of resources, such as mycorrhizal associates and light.

Introduction

All theories concerning metapopulation, source–sink, and metacommunity dynamics require dispersal, seed-bank dynamics, and seedling establishment to structure populations and communities (Hubbell, 2001; Leibold et al., 2004). Our understanding of dispersal (Howe & Smallwood, 1982; Nathan & Muller-Landau, 2000; Levine & Murrell, 2003), seed germination ecology (Baskin & Baskin, 1998; Fenner & Thompson, 2005), and seed-bank dynamics (Leck et al., 1989a), as well as the structure and dynamics of adult plants and communities, emphasize the need to bring the seedling life history stage to the fore. This is particularly important for the concept of recruitment limitation (Hurtt & Pacala, 1995).

Seedlings are clearly a vulnerable stage, shifting in short time periods from complete dependence on maternal reserves to physiological independence. Variation in seed size, carbon allocation patterns, and seedling structure and physiology has considerable influence on the potential for individual seedlings to survive to establishment. Dispersed across a variable habitat, mortality results from discordance in those characters and the environment, limiting potential establishment of seedlings. Their high mortality results from drought, herbivory, and disease (Moles & Westoby, 2006b; Fenner & Thompson, 2005; Kitajima, 2007), although many other factors also determine seedling success.

Reducing vulnerability may be accomplished by maternal investment in the seed that shepherds the seedling toward independence, or by facilitation by a nurse plant or other microhabitat (rock crack). Typically, the nurse plant is not the parent, raising the question of implications for community associations and long-term dynamics.