The study of succession has been hampered by the lack of a general theory. This is illustrated by confusion over basic concepts and inadequacy of certain models. This review clarifies the basic ideas of pathway, mechanism, and model in succession. Second, in order to prevent inappropriate narrowness in successional studies, we analyze the mechanistic adequacy of the most widely cited models of succession, those of Connell and Slatyer. This analysis shows that models involving a single pathway or a dominant mechanism cannot be treated as alternative, testable hypotheses. Our review shows much more mechanistic richness than allowed by these widely cited models of succession. Classification of the mechanisms of specific replacement, called for by existing models, is problematic and less valuable than the search for the actual mechanisms of particular seres. For example, the “tolerance” mechanism of succession has at least two contrasting meanings and is unlikely to be disentangled from the “inhibition” mechanism in field experiments. However, the understanding of particular species replacements through experiment and knowledge of the conditions of a particular sere and species life histories is a reasonable and desirable goal. Finally, we suggest the need for a broad mechanistic concept of succession. Thus, based on classical causes of succession that have survived recent scrutiny, we erect a framework of successional mechanisms. This framework aims at comprehensiveness, and specific mechanisms are nested within more general causes. As a result of its breadth and hierarchical structure, the framework performs two important functions: First, it provides a context for studies at specific sites and, second, is a scheme for formulating general and testable hypotheses. The review of specific successional mechanisms and the general mechanistic framework can together guide future work on succession, and may foment the development of a broad theory.

Models, mechanisms and pathways of succession

Summary 1. Seeding is an important management tool in aridland restoration, but seeded species often fail to establish. Previous research has largely focused on the technical aspects of seeding with little effort directed at identifying demographic processes driving recruitment failures. 2. In tilled plots, in each of 3 years, we estimated life stage transition probabilities for three species commonly used in sage steppe restoration. We also took similar measurements on seed sown by managers following four major fires. 3. Point estimates and associated Bayesian confidence intervals demonstrated germination probabilities that were consistently high, averaging 0AE72. However, estimates suggest only 17 and 7% of the germinated seeds emerged in the tilled plots and fire sites, respectively. Following emergence, survival across the seedling, juvenile and adult transitions averaged 0AE72. This suggests the transition from a germinated seed to an emerged seedling was the major bottleneck in recruitment. Although most individuals died during emergence, this was not always the principal source of variation in recruitment across sites. 4. Synthesis and applications. Processes occurring after emergence, such as mortality during spring and summer drought, may contribute to site-to-site variation in recruitment but are unlikely to be the main causes of restoration failures. Instead, recruitment may largely be determined by processes occurring during emergence, such as freezing and thawing of the seedbed, development of physical soil crusts and pathogen attack on germinated seeds. Using tools such as seed coatings and soil amendments to manage processes inhibiting emergence and developing seed mixes with higher emergence probabilities are likely to greatly improve restoration outcomes in the sage steppe and similar aridland systems.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2664.2011.02009.x

Demographic processes limiting seedling recruitment in arid grassland restoration

The development and dispersal of seeds as well as their transition to seedlings represent perhaps the most critical stages of a plant’s life cycle The endophytic and epiphytic microbial interactions that take place in, on, and around seeds during these stages of the plant’s life cycle may have profound impacts on plant ecology, health, and productivity While our understanding of the seed microbiota has lagged far behind that of the rhizosphere and phyllosphere, many advances are now being made This review explores the microbial associations with seeds through various stages of the plant life cycle, beginning with the earliest stages of seed development on the parent plant and continuing through the development and establishment of seedlings in soil This review represents a broad synthesis of the ecological and agricultural literature focused on seed-microbe interactions as a means of better understanding how these interactions may ultimately influence plant ecology, health, and productivity in both natural and agricultural systems Our current understanding of seed-microbe associations will be discussed, with an emphasis on recent findings that specifically highlight the emerging contemporary understanding of how seed-microbe associations may ultimately impact plant health and productivity The diversity and dynamics of seed microbiomes represent the culmination of complex interactions with microbes throughout the plant life cycle The richness and dynamics of seed microbiomes is revealing exciting new opportunities for research into plant-microbe interactions Often neglected in plant microbiome studies, the renaissance of inquiry into seed microbiomes is offering exciting new insights into how the diversity and dynamics of the seed microbiome with plant and soil microbiomes as well as the microbiomes of dispersers and pollinators It is clear that the interactions taking place in and around seeds indeed have significant impacts on plant health and productivity in both agricultural and natural ecosystems

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The seed microbiome: Origins, interactions, and impacts

Summary 1. Plant defence theory provides a robust framework for understanding interactions between plants and antagonists, and for interpreting broad patterns in the functional-trait composition of plant communities. However, this framework has been built almost entirely on traits expressed by seedlings and mature plants. 2. No equivalent seed defence theory exists that recognizes the distinct suite of natural enemies that seeds encounter, and the unique constraints to their response. Furthermore, most attention has been paid to insect and vertebrate seed predators active above ground, whereas microbes in soil also have large effects on seed survival, particularly for plants that recruit from soil seed banks. 3. We suggest that concurrent selection on seed dormancy and resistance to microbial antagonists should result in distinct seed defence syndromes. We predict that species with physical seed dormancy will rely on physical defences to exclude predators and pathogens, and rapid seed germination to escape pathogens at the emergence stage. In contrast, species with physiological seed dormancy will deploy a continuum of physical and chemical defences, depending on soil pathogen pressure and duration of seed persistence. Finally, seeds of some species persist in the soil in a non-dormant, imbibed state, and lack obvious chemical and physical defences. These seeds may be especially dependent upon protection from beneficial seed-inhabiting microbes. 4. Framing a general ‘seed defence theory’ may help to account for the distribution of seed dormancy types across ecosystems. We predict that physiological dormancy will be favoured in dry or well-drained environments where pathogen pressure is relatively low, germination cues are most unpredictable, and seedling recruitment success is most variable. In contrast, physical dormancy should be favoured in warm and moist environments where pathogen pressure is high, and where germination cues are a stronger predictor of recruitment success. Persistent, non-dormant seeds are restricted to relatively aseasonal environments where favourable conditions for recruitment can occur over most of the year. 5. Synthesis. Integrating seed defence and dormancy traits can provide new insights into selection on dormancy types, and will help elucidate major trends in seed ecology and evolution. Understanding how seeds are defended also may improve our ability to predict plant regeneration and help develop innovative management strategies for weedy and invasive species.

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Seed survival in soil: interacting effects of predation, dormancy and the soil microbial community

Despite impressive efforts at clearing stands of invasive Australian Acacia species in South Africa, insufficient attention has been given to understanding the role of seed banks in the invasiveness and long-term persistence of populations. We review information on seeds of these species, considering seed production, seed rain, and the dynamics of seeds in three layers: leaf litter, and upper and lower seed banks in the soil. Many factors affect the accumulation and susceptibility to destruction of seed banks and thus the opportunities for intervention to reduce seed numbers for each of these components. Reduction of seed banks is crucial for the overall success of the multi-million dollar management initiatives against these species. Classical biological control of buds, flower and young pods has reduced the seed production of many Australian acacias in South Africa. Fire can be applied to reduce seed numbers in the leaf litter and upper seed bank in some cases, although there are serious problems associated with high fire intensities in dense acacia stands. Other options, e.g. soil inversion and solarisation, exist to exercise limited reduction of seed numbers in some situations. There is little prospect of meaningful reduction of seed numbers in the lower seed bank. Preventing the accumulation of seed banks by limiting seed production through biological control is by far the most effective means, and in almost all cases the only practical means, of reducing seed numbers. This must be an integral part of management strategies. Several invasive Australian acacias are already under effective biological control, and further work to identify additional potential agents for all the currently invasive species and potentially invasive alien species is the top priority for improving the efficiency of management programmes.

/pdf/seed-banks-of-invasive-australian-acacia-species-in-south-elo5uxqe16.pdf

Seed banks of invasive Australian Acacia species in South Africa: Role in invasiveness and options for management

SUMMARY (1) An abundant mycoflora was found on the seed of Abutilon theophrasti, Datura stramonium, Ipomoea hederacea, and Polygonum pensylvanicum, which are colonizing annuals of early successional habitats. The fungal species differed among the plant species and most were specialists with respect to their hosts. (2) A more extensive mycoflora occurred within seed of Polygonum pensylvanicum and D. stramonium than within seed of I. hederacea and A. theophrasti. Ipomoea hederacea and A. theophrasti had greater resistance to fungal infection than P. pensylvanicum and D. stramonium. Fungi differed in their ability to penetrate seeds. (3) Seed germination and seedling development were affected, both quantitatively and qualitatively, by the fungal isolates. Polygonum pensylvanicum seedlings were more susceptible to fungal invasion than those of A. theophrasti or I. hederacea. (4) Extracts from all tissues of I. hederacea and from the roots of A. theophrasti had antifungal activity. Little activity was detected in extracts from P. pensylvanicum. (5) Plant densities of I. hederacea, D. stramonium and P. pensylvanicum remained fairly constant in the field, after seedling emergence, irrespective of density or species diversity. (6) Fungi mainly affected seed germination and the development of young seedlings; at later stages, herbivory became more important in plant survival.

B. L. Kirkpatrick

Papers

Influence of certain fungi on seed germination and seedling survival of four colonizing annuals