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

Disturbance regimes are changing rapidly, and the consequences of such changes for ecosystems and linked social-ecological systems will be profound. This paper synthesizes current understanding of disturbance with an emphasis on fundamental contributions to contemporary landscape and ecosystem ecology, then identifies future research priorities. Studies of disturbance led to insights about heterogeneity, scale, and thresholds in space and time and catalyzed new paradigms in ecology. Because they create vegetation patterns, disturbances also establish spatial patterns of many ecosystem processes on the landscape. Drivers of global change will produce new spatial patterns, altered disturbance regimes, novel trajectories of change, and surprises. Future disturbances will continue to provide valuable opportunities for studying pattern-process interactions. Changing disturbance regimes will produce acute changes in ecosystems and ecosystem services over the short (years to decades) and long-term (centuries and beyond). Future research should address questions related to (1) disturbances as catalysts of rapid ecological change, (2) interactions among disturbances, (3) relationships between disturbance and society, especially the intersection of land use and disturbance, and (4) feedbacks from disturbance to other global drivers. Ecologists should make a renewed and concerted effort to understand and anticipate the causes and consequences of changing disturbance regimes.

Disturbance and landscape dynamics in a changing world

Natural disturbances have been traditionally defined in terms of major catastrophic events originating in the physical environment and, hence, have been regarded as exogenous agents of vegetation change. Problems with this view are: (1) there is a gradient from minor to major events rather than a uniquely definable set of major catastrophes for each kind of disturbance, and (2) some disturbances are initiated or promoted by the biotic component of the system. Floras are rich in disturbance-adapted species. Disturbances have probably exerted selective pressure in the evolution of species strategies. Heathland cyclic successions and gap-phase dynamics in forests have been viewed as endogenous patterns in vegetation. When death in older individividuals imposes a rhythm on community reproduction, dynamics may indeed be the result of endogenous factors. However, documented cases of senescence in perennial plants are few and many cyclic successions and cases of gap-phase dynamics are initiated by physical factors. Forest dynamics range from those that are the result of individual tree senescence and fall, through those that are the result of blowdown of small groups of healthy trees, to those that are the result of large windstorms which level hectares of forest. The effect of wind ranges from simple pruning of dead plant parts to widespread damage of living trees. Wind speed is probably inversely proportional to occurrence frequency. Disturbances vary continuously. There is a gradient from those community dynamics that are initiated by endogenous factors to those initiated by exogenous factors. Evolution has mediated between species and environment; disturbances are often caused by physical factors but the occurrence and outplay of disturbances may be a function of the state of the community as well. Natural disturbances in North American vegetation are: fire, windstorm, ice storm, ice push on shores, cryogenic soil movement, temperature fluctuation, precipitation variability, alluvial processes, coastal processes, dune movement, saltwater inundation, landslides, lava flows, karst processes, and biotic disturbances. Disturbances vary regionally and within one landscape as a function of topography and other site variables and are characterized by their frequency, predictability, and magnitude. The landscape level is important in assessing disturbance regime. Disturbances and cyclic successions belong to the same class of events—that of recurrent dynamics in vegetation structure—irrespective of cause. Dynamics may result from periodic, abrupt, and catastrophic environmental factors or they may result from an interaction of the changing susceptability of the community and some regular environmental factor. In any case, the dynamics result in heterogeneous landscapes; the species adapted to this heterogeneity are numerous, suggesting their long time importance. The importance of disturbance regime as part of the environmental context of vegetation means that allogenic and autogenic models of vegetation are difficult to apply. Species composition can be seen to be a function of disturbance regime, as well as other environmental variables. Competitive replacement in succession occurs, then, only as disturbances cease to operate and can be viewed as allogenic adjustment to a new disturbance-free environment. Competitive divergence, separation of role, and competition avoidance may, in fact, underlie successional patterns traditionally viewed as the competitive replacement of inferior species by superiorly adapted climax species. The importance of ongoing dynamics is also difficult to reconcile with the concept of climax, founded as it is on the idea of autogenesis within a stable physical environment. Climax composition is relative to disturbance regime. Climax is only arbitrarily distinguished from succession. Climax as an organizing paradigm in plant ecology has obscured the full temporal-spatial dimensions important in understanding the vegetated landscape and the evolution of species which contribute to the landscape patterns. Whittaker’s coenocline concept is accepted with modifications: (1) natural disturbance gradients and Whittaker’s complex gradient are intimately related, (2) temporal variation in the community should be viewed as an added axis of community pattern, and (3) ongoing dynamics have important effects on specificity of species to site relations and the predictability of vegetation patterns. Recent work has suggested an r-K continuum in species strategy. In general, colonizing ability is seen as a trade-off against specialization. Frequent disruption of the community and the creation of open sites seems to result in mixes of species that are fleeting in time and do not repeat in space. Species in such mixes are often tolerant of wide environmental extremes but are compressed into early successional time if disturbance ceases. The composition of such communities is not predictable from site characteristics. Even communities with low disturbance frequency lack complete environmental determinism, and historical events are important in understanding present composition. Communities vary in level of environmental determinism and species differ in niche breadth and degree of site specificity. Management implications of vegetation dynamics are discussed.

Pattern, process, and natural disturbance in vegetation

ak (Quercus) represents one of the most dominant species groups in the eastern deciduous forest of North America (Table 1). In certain eastern regions, oak dominance reflects the importance of this genus in presettlement forests (Abrams and Downs 1990, Spurr 1951, Whitney and Davis 1986). Yet, in other regions, the current distribution of oak greatly exceeds that of the original vegetation (Abrams 1986, Howell and Kucera 1956, Nowacki et al. 1990). Thus, the development of oak species has occurred through a variety of ecological pathways and disturbance conditions. The dominance of oak in presettlement forests is particularly interesting because most oaks are considered early to midsuccessional species. Indeed, most recent studies indicate the potential for widespread oak replacement by more shade-tolerant species in mature forests (Christensen 1977, Lorimer 1984, Nowacki et al. 1990). However, this phenomenon may vary with regional and edaphic factors and be more pronounced on mesic rather than on xeric sites (Abrams 1986, Host et al. 1987, McCune and Cottam 1985, Nowacki and Abrams 1991). These observations have led researchers to ask the following questions about oak ecology in eastern

https://www.jstor.org/stable/pdfplus/1311781.pdf

Fire and the Development of Oak Forests

This book attempts to make growth models more accessible to foresters and others interested in mixed forests, whether planted or natural. There is an increasing interest in,
 and controversy surrounding the use of mixed plantations and natural forests, and rational discussion and resolution of management options require reliable growth models linked to other
 information systems. It is my hope that this book will help researchers to build better models, and will help users to understand how the models work and thus to appreciate their strengths
 and weaknesses. During recent years, vast areas of natural forest, especially in the tropics, have been logged or converted to other uses. Well-meaning forest managers have often been
 over-optimistic in estimating forest growth and yields, and this has contributed to over-cutting in some forests. Growth models can provide objective forecasts, offering forest managers the
 information needed to maintain harvests within the sustainable capacity of the forest, and providing quantitative data for land use planners to make informed decisions on land use
 alternatives. In this way, I hope that this book will contribute to the conservation and sustainable management of natural forests in the tropics and elsewhere. This is not a "How to do it"
 manual with step-by-step instructions to build a growth model for mixed forests. Unfortunately, modelling these forests isn't that easy. There is no single "best" way to build a model for
 these forests. Rather, many approaches can be used, and the best one depends on the data available, the time and expertise available to build the model, the computing resources, and the
 inferences that are to be drawn from the model. So instead of writing a "cookbook" with one or two recipes, I review and illustrate some of the many approaches available, indicate the
 requirements of and output from each, and highlight their strengths and limitations. The book emphasizes empirical-statistical models rather than physiological-process type models, not
 because they are superior, but because they have proven utility and offer immediate benefits for forest management. A more comprehensive treatment of all the options is beyond the scope of
 this book, which is intended to serve as a ready reference manual for those building growth models for forest management. Because of my limited linguistic ability, the material covered is
 more-or-less restricted to English-language material. I have not attempted to review all the published work on growth modelling (it would be a huge task), but have tried to highlight
 examples that may be applicable to mixed forests in tropical areas. I hope that the language and terminology used in this book will be accessible to all readers, especially those for whom
 English is a second language. The glossary may help to clarify some terms, and those that have a specific technical meaning are printed in italics the first time they are used. Readers
 should consult the glossary to clarify the meaning of these words unless they are sure of the meaning. Exercises are given at the end of each chapter to reinforce points made in the
 chapter. These are simple exercises, deliberately chosen so that they can be completed quickly with pen and paper or PC and spreadsheet, but within these constraints, I have tried to keep
 them realistic. Some exercises (e.g. 9.1 and 10.3) require more specialized statistical analyses, but many commercial statistical packages (e.g. GLIM) are suitable. Where possible, these
 exercises draw on real data, but some data were simulated to create interesting exercises with few data. Whilst my approach places more responsibility on the reader to choose and develop a
 suitable modelling methodology, I hope it will help readers gain a better understanding of modelling, which should in turn lead to better models and more reliable predictions. And I hope
 that better models will provide better information, greater understanding, and better management of mixed forests.

https://espace.library.uq.edu.au/view/UQ:8211/ModellingForestG.pdf

Modelling Forest Growth and Yield: Applications to Mixed Tropical Forests

Replacement of oaks (Quercus spp.) by other species after natural and humancaused disturbance is recognized as a common problem on average and productive sites in many parts of eastern North America. Oak seedlings are often numerous beneath mature stands, but seedlings are usually too small to compete effectively with tall saplings of other species. Reasons for the poor development of oak seedlings are not well understood. An experiment was designed to evaluate the impact of tall and low understorey vegetation on oak seedling development beneath mature stands on two sites in southwestern Wisconsin. Understorey stems taller than 1.5 m and scattered small canopy trees were removed on half of the main plots, with the other plots retained as controls (...)

http://www.jstor.org/stable/pdfplus/2261291.pdf

Tall understorey vegetation as a factor in the poor development of oak seedlings beneath mature stands

Analysis of the frequency of past moderate and high-intensity disturbances has been hindered in forests of complex age structure by methodological problems. A methodology is proposed for developing a disturbance chronology in such stands by identifying the probable date of canopy accession for each sample tree. Canopy accession dates are based on an evaluation of radial growth pattern and early growth rates of existing canopy trees. Canopy disturbance intensity is defined as the percentage of sample trees with canopy accession events in each decade. Rotation periods for disturbances of various intensities are calculated from the chronology. The method was evaluated using 893 increment cores from 70 plots in northern hardwood stands of western Upper Michigan. The estimated average disturbance rate for all plots and decades was 5.7–6.9% of land area per decade, with an implied average canopy tree residence time of 145–175 years. These estimates are similar to those obtained by on-site estimates of canopy tr...

A methodology for estimating canopy disturbance frequency and intensity in dense temperate forests

The frequency of natural disturbances and their influence on the forest landscape mosaic were investigated on three large tracts of primary forest in Upper Michigan. Seventy 0.5—ha plots were randomly distributed in a total forest area of 23 000 ha dominated by sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Radial increment patterns were used to estimate canopy accession dates for each of a number of randomly selected overstory trees on each plot. From these data a disturbance chronology, representing the percentage of stand area occupied by cohorts originating during each decade over the last 130 yr, was compiled for each plot. Average rates of disturbance or canopy mortality are estimated at 5.7 to 6.9% per decade. The corresponding average canopy residence time of a tree is 145—175 yr. No significant differences were detected in average disturbance rates among the three study areas, between plots near the coast of Lake Superior and inland plots, among several different aspects, and among several different slope positions. Natural rotation periods increase exponentially with increasing disturbance intensity, which is defined as the approximate percentage of the plot area converted to gaps during a disturbance episode. Estimates of rotation periods range from 69 yr for ≥10% canopy removal to 1920 yr for ≥60% canopy removal. Spatial autocorrelation analysis indicated that plots with light and medium disturbances (<40%) are randomly distributed over the landscape. Plots wit heavy disturbances (≥40%) are clustered with a patch radius of ≈2 km, consistent with the sizes of thunderstorm downbursts. The data indicate that light and medium disturbances dominate the disturbance regime. The majority of stands on the landscape are composed of several major and many minor age classes. Even—aged stands with one predominant age class are uncommon. The age distribution of individual patches or cohorts in the two larger study areas (14 500 and 6073 ha) follows a nearly uniform distribution. None of the three study areas had more than 15% of the forest area converted to gaps in a single decade. The two larger areas meet most of the criteria that have been proposed for equilibrium landscapes.

Natural Disturbance Regimes in Hemlock-Hardwood Forests of the Upper Great Lakes Region

The frequency and intensity of natural disturbances in a montane old-growth forest of coniferous (Tsuga) and broad-leaved deciduous trees (Liriodendron, Betula, Castanea, Fagus, Acer) were investigated by means of diameter frequency distributions, diameter-age regressions, and anal- ysis of radial growth patterns. The seven species investigated are represented by all-aged populations regardless of the degree of shade tolerance. The age distributions of most species, however, are irregular with several prominent peaks. Apparent coincidence of these peaks among some species suggests a common underlying influence. Radial growth patterns reveal higher than average numbers of abrupt and sustained increases in growth rate in most decades of peak recruitment which, along with other evidence, suggest that disturbance was the principal causal factor. There is evidence for eight partial disturbances in the past 250 yr, each of which generally removed < 10%o of the overstory trees. These fairly light disturbances appear to be sufficient to cause major deviations from the theoretical equilibrium diameter distribution of the forest.

Age structure and disturbance history of a southern appalachian virgin forest

Land survey records of 1793-1827 containing forest data for 1.65 X 1Off ha of northern Maine were analyzed for species composition, successional status, and frequency of large-scale disturbance. Quantitative data consists of 1,448 sample trees spaced 1.6 km apart along a 9.7- X 9.7-km grid. Species which each comprised > 10% of the total were Picea spp., Fagus grandifolia, Abies balsamea, Thuja occidentalis, and Betula lutea. These forests appeared to be largely in a climax state as indicated by the dominance of shade-tolerant species and the small percentage (8% ) of intolerant or early successional species. However, 9.3% of the tract was burned land and birch-aspen forest at the time of the survey, mostly the result of large fires in 1803 and 1825. Windfalls occurred along 2.6% of the surveyed distance. If the amount of disturbed forest at this time was typical of the natural disturbance regime, then the average recurrence interval of fire and large-scale windthrow for a given site would be 800 and 1,150 years, respectively. Data on the structure of remnant virgin stands in the region likewise suggest that the time interval between severe disturbances was much longer than that needed to attain a climax, all-aged structure.

/pdf/the-presettlement-forest-and-natural-disturbance-cycle-of-1x2uidu1wy.pdf

Craig G. Lorimer

Papers

Tall understorey vegetation as a factor in the poor development of oak seedlings beneath mature stands

A methodology for estimating canopy disturbance frequency and intensity in dense temperate forests

Natural Disturbance Regimes in Hemlock-Hardwood Forests of the Upper Great Lakes Region

Age structure and disturbance history of a southern appalachian virgin forest

The presettlement forest and natural disturbance cycle of northeastern maine