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Plant succession: theory and applications by C.W.D. Gibson and V.K. Brown ’The most stable association (of plants) is never in complete equilibrium, nor is it free from disturbed areas in which secondary succession is evident’ (Clements, 1916). In modern times, these ’most stable associations’ are the exception rather than the rule, and are only found in the remoter parts of the world where undisturbed communities still exist. A natural state of change (such as after management is abandoned by humans) is more usual. Indeed much of our farming, forestry, land reclamation schemes and even nature conservation relies on preventing or in some way directing this natural process of change. A good understanding of the pinciples driving this natural succession, and their efficient application, is thus vital to anyone involved in land management. A striking example, which is developed later in this review, is provided by the reclamation of industrial waste tips; initial safe construction or reconstruction is the province of engineers, physicists, hydrologists and soil chemists, whereas long-term stability and successful later use of the land for agriculture and amenity, can only be achieved by applied ecologists using the basic principles of plant succession (Bradshaw and Chadwick, 1980). Most applications to date have been on land, so we have restricted this review to a discussion of terrestrial successions, with apologies to those interested in the application of succession theory to aquatic environments. The term ’ecological succession’ is usually applied only to directional change over a period of years; short-term cyclic seasonal changes and long-term climatic changes are regarded as being imposed upon an underlying successional pattern. Early observations of succession (e.g. Cowles, 1901; collation by Golley, 1977) showed that change often proceeded through a series (sere) of recognizable plant associations (seral stages) towards a state where little further change occurred (climax) without outside disturbance. Such disturbance usually gives rise to a repetition of some or all seral stages. Increasing numbers of observations and theories about how succession works led to a vast number of technical terms (for a review see Golley, 1977): those in common current use are defined in Table 1. The time scale of a sere can vary considerably; in some cases an ecologist can hope to see the whole process during a lifetime (e.g. Oosting, 1942), in others (e.g. Olson, 1958 on the sand dune vegetation of Lake Michigan - 1000 yr) there is the potential for considerable genetic change among the smaller organisms in the Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 474 Table 1 Some terms commonly used in the description of succession community, and certainly for climatic change, before the ecological process has run its course. By contrast, changes significant for management by humans can take place very fast indeed (e.g. Jones, 1933 found that the species composition of pasture leys could be changed drastically over a period of months). Succession at the community scale is based on the interactions between individual plant species, animal species and their environment. We therefore start this review by enumerating those types of interaction which have been shown to be important in directing succession. There follows an explanation of some current theories and models which attempt a general understanding of the process. The last section is dominated by a series of examples to show the range of ways in which succession theory has been applied to land management, and an indication of the ways in which we believe such applications could usefully be extended in the future. I Interactions We have divided interactions important in succession into those to do with the physical environment, those between different plant species and those involving animals as well as plants. A further important distinction concerns the scale on which succession is affected. This includes temporal scales (e.g. whether the whole course of a succession is affected or only one part), spatial scales (e.g. regional Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 475 climate vs microclimate under a bush or in the open), and effect scales (is an interaction strong enough to remove a species from consideration or so weak that it only affects the growth rate of a few individuals?). We have attempted to show the relative importance of different types of interaction in this section, in preparation for the development of general models of succession. 1 Environmental influences Regional climate was noted as being of such importance by Clements (1916) that it formed the basis of his ’climatic climax’ as the most fundamental concept in succession. However, applied ecologists will usually need to concentrate more on the modifiers acting within this, i.e. factors resulting in further restrictions on the species list allowed by regional climate. These smaller scale variations can be very important. Although climatic change on a regional scale is usually too slow to be within the scope of this review, local variations in climate may appear to change the adaptive range of a species. For instance, Pigott (1968) found that the calcicolous thistle Cirsium acaulon is restricted to south-southwest facing slopes in Yorkshire, the northern limit of its European range, whilst in southern Britain it is found at all aspects. At its southern limit in central Spain, it is again highly localized. This species needs regular temperatures of 20°-25°C to mature seeds and, whilst it is fairly drought-tolerant because of its deep root system, it is no xerophyte and cannot tolerate very hot dry soils. This effect is a product of the microclimate on the scale of the plant (within the limits ultimately set by regional climate). The interaction between microclimate, climate and the plants themselves is well shown by the ecology of a number of species in southern Britain. In East Anglia, plants such as the bluebell Endymion non-scriptus and the primrose Primula vulgaris are restricted to woodland sites which are at least several centuries old, and disperse from these only very slowly, under a relatively ’continental’ climate. In the ’Atlantic’ climate of western Britain, the same species are quick colonizers of secondary woodlands and even of pasture edge and hedgerow (Rackham, 1980). Thus the overall distribution and the apparent position in succession (fast-dispersing early colonizers vs slowdispersing late colonizers) may be set by an interaction between regional climate, microclimate modified by other species (e.g. shade) and the particular exaptations (sensu Gould and Vrba, 1982) of the plant itself (a need for relatively high humidity which is a characteristic of an individual rather than strictly an adaptation to its local environment). The incidence of fire as an ecological influence is dependent on regional climate but at present fires are usually anthropogenic. The effects of fires, and the course of postfire succession, depend considerably on the available plant species. In some regions of the world, notably southwest Australia, many of the plant species present are well adapted to fire, but in other places where fires have only been regular for a relatively short time, such as in parts of the southwest USA, there are fewer species which are specifically fire-adapted. In the Australian flora these Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 476 specific adaptations include thick corky bark, the ability to form epicormic shoots, regeneration from underground storage organs, and fruits which release their seeds after being burnt on the outside (e.g. Carlquist, 1974, especially for Proteaceae). Banksia (Proteaceae) seeds will also tolerate very high temperatures (Siddiqui et al., 1976). As a result, most of the species which could occur in the plant community have propagules already present immediately after a fire: the subsequent course and end-product of succession depends on the species present before burning and these species’ times of recovery (Purdie and Slatyer, 1976) and other lifetraits such as longevity. In some chapparal stands in southwest USA the story is similar (Biswell, 1974) but other, forest, vegetation types such as ponderosa pine forests contain fewer fire-tolerant species and here the intensity and frequency of burning and the ability of different species to recolonize a burnt area have more critical significance (Weaver, 1974). The variety of dispersal mechanisms among plants means that any discussion history of colonizing abilities is inextricably linked with interactions between plant species and with plant-animal pollination and seed dispersal mechanisms, outside the scope of this general review (but see Faegri and van der Pijl, 1966). However, purely physical barriers can be extremely important in determining the importance of colonization to the community scale. On oceanic islands, dispersal of some types can take so long that evolution has produced plants of similar ecology from earlier colonists (Carlquist, 1974). In more familiar regions, the problem is best illustrated by a contrast between the much-studied ’old-field’ successions (abandonment after agriculture) in temperate regions and studies in undisturbed forests. In most cases where succession has been studied in a relatively small patch left undisturbed in the midst of agricultural land, the successional process is manifested as a replacement series in which colonization is very important, especially in the later stages, in combination with biotic modification of the soil (for a review see Golley, 1977). At the other extreme, Piroznikow (1983), working in the largest area of virgin forest left in Europe (Bialowieza), found that even the soil seed bank was restricted and contained only forest species; after a small disturbance these late successional species, and only these, would be available for recolonization. In a range of habitats not including virgin forest, Thompson and Grime (1979) found results intermediate between these extremes. Clearly, material for germination in any particular place comes from immigrants (colonization), the immediately preceding seed rain and the seed bank as determined by seed longevity and past factors (individual species range from a requirement for immediate germination to an ability to survive many decades (Gross and Werner, 1978)). Interactions between these factors determine what happens in a given habitat (Thompson and Grime, 1979). Edaphic factors (connected with soil environment) put further constraints on the potential species list in a given habitat. pH, soil structure, water levels, temperature, and the concentrations of key nutrients all provide limits to which plant adaptations vary in degree and in the part of the spectrum over which they can survive. However, plant’s resource spectra are usually labile, being influenced by other biotic factors, e.g. by other plants which modify the soil environment, Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 477 resources, or by animal-induced stresses, and cussed below in the context of these biotic interactions. by direct competition for 2 are dis- Interactions between plant species Competition between plant populations is well attested (Jackson, 1981) although its precise importance in relation to other factors is understood in only a few communities. Most pertinent to the present review are those studies which show competition giving rise to the replacement of plant species by others, rather than demonstrating stable coexistence. In some cases, ’early successional’ species need open soil patches (e.g. Aphanes arvensis - Grubb, 1976) or even light reaching the seeds (e.g. Verbascum thapsus - Gardner, 1921; Gross and Werner, 1978) for germination. Annuals and biennials such as these are usually denied such microsites for seed germination by the closed litter cover established by perennial grasses after a few years of old-field succession, but they may persist in local conditions of shallow soil or when seeds are exposed by the scrapes of burrowing animals (Grubb, 1976). Later on in succession, although germination may take place, seedling establishment, reproduction and/or dispersal may be affected by direct shading. Such factors contribute to the depleted seed bank found in old European forests. Piroznikow (1983) noted that the surviving forest floor species were those capable of persisting by vegetative reproduction under deep shade and that some of these were not represented in the seed bank. In Britain, although Primula vulgaris can survive in deep shade, it cannot set seed, and new seeding depends on disturbances to the forest canopy such as coppicing or natural tree fall (Rackham, 1980). Soil characters can also modify the outcome of competition between plants. Nutrients and water are both obtained from the soil, and it is virtually a tautology to say that competition, where important at all, is for the limiting soil factor(s). The position is not always so simple. Many plants only tolerate restricted pH ranges: among these, ’calcicoles’ are confined to chalk or limestone soils, and ’calcifuges’ to more acid sites. Despite these field restrictions, many species tolerate a much wider range when grown in culture (Rorison, 1969). Plants are often restricted to acid soil by the interference of Ca++ ions with the plant’s ability to take up Mn, Fe, P and other essential nutrients. Conversely, acid soils can hold large quantities of A1+++ and Fe+++ ions in solution, which are both directly toxic (Al +++) and tend to precipitate H2po4, preventing uptake by the plant. In acid waterlogged soils, lack of oxygen near the roots must also be overcome, e.g. by enhanced transport from the aerial parts (Fitter and Hay, 1981). Differences between field and laboratory conditions can often be explained by the modifying effects of competition from species better at one extreme or the other (Tansley, 1917; Rorison, 1969). Such interactions between soil conditions, plant competitors and the effects of plant on the soil can give rise to succession by nucleation (Austin and Belbin, 1981; Game et al., 1982) where initial patches Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 478 start systems of concentric rings of vegetation types which eventually coalesce: in circumstances these patches can cyclically replace each other in time and space (Godwin and Conway, 1939; Watt, 1947). A special case of competition is when plants produce substances which are toxic to other plants (allelopathy). Many species produce such substances (classic examples come from American chaparral e.g. Biswell, 1974; Harborne, 1977) but there has been argument over the function of these allelopathic chemicals - are they substances produced by one plant to poison another, or are they merely waste metabolites which are unavoidably produced and happen to have this effect (e.g. Harper, 1977)? In one sense this is immaterial; such chemicals are produced, they do have inhibiting effects on most plants, often including the individual which produced them, and they are destroyed by fire, releasing successional stages more diverse and productive and with a higher biomass than this ’allelopathic climax’ particular (Biswell, 1974). . In direct contrast are cases in which one plant alters the environment to the benefit of another species. Nitrogen fixing by bacteria in root nodules (e.g. Hansen and Dawson, 1982) can not only accelerate succession, but also produce plant species which would be unlikely in other circumstances (see ’Applications’ below). Hodgkin (1984) found a simpler, but less intuitive, mechanism operating during scrub establishment on Welsh sand dunes. Hawthorn (Crataegus monogyna) bushes establishing in phosphorus-poor dune grassland have deep roots which access phosphate sources unavailable to the usual grassland plants. Seasonal leaf fall gradually enriches the topsoil and subsequently other plants demanding high phosphate levels can establish at the expense of the original flora, even if the scrub is removed. However, the initial scrub establishment often depends on interactions with grazing animals, and an explanation needs an understanding of the next level of interactions. 3 Animal-plant interactions . profound and varied effects of grazing on vegetation dynamics are well known (e.g. Harper, 1977), and the current characteristics of some plant communities may reflect their history of grazing pressures (Duffey et al., 1974). However, the impact of herbivory on the establishment and development of natural vegetation is seldom documented. Traditionally, many botanists have believed that small herbivores (e.g. insects) have little impact on succession, and that large animals are best regarded The interrupting the process rather than radically changing its dynamics. Animal pollination and seed dispersal have been accepted as important in special cases (e.g. Godwin, 1936). It is becoming apparent that even small herbivores may change the course and/ or dynamics of succession, while large grazers may set plant communities on a wide variety of paths, dependent on the timing and intensity of succession and the particular herbivore species. In a classic early study Jones (1933) was able experimentally to direct sown agricultural leys towards grass swards, clover swards or high diversity (’weedy’) mixtures by varying the intensity, and more especially the as Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 479 timing, of sheep grazing. Major changes took months rather than years. Sheep can also set the form and persistence of micro-patterns within the sward (Bakker et al., 1983). The pattern of change in upland vegetation on acid soil in Britain is also influenced by the intensity of sheep grazing (Miles, 1978). Under low grazing pressures succession to woodland takes place via grassland (often with bracken) or heath phases. When grazing pressures are increased the regeneration of shrubs and tree saplings is prevented either by grazing impeding competition with herbaceous species or by mechanical damage. The secondary effects of vertebrate grazing can be considerable (e.g. Spedding, 1971). The influence of grazing on the plant succession of rangeiands is reviewed by Ellison (1960) and the dangers of overgrazing in arid lands cited as an example of interruption/disturbance of the normal successional pattern. In woodlands there is less evidence for the effect of browsers, except as negative influences which prevent regeneration in most years (Adams, 1975), but the potential for browsers in determining forest species composition is suggested by the known feeding preferences of these animals among tree saplings (Peterken, 1981; Rackham, 1980). While the effects of vertebrates on plant succession may be assessed by either intuitive observation or enclosure/exclosure experiments, the effects of insect grazers can be studied by experimental application of insecticides (to which grasslands and early successional associations are more amenable than forest) and by examining the effects of natural outbreaks of forest defoliators (usually the only course with forest insects). Among the few grassland studies, both McBrien al. (1983) and research by Brown and coworkers (reviewed in Brown, 1984) demonstrated strong effects. In the latter work, the exclusion of insects by the regular use of non-persistent insecticides permitted direct comparison between insect-grazed and insect-free plots. (Grazing by other herbivores was prevented.) In the absence of insect herbivory, plant species accumulated more rapidly, the vegetation cover was greater, grasses invaded (and replaced herbs) earlier and the structural complexity of the vegetation was higher. Recent single plant species studies in natural early successional grassland (e.g. Brown, 1985) have indicated a significant reduction in number of individual plants, a decrease in growth rate and a lowering of reproductive potential when exposed to ’natural’ populations of insect herbivores. The reduction in number of plants was mainly due to seedling mortality. Whelan and Main (1979) have indicated that insect attack on seeds and seedlings (the most vulnerable stage in the life history of a plant (see Harper, 1977)) may influence plant succession as can rodents in a similar system (Bond, 1984). Bews (1920) suggested that seed predation by insects in the coastal forest of Natal might alter the course of succession by the release of previously suppressed tree species. The experimental manipualtions by McBrien et al. showed that the domination of old-field successions by Solidago spp (goldenrods) could be broken by herbivorous beetle outbreaks, thus accelerating succession towards forest conditions. This effect was experimentally reversed by killing beetles on insecticide plots. Among forest successions, Bess et al. (1947) showed that gipsy moth outbreaks possible et Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 480 could either accelerate or decelerate succession depending on outbreak intensity. At moderate intensities, late-successional tree species, which happen to be less palatable to the moth, gained an advantage due to defoliation of overshading earlier successional species. At very high intensities, all tree species were defoliated, allowing the light-demanding but fast-growing successional saplings to reestablish a new succession. The purist might argue that this represents the behaviour of an introduced pest, not a natural succession. Among native insects in native forests, tree death caused by mountain pine beetle (Dendroctenus ponderosae) in the western USA had different effects according to interactions with fire and the natural climax forest of particular regions (Amman, 1977). In natural stands of mixed lodgepole pine, subalpine fir and Douglas fir, frequent fires hold the forest at a lodgepole pine stage. Less frequent fires allow outbreaks of the beetle in mature lodgepole pines, permitting the shade-tolerant juveniles of the other tree species to take over. In some regions which differ in both soil and climate, the pattern is again changed, and pure lodgepole stands persist in whatever circumstances, but with their age class distributions set by fire and beetles. In Europe, succession of alpine larch forests towards a larch/cembran pine mix is delayed or accelerated by larch budmoth (Zeiraphera diniana) in a similar manner to the gipsy moth system described above (Baltensweiler, 1975; Baltensweiler et al., 1977). Although have concentrated on the effects of grazing animals on plant succession, it must be stressed that plants have an important bearing on the nature, diversity and abundance of the animal species present at any particular stage in succession. Among the possible mechanisms for these plant-animal effects, plant architectural changes during succession have at least as much influence on the associated succession of animal species as do changes in plant species composition we (e.g. Southwood et al., 1979). II Modelling successsion From the above it is clear that no single model could explain or predict all plant successions. Despite this, there are several ways in which general models can help the ecologist. Conceptual models can be used to identify sets of general patterns likely to be found under particular, predictable, circumstances. Detailed models (often referred to as ’component models’ - see Southwood, 1978) can predict the outcome of a particular treatment in a particular place. Much simpler models, nevertheless containing the most important features of a system, can be used in a strategic manner to explore the possible range of consequences of a particular feature of a system or treatment of it. In practice, component models are usually too expensive (because of the complexity of data gathering and analysis) for most applied uses of plant succession, and only conceptual models and simple mathematical frameworks are discussed below. Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 481 1 Conceptual models For many years the field of plant succession was a battleground between the proponents of different single models which purported to explain the whole field. The current view is that there are several broad types of succession which dominate under recognizable circumstances. The development of these ideas was due to many people (see Golley, 1977) but most current ideas are encompassed by the works of Drury and Nisbet (1973), Egler (1954), Horn (1974) and Connell and Slatyer (1977) (Table 2). In all these models the presence of a wide range of life history strategies is acknowledged (see e.g. Grime, 1979; Horn, 1978; Southwood, 1978; Stearns, 1977; 1983 for a variety of views on their generation). Among these, very early successional species have a short life span, high seed production and are good dispersers, whilst very late successional species tend to have the converse traits. The most extreme general succession model in one direction (Table 2, column 1) is one in which most of the species are present from the start and the course of succession depends solely on their relative growth rates, generation times, persistence and other life history traits. The ’initial floristic composition’ model is most likely to apply under secondary succession initiated by small disturbances in a large area of climax vegetation (e.g. Piroznikow, 1983 - although here early successionals may be absent altogether), or under larger-scale disturbances where most species are well adapted to the disturbance and can persist as seeds or other propagules (e.g. Purdie and Slatyer, 1976). Connell and Slatyer (1977) envisaged three other types of successional sequence in all of which ’early successional’ species are important. In a ’facilitation’ succession (Table 2, column 2), equivalent to Clements’s (1916) ’relay floristics’, the early stages are dominated by species which change soil conditions, by soil formation or debris trapping, by changing drainage properties and/or by nutrient enrichment (Crocker and Major, 1955; Lawrence et al., 1967). Such changes inevitably make the site less suitable for the pioneer plant, but happen to make it more suitable for other plant species - these are the later successionals. Such succession proceeds until and if the final colonists change the soil conditions little and/or make the substrate suitable for themselves alone. These processes are likely to be especially important in primary successions where soil formation takes place, and in some cases may generate cyclic changes when no species can stabilize the system (e.g. Godwin and Conway, 1939). Connell and Slatyer’s second model (‘tolerance’ - Table 2, column 3) is one in which early successionals are merely the first colonists and subsequent behaviour is directed solely by competitive behaviour under the same soil conditions. These authors stated that ’there is little evidence’ for this model; in many ways it is the most parsimonious of the four and could be resorted to in cases where nothing special appears to be happening! It would seem to be thoroughly inappropriate for primary successions, but might be found where only a small disturbance has taken place but nevertheless not all the actors in succession are present from the start, or encompass a wide range of life histories even when present from the start (Pickett, 1982. Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 rr- I *. .v U) 1 ’o c Cd C6 u za _fG 7i 0 CD U 1 E a) r Q) 0 U C f0 T3 -2 I u ,I:$ tl 1J c~ 0. x f0 ’C~ E8% x 0 .2 ’Gl Q) ...... C) u u 1J C co all i5 w ’ 0 4 C :J ~# en u o 0 Q) E a. ~3 f z x m LZ. u U E c i5 0 Q) U ’5 en N r4 t0 U .2 .0CO t0 C H Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 483 The final model (Table 2, column 4) is an ’inhibition’ process, where even the first colonists are good competitors and will not be replaced until they die of other causes. They are only replaced at all by virtue of their relatively short natural lifespans. Subsequent establishing species are even more intransigent and the community is eventually dominated by those species which combine a long lifespan with an ability to replace themselves in conditions of harsh intra and interspecific competition. The best examples come from the chaparral systems discussed earlier, in which the ’moribund’ climax vegetation is usually broken periodically by fires; otherwise productivity and regeneration of even the climax vegetation continue to decline (Biswell, 1974). It is difficult to imagine primary successions working like this, and secondary successions resembling the model could best be looked for under conditions of naturally low productivity and nutrient/water stress. Grime (1979) noted the same phenomenon and thought it worthwhile to erect a special category of life history strategy (’stress tolerator’) to describe species similar to Connell and Slatyer’s (1977) climax species for inhibition successions. 2 Simple mathematical frameworks It can be argued that a good mathematical model is one which is simple to use and which is wrong in helpful ways i.e. the manner in which it breaks down helps us to form new theories about what is really going on. Mathematical models which are right are usually too complex to use cheaply and apply only in restricted circumstances. Starting with the interactions between plant species, there are several appropriate starting formats for modelling plant succession. Shugart et al. (1973) examined succession as a series of linear differential equations describing the component interactions between plant species. They were restricted to a linear framework by the rapidly-increasing complexity of including non-linear effects. Although this line of modelling has proved extremely powerful for exploring the more general field of community stability (for a review see Pimm, 1982), there has been little subsequent use in the field of plant succession. A major problem is that it is difficult to collapse the interactions between collections of associated plant species into single linear differential equations, and without this simplification a vast number of separate equations are usually involved and the interaction between each species pair must be determined - a task of impossible complexity. Difference equations, although a useful tool in building theories about population dynamics (May, 1981), have not been used in succession modelling. As well as the problem mentioned above, there is a difficulty in matching the time frames of each plant species which would restrict their use. Succession can, however, be regarded as a process by which each plant species, or closely associated set of species, has a particular chance of replacing an individual belonging to any other particular species (or species group). Thus matrix models in which the base matrix is a table of probabilities of replacement of each plant Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 484 are an intuitively attractive way of representing plant an added There is succession. advantage in that the time frame is flexible: the time of the matrix can be step adjusted to e.g. the shortest generation time of any plant of or species set species. If replacement probabilities are fixed, depending only on the species involved, the model forms a Markov chain, whose mathematical properties are well understood (Bailey, 1964). Horn (1976) pointed out that these models could mimic many of the features found in real successions (Table 1), depending on the values given to different replacement probabilities (Figure 1). Disturbances from other sources such as fire, grazing, tree falls or agriculture could be regarded as changing either species composition, followed by the same replacement probabilities, or changing the probabilities themselves (Figure 1(5)). Unfortunately, if these transition probabilities change, the model becomes an order more complex and the conveniences of matrix algebra are lost. Usher (1979; 1981) and Austin and Belbin (1981) found that, in all successions where suitable data had been gathered, individual transition probabilities changed with time (the process is non-stationary) and/or with past cover states (the process is higher than first order). These authors concluded that not only did Markovian models give an inadequate description of succession, but the necessary data gathering for their more complex structure was too cumbersome to be useful when the biology of the individual plant species was known or could be assessed. Under some circumstances these models can be of practical use. On the one hand, the original demonstration by Horn (1976) that many of the classic patterns in succession could be explained by very simple processes stimulated ecologists to examine the real processes involved in a new light (e.g. Austin and Belbin, 1981; Game et al., 1982; van Hulst, 1979). On the other hand, one is often faced with a situation where little is known about the component plant species or their interactions, and the way in which a first-order, stationary, model breaks down can give useful insights into the possible processes involved. Gibson et al. (1983) studied succession after release from giant tortoise grazing on Aldabra atoll. The succession was both non-stationary and high-order, but the manner in which the simple models broke down, and the possible range of consequences of this, showed that succession accelerated with time, due to both colonization and facilitation effects, that individual species groups could both facilitate and inhibit others, depending on the receiver species, and that previous fears that giant tortoises overgrazing would devastate the island’s vegetation (Hnatiuk et al., 1976; Merton et al., 1976) were probably unjustified as the vegetation types thought vulnerable were part of a completely different successional series to those suffering high grazing pressure. Thus although there is, and probably will be, no simple general model which explains all plant succession, there are a number of tools available to the applied ecologist which can help identify the processes most important to the problem and goals in hand. species by each plant species, Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 Figure 1 A variety of transition matrices which mimic a variety of successions (all contain four possible states). 1 ) A slow but constant rate set of transitions from early to late - a possible format for a tolerance model. 2) Here we have mixed fast early stages with a very slow late one - there could be facilitation by A and B followed by inhibition by C. 3) This is a polyclimax version - climax states C and D are equally stable and equally likely to be entered from B. 4) Cyclic change - in practice the oscillations will eventually damp down to an even mixture of all four states. 5) One way of modelling a continuing disturbance such as grazing. Before grazing (5a), the succession resembles (1) but with a climax at C not D. Grazing changes the transition probabilities themselves (5b) such that C is now a transient state on the way to D and can indeed be bypassed by going straight from B to D. Grazing has also increased the chance of the early states being replaced. This is by no means the limit of the models’ versatility - it only serves to illustrate the range. Equally, the mimicking ability does not mean that the models are true (see text). Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 486 III Applied uses of succession To the inhabitants of industrial nations, a number of spectacular examples of succession theory applied to practical problems are in easy reach, although the layman may not recognize them as such. Ideally, they will not be recognizable successful applications to the revegetation of industrial waste tips, stabilization of sand dunes, reservoir and new construction landscaping will present themselves as indistinguishable from the surrounding agricultural and seminatural landscape. The ecology of land restoration is now a well-established field and an extensive literature is developing on the subject (see e.g. Bradshaw and Chadwick, 1980; Cairns et al., 1976; Goodman and Chadwick, 1978; Holdgate and Woodman, 1976). Although the practical transition from a bare, unsightly and often dangerous landscape to something more useful to people often involves a great deal of intricate and difficult research, the theoretical answer can be quite simple: a means must be found to initiate and accelerate primary succession from conditions inimical to most forms of life. Two examples will suffice to illustrate the general point. Open-cast kaolin (china clay) workings in southwest Britain (Cornwall) left conical tips of micaceous waste which supported only a sparse vegetation even after a century of abandonment (Skeffington and Bradshaw, 1982). Modern disposal produces tips of waste and overburden which are better designed for stability, but some of the older tips had to be reshaped before ecological work could begin (Allaby, 1983). The key problems in the establishment of succession were identified as nutrient (especially nitrogen) shortages and the free leaching properties of the original waste i.e. although seed could be sown mixed with fertilizer, repeat applications would be prohibitively expensive unless an effective nitrogen cycle could be established within the tips. This was achieved using the exotic (to Britain) tree lupin (with a very high rate of nitrogen fixation by associated bacteria in root nodules) in combination with initial mulching and later sowing of other species. In a very few years a sward develops which helps stabilize the tip surface, starts to build soil structure and can now support sheep grazing as well as starting to accumulate a wide variety of other plant and animal species. Wastes contaminated with heavy metals pose a further problem; it is not enough to identify the key nutrients limiting the start of primary succession, for the wastes are often toxic enough to kill most plant species. If the associated engineering problems can be solved; it is sometimes possible to seal the wastes in with a suitable layer of overburden (Bradshaw and Chadwick, 1980). In Welsh lead workings, a cheaper and equally effective method has been adopted (Bradshaw et al., 1978). They fould that on some of the older workings with very high lead concentrations (due to less effecient extraction processes in the past), one or two plant species could be found growing in very high lead concentrations. On examination these were found to be genetically resistant strains, and by pooling the species from a large number of spoil heaps around the country, several dozen species could be accumulated (Smith and Bradshaw, 1979). Unfortunately, few of these species Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 487 had nitrogen-fixing symbionts, and so establishment of a viable sward and the initiation of soil formation had to be accompanied by applying fertilizers and/or sewage sludge. Thus the problem was partially solved by artificially enhancing colonization (bringing the plants in) combined with identifying the key nutrients involved in initiating succession. The solution is not perfect; although the lead tips so treated are no longer unsightly and are safe for amenity purposes, the lead levels in tolerant plant species are too high to permit agricultural use (Bradshaw and Chadwick, 1980). Low-input agriculture and forestry worldwide is plagued by problems caused by inadequate understanding and/or application of processes of plant community change. We feel that this is a neglected area of research and one in which investigation of particular problems would reap enormous dividends. Such problems are most likely to be encountered in third world countries where social, economic or environmental conditions make the high-input agriculture of the developed world difficult or inappropriate. To date, we know of few studies where particular problems have been solved (but see Kruger, 1984) and a general example is worked out below to indicate the sort of applications that are possible. A common problem in semiarid areas has been the reversion of grasslands and savanna to woodland following use as grazing range by domestic stock. Walker al. (1981) proposed a model which accounts for this process and suggests relatively simple solutions. They showed that the balance between water permeability properties of the soil, deep-rooted plants such as most of the woody species, and shallow-rooted ones such as the pasture grasses, and grazing pressure, resulted in two alternative equilibria. Given a range of semiarid seasonal rainfall regimes, under light grazing the grasses formed an extensive shallow-root mat which effectively suppressed most shrub seedlings and prevented water from reaching the deeper layers of the soil where established shrub roots lay. If grazing pressure was increased, there came a point where the shallow grass root mat is weakened and more water (also probably leached nutrients) reached deeper layers of the soil. Young grasses cannot establish because of the grazing pressure, but young woody plants manage to burst past the grazing levels because of their increased access to water underneath. Once the woodland is established, it will remain as by this time the canopy has closed and most water passes through the regions where young grass roots lie. The system can only be broken by artificial removal of the shrub cover (e.g. by burning) combined with the exclusion of herbivores and possibly reseeding: the season of the year at which this is done is also crucial (Kruger, 1984). The problem need never arise if stocking densities are carefully managed at the start. Even in intensive agriculture, an understanding of the interactions between different types of grazers and the development of the sward can be important in devising the most efficient management. Moore and Clements (1984) found that frit fly (especially Oscinella frit) infestations were lower when a ryegrass ley was cut than when it was rotationally grazed by sheep. Grazing produced higher numbers of tillers, and the individual tillers were more susceptible to frit fly et Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 488 attack, for reasons which remain uncertain. The natural succession after sowing a ley has always been an expensive problem; either from the need for frequent reseeding or from the decline in productivity associated with a more diverse sward. In many areas of Britain modern practice is to keep grazing animals off during the spring and summer months, taking hay or silage cuts instead. The pastures are only grazed from later on in the summer. This is a direct outcome of work such as that described above and going back to Jones (1933). His findings showed, among other things, that the traditional practice of pasturing as many sheep as the area could hold during the winter and leaving the same number on in the summer quickly turned grass/clover mixture into high diversity, weedy, low productivity swards. Pure ryegrass or clover-ryegrass mixtures could be maintained by changing the phenology of grazing to match the phenology of the desired species, as is done now. Wildlife conservation, especially in areas with a long history of human use, is often a matter of the manipulation of habitats to accord with a particular successional stage. Here it is rarely enough to know only a few key processes in the succession; the rarity of a species and/or a particular community may depend on the fine details of a relationship between climate, edaphic factors, the species available for colonization, and past human management. Two studies in the conservation of butterflies in Britain, one unsuccessful, the other a qualified success, serve to illustrate the point. The native Large Blue (Maculinea arion) is now extinct in Britain, the Adonis Blue (Lysandra bellargus) persists in a few places. Both species’ larvae start life as phytophages of common plants of limestone grasslands (Thymus drucei and Hippocrepis comosa respectively). The Large Blue is then taken into a nest of Myrmica sabuleti by foraging worker ants, wherin it completes its development as an ant predator. The Adonis Blue remains phytophagous but is tended by ants of several species (Thomas, 1983) and may pupate in an ants’ nest. In both systems, the animal part is dependent on the microclimate of short, heavily grazed limestone swards in the south of England: the Adonis Blue is further restricted to southfacing slopes. Changes in farm economy in the early twentieth century were followed by the removal of heavy rabbit grazing by myxomatosis in the 1950s. The consequent decline in both butterfly species was noted early, but conservation efforts were frustrated by inadequate knowledge of the unusual plant communities and grazing management on which they and their ant associates depended (Thomas, 1980; 1984). By the time that this was understood, the Large Blue had declined to a point where a combination of years with atypical weather drove it extinct (Thomas, 1980). The vegetation needed by both species can be recreated, and has been, by reimposing the right grazing regime in the right place, but remaining colonies of the Adonis Blue are usually too scattered for effective colonization; this has been done successfully by artificial means (Thomas, 1984 and personal communication). This story of particular species reflects both i) the need to understand the processes behind change in the whole community for conservation to be effective and ii) the potential for restoration of any community whose component species are still available provided that the mechanisms which drive secondary succession Downloaded from ppg.sagepub.com at PENNSYLVANIA STATE UNIV on September 20, 2016 489 (e.g. on abandoned agricultural land) towards it are well understood and can be reapplied. We see this as one of the areas within succession study with great potential for future applications, which are only beginning to be used. Great potential for future development also lies in the management of low-input systems of agriculture and forestry, especially in third world countries where reliance on continued input of fertilizers and pesticides from outside can be too expensive or not feasible for social reasons. 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