Succession is the term used to denote the sequence of orderly changes which take place in vegetation. It must in the first place be made quite clear that in the majority of cases plants live together in well-defined communities. Even the arable crop of our English countryside will but exceptionally be a pure crop, because of the fact that 'weeds' of one sort or another will always be found, even in the most carefully cultivated plot of land. Compared with the arable field, the grass field is more conspicuously a mixture of crops. Certain grasses and clovers will generally have been explicitly sown, and in addition, other and unsown plants will invariably make their appearance in the sward. The basis of our vegetation, therefore, is the association of plants living together more or less in harmony with one another. The less harmonious the relation, the more unstable the association, and the more likely do rapid and fundamental changes occur in the composition of the pasture (i.e. of the plant association).
Changes in the composition of the plant association usually take place gradually. Furthermore, and this is an important point, the trend of any such changes follows an orderly sequence. Succession, therefore, is the term employed to denote this orderly sequence of associations of plants. Normally, the unstable association will change not only in the comparative abundance of its constituent species, but also by the infiltration into it of new species. If the new environment is suitable, these new infiltrations will flourish, and they may become the chief species in the association -- they will then be referred to as the dominant species. During this new infiltration and change towards dominance, the composition of the plant association will obviously have markedly changed; many of the characteristic plants of the previous association will have been ousted, while others may have been reduced to negligible frequencies. In short, a new plant association, with a new set of dominant species, will have replaced the old, and a phase in ecological succession will have been completed.
It has been said in a previous chapter that the natural vegetation over the greater part of Britain is forest, and that our grasslands are being maintained only as the result of human and animal activity. Were these humans, together with all domestic and wild animals, to be removed from the grasslands of this country, then a gradual replacement of grass by forest would take place. This replacement would occur in an orderly series of succession phases. Grassland regarded in its widest sense in this country may, therefore, itself be regarded as a distinct phase in the succession from bare soil (as recently ploughed land) to forest. The next point to note is that within the general conception of grasslands as they occur in this country, there are a number of pasture types many of which are distinct associations in themselves. Often these pasture types grade the one into the other, and in many cases they show decided trends in succession. The precise nature of the successional changes will depend partly upon soil and situation, although human activity involving management of the pasture may have an even greater influence upon the vegetation.
Newly sown pastures are normally much less stable as grassland associations than are older-established swards. This is due, in great part, to the fact that the seeds mixture will advisedly contain a mixture of grasses and clovers: the constituent species will have differential attributes in relation to rapidity of soil germination, seedling establishment and rate of vegetative growth. Some species, such as Italian ryegrass and broad red clover, are able to grow rapidly and to come quickly to maturity, whereas other components of the mixture, including white clover, timothy, dogstail and say cocksfoot, are slower to establish themselves. Generally speaking, the short-lived species and strains employed in the seeds mixture are those which make the most rapid growth in the early stages of the ley while the more persistent elements of the mixture are slower to get away from seed.
These facts are important, and form the background of knowledge dealing with the proper management of leys. In a short-duration ley, where persistency is not demanded, all the ingredients of the seeds mixture will be rapidly establishing and short-lived plants lasting for one or two years and providing vigorous growth during this period. Characteristically, Italian ryegrass, early maturing forms of perennial ryegrass, the red clovers and trefoil provide the basis for such mixtures giving excellent grazing, silage crops. and/or hay. If, however, a long-duration ley is required, additions must be made to the seeds mixture. These additional species and varieties will include the pasture types in ryegrass, cocksfoot, timothy and other grasses, with always wild white clover. These longer-lived ingredients of the seeds mixture, because they are slower to become established, may easily be crowded out by the more vigorous ryegrasses and red clovers during the first year after sowing. To counteract the adverse influences of the quick-growing species, the latter may, of course, be omitted from the seeds mixture, reliance being placed wholly on the less aggressive but longer-lived elements. In practice, however, the quickly establishing species have an important value. In fact, their inclusion in the mixture may be of decided advantage not only in providing so much earlier keep, but also in acting as nurse to the species of slower seedling growth. The shelter offered to small seedlings by Italian ryegrass in a newly sown grass field is of appreciable practical value. When the new seeds are grazed off, the ryegrass provides the bulk of the herbage, and the influence of treading, urinating and dunging by the grazing animals assists the further establishment of other seedlings as well as putting renewed vigour into the ryegrass itself.
The new ley is, therefore, a very delicately balanced association, with the consequence that even minor changes in the management of the grazing will result in a directly comparable change in the balance of species making up the ley. Allowing the ryegrass to go away to a hay crop may cause the complete suppression of other species, while grazing excessively hard during the first year will kill out the Italian ryegrass or so seriously weaken it that it can no longer become aggressive towards other species growing in association with it. The increase of soil fertility during the early phases of pasture establishment reacts directly upon the rate of growth and on the relative aggressiveness of the different species. Normally, growth and germination in the soil will be most rapid under conditions of high fertility. Italian ryegrass and other quick-growing plants will, under these conditions, be correspondingly more aggressive, and therefore, in order properly to establish the sward, the grazing should be heavier and greater care taken to keep the ryegrass under control.
As the ley proceeds into its second year, so it becomes more stable as a plant association. If properly managed, all the sown species will have become fully established during the first year. The balance as between the different species is now less delicate than in the first year, and therefore the management may be the more varied. The taking of hay, for example, is not likely to have such harmful consequences in the second year as in the first. Conversely, excessively hard grazing in the second year can more easily be counteracted than can excessive grazing in the first year. Leys in the third and subsequent years, if properly constituted in relation to seeds mixture and manuring, may attain to reasonable stability in respect of botanical composition. They may retain this stability for a number of years, and as they proceed towards the permanent pasture phase the botanical composition (under a given set of management and manuring) need not change to any considerable degree.
Permanent pastures as a whole change less in botanical constitution from year to year than do the leys. Long-established pastures and meadows, therefore, attain to some standard of equilibrium, and so long as the same management (including the maintenance of soil fertility) is superimposed on them year after year, permanent grasslands will change very little from year to year. If the management is drastically changed, however, this is quickly reflected in an altered botanical composition. In other words, a new plant association replaces its forerunner and ecological succession takes place.
The natural trend of succession is always towards woody growths, normally the pre-phases of forest development. The effect of grazing or of mowing is to maintain a grassy herbage. In many parts of the Midlands, as, for example, on the Lias clays, the pastures do not carry sufficient animals to keep down thorn and briars. In order to control the bushes it has become the practice in some districts either to take hay in alternate years or to take hay in every year. In other cases (more common, however, in past days than in recent years), small thorn bushes on pastures were kept under control by hand-cutting (slashing).
Taking hay every year, accompanied by little or no grazing, gives rise to a characteristic herbage. Grazing only (in practice and all too often this means hard grazing in winter and spring and altogether understocked grasslands during the summer flush periods) on the other hand promotes a different type of herbage. Thus, on the Lower Lias clays of Warwickshire, continual hay without manuring (and we are talking now of permanent grassland) gives rise to swards in which red fescue, stunted cocksfoot, torgrass, wild red clover and a variety of weeds are the prominent features. Hay year after year on the acid soils of the Welsh lowlands gives an Agrostis pasture with cocksfoot, ribgrass, yellow rattle and other weeds, and with but little clover. In the Cumberland valleys the hay-every-year-fields are full of cocksfoot, together with a large weed content induced in part by heavy dunging of the meadows.
Continued grazing without regard to the finer points of good pasture management promotes in the west and north the formation of Agrostis pastures with or without white clover. Among the weeds, woodrush (Luzula), buttercup, daisies, catsear and the like are prominent features of the pasture fields. In the Midlands and the south-east, tussock grass (Deschampsia caespitosa), blue rush (J. glaucus), sedges, torgrass, agrimony and small thorn are characteristic weeds in the Agrostis or Agrostis-with-fescue pastures. Anthills are a very common feature on sub-derelict pastures. Once anthills are allowed to form unhampered, the use of the mower becomes impossible until the anthills are removed. Without the influence of the mower, thorn spreads rapidly.
Over much land in the Midlands and the south of England the occurrence of anthills on fields, is rapidly succeeded by thorn and complete dereliction follows. From the standpoint of succession, the gradual and diminishing influence of the domestic animal on the already low carrying capacity pasture has ended first in a wilderness of thorn, briars and brambles, and ultimately (and in the more or less complete absence of livestock) in a dense cover of woodland. Given time, such woodland would give way to mature forest of oak, birch, ash or beech, depending on soil and locality. The all-too-numerous instances of whole blocks of one-time fields now reverted to wilderness on the heavy clays are in themselves very instructive. If these areas be carefully examined, they are found to include small clearings, usually closely grazed by rabbits. Indeed, these small patches of grazed pasturage owe their maintenance as such directly to the rabbits, and are a clear example of the manner in which the grazing animal maintains grassland and prevents the natural succession (or reversion) towards forest.
Grassland succession on pasture fields is closely linked up, therefore, with the management and the manuring of the fields. Thus, when phosphates are applied to an ordinary lowland pasture, say of the Agrostis-with-fescue type, there is a marked increase in clover content, which induces better grazing, and the field 'improves'. What has happened is that the succession, instead of being allowed to proceed towards fescue-with-Agrostis (which is closer to the woodland and scrub phase than is Agrostis-with-fescue) has been reversed, and if the treatment of combined manuring and closer grazing is repeated, the trend of succession will be such as to reduce the fescue, with a corresponding increase in clover and Agrostis. Given time and the continuation of improved grassland management, ryegrass, cocksfoot, timothy, rough-stalked meadow grass may each make spontaneous appearance, and may even dominate the pasture. Such a pasture is of greater agricultural value than the original Agrostis-with-fescue which it has replaced. What has been accomplished is a reversion of the natural processes of succession, ending in a sward of enhanced grazing value.
Our attempt in grassland maintenance, therefore, is to retard succession towards forest and to maintain grasslands at their most highly productive stage. This can be done on permanent grass by well-informed management, together with a measure of manuring as is demanded by the soil. On naturally fertile soils, pastures of the highest grade (composed chiefly of ryegrass and white clover) can be maintained indefinitely, but only where the management is such that the sward is grazed and all roughage is periodically removed. On such soils the demand for added plant foods is small, and especially as on the fatting pastures of the Midlands, where mature animals are being grazed. To make similar quality permanent grasslands on less fertile soils would naturally make for heavier demands on manuring, and particularly in districts where dairying and stock-rearing replace the mature fatting beast.
The influence of treatment upon the composition of pasture is very well shown by examination of roadside verges throughout the country. Usually the strip of pasture next to the highway contains ryegrass and white clover in abundance. Away from the road edge, the proportion of coarser grasses increases sharply, and there is little or no clover. The following comparison from data by Davies is of interest in this connection:
- A - Road Edge B - Middle Zone C - Inner Zone (towards hedge base) Perennial ryegrass 33 per cent 20 per cent 4 per cent White clover 14 7 3 Cocksfoot 3 7 20 Yorkshire fog 2 10 21 Agrostis (= bent) 15 21 19
This statement is based on the average of a number of roadside verges examined in different parts of England and Wales. The narrow strip of grass adjoining the carriage-way contains a large proportion of ryegrass and white clover, but very little cocksfoot or fog. Away from the road the proportion of ryegrass and clover decreases, and there is a marked preponderance of cocksfoot, bent and fog in the sward. This series of readings has a direct bearing upon succession in grassland, and in relation to the management afforded to the three roadside sections. The inner zone of roughage and with little clover lies in proximity to the hedgerow, which itself may be regarded as representing the woodland and scrub phase which follows after open grassland. At the base of the hedge numerous tall-growing weeds -- themselves semi-woodland plants -- are to be found.
There is also marked similarity between the vegetation of the much-trampled roadside grass verge and that of the footpath and sheep-track. The same is true of the plants growing around the gateways into fields. Thus, the footpath is predominantly ryegrass and white clover, and this is appreciably true whatever the composition of the grass field through which it passes. Characteristically, the gateways through which livestock pass in and out of fields are associated with a zone of more or less bare ground (where treading is excessive) around which is a zone of ryegrass and white clover, or, on certain clay soils, timothy with white clover. Beyond the zone of ryegrass and white clover the vegetation merges into that of the body of the field.
We find the same degree of differences when we look at the composition of contrasting fields within the same locality, and these differences in permanent grass are to be related to differences in management. The diagram below will serve to illustrate the connection between grassland type and treatment of the pasture.
It will be instructive to consider the statement hereunder showing the main species contributing to four contrasting pastures in Warwickshire:
Grade of Pasture A - 1st Grade Ryegrass B - 2nd Grade C - 3rd Grade D - Agrostis-fescue with torgrass Reference Number 83/B 4 83/A 3 82/B 12 82/G 11 Perennial Ryegrass 44 25 8 - Cocksfoot 2 - 4 - Rough-stalked Meadow Grass 15 10 - - Agrostis 10 35 44 16 Red Fescue 12 15 7 16 White Clover 12 7 8 6 Torgrass - - - 20 Crested Dogstail - 3 10 8 Other Plants 7 3 23 30 Total 100 100 100 100
Sample A represents a first-rate ryegrass pasture on which cattle beasts can be fattened without supplementary feed. Sample B is also a feeding pasture, but of less good quality, and of lower agricultural value. The third sample represents the type of ordinary pasture not capable of fattening, but which will carry store stock in good condition. Sample D is from a derelict pasture, in which torgrass is the most abundant species. This pasture is of very limited grazing value, and is typical of considerable acreages on the Lias clays of the Midlands. We would stress that these examples of sharply contrasting botanical composition are, in fact, closely related the one to the other. They represent four different phases in grassland succession -- phases induced by the management and the manuring (or lack of it) afforded to the four parcels of grassland. It would be possible, by consistent bad management, to make the best pastures deteriorate to the standard of the poorest. Conversely, by excellent management, coupled with efficient manuring over a sufficient period of years, many of the worst pastures could be improved to a very high standard without ploughing. It is the successional phases of permanent grassland with which we are here dealing. It will be shown in subsequent chapters that these desirable changes however can be brought about very much more rapidly under the influence of the plough and with the establishment of high-class temporary leys properly managed and adequately manured.
DAVIES, WILLIAM (1938). 'The Ecological Relationships of the Grass Verge and Other Excessively Trodden Habitats.' Journ. Ecology. Vol. 26.
Next: 4. The Basis of Ley Farming
Back to Contents
Back to Small Farms Library index
Community development | Rural development
City farms | Organic gardening | Composting | Small farms | Biofuel | Solar box cookers
Trees, soil and water | Seeds of the world | Appropriate technology | Project vehicles
Home | What people are saying about us | About Handmade Projects
Projects | Internet | Schools projects | Sitemap | Site Search | Donations | Contact us