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Phytosociological survey and pasture-range evaluation


(*) A. Gaston, Agrostologist, I.E.M.V.T.

The phytosociological survey
Evaluation of pastures (range grazing)
Evaluation for management
Control of ecological changes


The elaboration of a survey is analysed to show points which may be subject to different methodologies, and from the point of view of agrostological utilisation. The methods used should lead to a vegetation description, understandable to the reader, regardless of the method.

Evaluation of pastures is studied, from the point of view of the productivity at the time of the inventory and of the evolution of productivity under exploitation, emphasis being placed on this last point.

In addition to measures of biomass this evolution of productivity can be studied by ecological methods, complemented by photo interpretation.

Use of precise methods of evaluation which allow us to follow the evolution of productivity with time, or better, to forecast for short periods, is the essential guarantee of good management.

The phytosociological survey

There is an abundant literature on phytosociological survey, so the subject will not be discussed here, even in mere summary form.

A phytosociological survey meets the need for a description of a plant population, a description which can be made concrete in the form of a map. The phytosociological survey represents what exists.


Selecting the station is the first problem and the value of the survey depends on this choice, for the species noted will be representative of the station.

Transcribing the observations will be the second problem, for this must be the true reflection of reality, as precise and comprehensive as possible. To this end all the criteria immediately available should be taken down at once on the site, others (analysis of the soil, for instance) being noted later on. Observations should all be reported in the same way, the observer following a strict discipline, firstly in the order of his observations and then in codifying their transcription.

A phytosociological survey shows all the conditions and the vegetation at the station.

The conditions can hardly be subject to discussion. There is only one way of expressing the altitude and the aspect. The way of expressing percentage of area covered by stones, for instance, is generally accepted by everyone. Likewise, the results of pedological analysis are not subjects of controversy.

Vegetation poses more problems, especially in relation to the relative abundance of the species. To express this, one should use a method which is clear and representative. Counting and measuring every individual of each species is out of the question, hence it is necessary to use a convention representative of the abundance and dominance. The floristic part, the cover (ground level and at the canopy), and the vegetation strata are not subject to serious dispute.

The methodology may take various forms, such as a synoptic table or mechanical data processing. Each of them demands comprehensive surveys correctly recorded, every observation being noted in a strict way so that there cannot be any possible discussion. The scale of work may justify the use of a synoptic table if the number of surveys is low, with the possibility later of using another methodology. Whatever means are used, the technique should lead to a life-like description of each unit of vegetation.

To conclude, it appears that:

- the survey may be approached by means of various methods, the main point being to make a judicious selection of location;

- the codification of the observations sets problems only in relation to abundance and dominance;

- the technique can be achieved by various methods.

The outcome of the survey, that is to say the description of the vegetation, should not be affected by the use of one or another method; still it would be advisable to try to conceive of a methodology accepted by everyone.

It seems suitable now to place the survey and the work which inspired it in context.

Phytosociological study should end in a description of the vegetation allowing the reader to reconstitute the vegetal landscape, and anyone in the field with a fair knowledge of the flora to isolate the unit defined, this being made easier by means of a map.

This unit must represent an average survey, obtained by comparing similar stations. It must provide a list of typical species with their respective importance so that the reconstitution of the vegetal landscape can be made. The physiognomic and geomorphic characteristics which were not overlooked during the survey on the ground should again be mentioned.

In the case of an agrostological study, the phytosociological part of the work being basic, it must be carried out in the best conditions, everything being adapted to its purpose and to the means at the disposal of the surveyor, but, equally, adapted to the conditions of the region. For some very homogeneous regions, a low density of surveys will be sufficient. Others will need a very high density for a good understanding of the vegetal population. So, two factors interact on the quantity of surveys necessary, the area of the zone and its complexity.

Here, the operator has to make a choice and decide whether increasing the number of surveys will be useful. The choice is not completely subjective, for it can be supported by the study of aerial photographs and the comparison of surveys of a similar site. Even statistical comparison may be used. Once a certain stage has been reached, accumulation of additional information does not add anything more to the final result.

A second choice, indicated by the scale of work, is made at the level of the ecological station. In the Sahelian zone for instance, a pastoral inventory may neglect casual and rare micro-fasciae. The resulting description of the vegetal unit will not be very far wrong, for a limited number of surveys can account for the vegetation of a vast zone. Likewise, omitting the micro-fasciae will not damage the value of the general description.

Evaluation of pastures (range grazing)

Once the units of vegetation are defined and transferred onto a map, the second part of the agrostological work will consist of the estimation of the forage value of the pastures. This can only be known by steps.

The forage value at the moment of study is the easiest to obtain. A representative sample is taken of the edible biomass in relation to the biological cycle; so sampling will begin during the rains. In the Sahelian zone the optimum forage value occurs at the same time as the optimum biomass, whereas in the Sahelo-Sudanian zone it precedes it.

Regular samplings throughout the whole year enable us to follow the value. In the Sahelo-Sudanian zone this evolution may be complicated locally by the phenomenon of perennial species sprouting again after grazing or burning. So, the value under exploitation and farming conditions must be included in every programme of pastoral development. The types of dominant pasture will be the objects of continuous observation and their value followed within control areas and in totally enclosed zones. If one is dealing with extensive exploitation, the evolution of the cattle herd should be controlled to balance the actual stock and the potential stock carrying capacity. It is essential, after every rainy season, to know the biomass available for the coming year, the pluviometric regime having the determining influence, especially in the Sahelian zone.

Once that value is known, it enables us to classify the different natural pastures of an area and to integrate them in a development programme, because their productivity will be deduced, and the pastoral zones will be separated from the agricultural ones, the latter having the possibility of including some peasant cattle-raising.

Evaluation for management

From the beginning of a pastoral operation one must set up the means of measuring the evolution of the forage value under the effects of exploitation. It is essential to know the potential forage available for the coming year and to know how it is going to evolve during that year. Statistical methods must be employed.

These means of control should help us to forecast the evolution of the pasture for the following years. This can only be known by comparing various biomasses and by studying annually the vegetation of control areas, a process which allows us, for instance, to follow bush-formation or changes in the grassland flora.

To this procedure must be added experimentation, consisting of trials of various stocking rates to know their influence upon the value of the grazing. Only a detailed method of estimation, supported by experimentation will lead to good management of the pastures. One must remember that the value is never steady and must be constantly reassessed.

Control of ecological changes

Farming a pasture, whether intensive or extensive, entails ecological modification, that is to say qualitative and quantitative variations in the flora. These modifications can be measured by means of phytosociological surveys, enumerations and measures. The area of work being limited, more precise methods may be used, which can imply counting and measuring each individual. The total biomass comes into the phytosociological study and the biomasses of the species, or edible parts of species, in the evaluation of the pasture.

Controlling these modifications is a question of scale; a small farming exploitation will be easily controlled on the ground by the methods previously described; a medium-sized exploitation may be controlled by cautious extrapolation. Vast zones, such as the Sahelian ensemble, likely to undergo ecological modification, will be controlled by measures on the ground in representative stations, and extrapolated by "photo-interpretation".


It is important to harmonize the methods of selecting the survey locations, of noting abundance and dominance, as well as the methodology, in order to obtain a clear description of every unit of the pastoral vegetation, because it is necessary for the grassland management specialist to be able to recognise them on the ground.

The knowledge of the value of the pasture is indispensable for good management evaluation. It should be the object of regular investigations. The methodology of evaluation and control of the evolution of the pasture should also be subjects for harmonization of the methods. Evaluation offers the grassland management specialist a starting-point but he must provided with a practical means of control.

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