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Grazing index method procedures


Du Toit PCV, Botha WvD, Du Pisani LG, Blom CD, Oliver DJ

Grootfontein Landbou-ontwikkelingsinstituut,

Privaatsak X529, Middelburg (OK) 5900

 


Abstract

In the past, veld condition in the Karoo was assessed by using the ecological index method. This recently changed to the grazing index method, on account of the differently estimated grazing index values being used. The principles governing the method of survey remain the same. Through the application of the individually estimated grazing index values of the species, the index of veld condition is computed from line-point data. Appropriate management of the line-point data yields acceptable indices of veld condition, from which current grazing capacities can be estimated for assessed sites. Additional index words: grazing index method, line-point surveys, canopy spread cover, veld condition index, index of agreement, z-index.

 

Introduction

Due to time constraints when doing botanical surveys, it was generally felt that recording 100 point observations per survey, sufficed. However, when recording the canopy spread cover a 100 % cover was rarely recorded. This led to insufficient data points on which to base the important calculations of the veld condition score of the surveyed site and its derived current grazing capacity, with obviously large attendant errors. Various suggestions regarding the technique, number of point observations to be taken during a survey and the treatment of the collected data were put forward and are considered in this paper.

 

Discussion

The technique

Owing to the fact that the line-point method of survey and its concomitant data treatment, leading to the computation of current grazing capacities is , well known and has become firmly entrenched in the day to day survey activities of a large number of research and extension personnel in the Karoo and adjacent dry grassveld areas, it was decided to support the existing technique (Du Toit 1995), instead of the alternative technique promoted by Westfall et al. (1994).

The method of line-point survey is loosely based on the descending point method of plant survey (Roux, 1963). However, instead of the Tidmarsh botanical survey wheel (Tidmarsh & Havenga 1955) with its attached descending point (Roux 1963), a length of non-stretch rope, suitably marked at 1 m intervals and a handheld rod is now used to conduct the survey. The theory and principles governing the methodology of point surveying remains the same and this method is comparable with the wheel point and descending point methods of vegetation survey (Roux 1993).

On account of the patchiness usually encountered in vegetation, it is advisable to do long-line surveys, instead of a number of parallel lines of 50 to 100 points each, restricting the survey to a small area of the plant community. Long-line point sampling eliminates the possibility of unrepresentative sampling of the vegetation to a large degree.

However, if the intention is to specifically sample a community where it is suspected that selective grazing is taking place or where there is some other form of disturbance, a number of parallel lines should be laid out to carry out the survey. The effects of the selective grazing or disturbance can then be studied.

Line-point surveys are therefore to be carried out (Du Toit 1995) and an estimate of plant cover and botanical species composition obtained by recording the canopy spread cover, i.e. the perpendicular projection of the circumference of the canopy of the species onto the ground (Goebel et al. 1958, Du Toit 1996). The method of measuring the canopy spread cover, is favoured on account of the shortcomings in the measurement of basal cover (Mentis 1982; Vorster 1982; Palmer et al. 1990, Westfall et al. 1994) and the recording of the nearest plant which favours annuals, small plants and seedlings (Vorster 1982; Du Toit 1995).

Measurements of canopy spread cover provide a more realistic botanical species composition of the permanent vegetation in karoo veld (Roux 1963; Vorster 1982) and provide more useful information than does basal cover measurements (Westfall et al. 1994). Indeed correlations of paired sets of surveys where canopy spread cover (Vorster 1982; Du Toit 1995) as well as the nearest plant (Foran et al. 1978) were recorded in order to establish the botanical species composition, rendered a mean coefficient of determination of R2=0,8987 (Table 1).

It was found that where the coefficient of determination was below 0,7, the sites were recently grazed, or there was some form of disturbance. As far as the karoo vegetation is concerned, the method is robust enough to record different veld composition scores, by measuring canopy spread cover before and after grazing events. In the Karoo the nearest plant method will therefore not be used in combination with canopy spread cover measurements.

The number of points observed Owing to the fact that time is usually at a premium during botanical surveys, the recommendations of Tidmarsh & Havenga (1955) were noted, but it was decided that 500 point observations per survey will be recorded [Table 2). This number of point observations has an acceptably low degree of error in veld with varying degrees of canopy spread cover percentages and it was felt that spending twice as long per survey in order to minimise the error by a further 3 %, was unjustified.

 

Treatment of the survey data

In the line-point survey, the number of strikes on a species is calculated as a percentage of the total number of point observations made. Note very specifically that the number of strikes on a specific plant species are not expressed as a percentage of all the strikes obtained. Therefore, the sum of the individual plant species' percentages obtained, seldom totals to 100, because the number of strikes observed are fewer than the total number of point observations made. This value, i.e. the total number of strikes recorded, is an indication of the canopy cover of the site (Du Toit 1995). The percentage values obtained for the individual species are then multiplied by their respective grazing index values (Du Toit et 01. 1995). The products are summed and the index of veld condition obtained. From this veld condition index value the current grazing capacity of the site can be computed (Du Toit 1995). Different sites can be compared by reference to their veld condition indices or by computing the Z-index of agreement between the sites according to Roux (1993). In this way the reaction of the vegetation to different grazing treatments or the reaction of the vegetation to the rainfall received during different years, can be quantified. Statistical differences between sites can also be computed by using the tables published by Roux (1993).

 

Conclusions

The line-point method of vegetation survey is well known and widely applied in the Karoo and the adjacent sweet grassveld areas. With appropriate management of the data collected by means of the line-point method of vegetation survey, comparable and acceptable indices of veld condition are calculated, by applying the individual grazing index values of the different species. From these calculated veld condition index values, agronomically acceptable current grazing capacities can be estimated (Du Toit 1995).

 

 

References

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