A cover meter for canopy and basal cover estimations

A simple, inexpensive, pocket-sized cover meter for estimating both canopy and basal cover is described. The cover meter is based on the visual superimposition of canopy-to-gap ratio scales, on to vegetation. Tests indicate that, in certain vegetation types, accuracy is comparable to the wheel-point apparatus in estimating basal cover. Canopy cover is estimated in classes according to the Domin-Krajina cover-abundance scale.


INTRODUCTION
The accuracy of visually estimating canopy cover in Braun-Blanquet vegetation analyses is dependent on the observer's experience in estimating cover. The B raun-B lanquet cover-abundance scale (Mueller-Dombois & Ellenberg, 1974) has large class intervals to facilitate cover estimation. The large class intervals, however, do not permit detection of significant differences in vegetation cover. Consequently, interpretation of Braun-Blanquet tables places greater emphasis on presence o r absence of species than on species cover.
In a recent vegetation study (Westfall, 1981) the Domin-Krajina cover-abundance scale (Mueller-Dombois & Ellenberg, 1974) was used to determine veld condition and trend, because of the smaller class intervals than in the Braun-Blanquet coverabundance scale. However, the smaller class intervals increase the difficulty of cover estimation. This difficulty led to the development of a simple, inexpensive, pocket-sized cover m eter for canopycover determinations. The success of the cover meter was such that its application was extended to basal-cover determinations. DESCRIPTION

AND USE O F T H E C OV ER METER
The cover meter is based on cover estimation by the canopy-to-gap ratio method (Edwards, 1983) where cover is given in terms of the ratio of the mean canopy diameter to the mean distance, as number of canopy diameters, between the canopies of the plants. Two  observed by the free eye. A filter, made of paper or exposed film, and placed in front of the slide, reduces the light input which can facilitate superim posing the two images. The observer selects a bar and corresponding space to the right of the bar which, when superimposed on the canopy or basal diameter and gap, produce the best fit, and reads the appropriate cover percentage, canopy: gap ratio ( 0 ) o r Domin-Krajina cover-abundance value (Figs 1 & 2). The proportions between bars and spaces for a given percentage cover remain the same, so that backward or forward movements by the observer to match the images, are limited. The observer should, however, remain at right angles to the canopies or basal diameters observed.
The cover meter was tested to determine limits of accuracy and minimum samples required.

STUDY AREAS
The estimates of basal cover were determined in grassland plots at the Botanical Research Institute in Pretoria, during March 1983. The canopy cover estimates were determined in Acocks's (1975) Mixed Bushveld at the Nylsvley Nature Reserve, near Naboomspruit, Transvaal, also during March 1983. The stands were all selected to provide variation in total cover.

Basal cover
Basal cover was determined for each of the three 20 m x 20 m grassland plots by: (i) A wheel-point apparatus (Tidmarsh & Havenga, 1955), with points 0,38 mm in diam eter and 1 m apart. A sample of 1 000 points was taken in each plot.
(ii) The cover m eter, with 30 random readings in each plot based on a grid with lines 1 m apart. Grid co-ordinates were selected by random numbers (Fisher & Yates, 1949). The nearest grass beneath the grid intersection points was selected together with the nearest neighbour intercepted by the grid lines. The observer faced in a constant direction for all recordings and used the right-hand grid line to determine the nearest neighbour, for convenience.
Although grass, forbs and shrubs were recorded separately for each method, only the grass component is considered here.
The basal cover estimates with the cover meter were tested for normal distribution according to Southwood's (1966) method as used by Van Ark (1981). The number of samples required for different limits of accuracy were then determined for the three basal cover plots individually and jointly, according to the formula (Van Ark, 1981): where N = number of samples t = table value for the t-distribution with (n-1) degrees of freedom S = standard deviation with (n-1) degrees of freedom d = level of accuracy expressed as a decimal x = mean.

Canopy cover
Eight stands, each with a different degree of canopy cover, were selected from 1: 4 000 scale aerial photographs. A plot of 2 cm x 2 cm was demarcated on the aerial photographs in each stand, representing approximately 80 m x 80 m in the field. A single cover meter reading with the Domin-Krajina cover-abundance scale was recorded of a visually representative sample of the whole plot within each plot. Only trees above 2 m in height were taken into account. The 2 cm x 2 cm demarcated areas on the aerial photographs were traced on to film together with the tree (above 2 m tall) canopies. The traced canopies were cut out and weighed, the remaining film was weighed as also the total 2 cm x 2 cm film with canopies as a control. The results were expressed as percentage canopy cover.  ( .......) and 2 (ooooo) are approximately normally distributed but plot 3 (xxxx) is not a normal distribution.

Basal cover
The comparison of total grass basal cover with the wheel-point method and cover meter is given in Table 1. The values for the cover meter are the means of 30 samples. The cover meter samples are approximately normally distributed for Plots 1 and 2 according to Southwood's (1966) method of deter mining normal distribution- (Fig. 3). Spatial heter ogeneity of grass tufts accounts for the lack of a normal distribution of samples in Plot 3 (Fig. 3). The minimum samples required with 95% confidence limits for different limits of accuracy with the cover meter are given in Table 2 for each of the three plots.

Canopy cover
The comparison of tree canopy cover, for trees above 2 m in height, with the canopy cover determined from aerial photographs and the cover meter is given in Table 3. The accuracy of the cover m eter, as in the wheel-point apparatus, (Tidmarsh & Havenga, 1955) decreases for a given number of samples, with decreasing basal cover (Tables 1 & 2). For 10% basal cover, 26 samples are required with the cover meter for a 20% limit of accuracy (Table 2). The wheel-point apparatus requires 1 000 points for a similar limit of accuracy (Tidmarsh & Havenga, 1955). However, 26 cover-meter samples takes approximately a third of the time required for 1 000 points. A 20% limit of accuracy means that for 10% basal cover there is 95% certainty that the results will be in the range of 8 -1 2 % . The grass canopies in the three grassland plots were knee-high. This did not detract from estimating basal cover with the cover m eter, because the gap between basal tufts, being greater than the bases, was of greater importance in estimating basal cover than tuft basal diameter. Where the basal tuft size is greater than the spaces between tufts, tall grass could prevent use of the cover m eter. In practice, it was seldom possible to obtain perfect fits of basal diameter to gap and bar to space. The best fit was recorded with emphasis placed on the gap and space fit, where the ratio of basal diam eter to gap ratio was greater than one.
The discrepancies of the cover m eter in estimating canopy cover in classes for the 38% and 56% cover values obtained from aerial photographs (Table 3) may be attributed to incorrect selection of a representative sample. In phytosociological surveys, the problem of a single cover value representing an entire community does not arise, because cover is expressed as a raftge of cover values represented by each relevé in the community. The range may be summarized as mean cover degree (Mueller-Dombois & Ellenberg, 1974). As in the case of basal cover, the best fit between canopy to gap and bar to space is obtained when emphasis is placed on the gap and space fit, where the canopy to gap ratio is greater than one.
Although visually superimposing two different images requires a little practice, the cover meter is an extremely simple and portable device for rapidly estimating both basal and canopy cover. The accuracy of the cover meter depends on the user's requirements which may be a quick estimate of cover or a more time-consuming assessment. Unlike the single wheel-point apparatus, the cover meter can be used by a single observer and is also very inexpensive.

ACKNOW LEDGEMENTS
The authors thank Dr D. Edwards for his support, Dr J. C. Scheepers for comments and suggestions and Dr H. van   The association-analysis resulted in a classification of 23 final groups of sample plots, of which only five groups showed high floristic and ecological homogeneity. O f the remainder, eight groups contained some anomalous, misplaced plots, and ten represented small, isolated fragments o f natural units. The original data were then analysed using Braun-Blanquet methods to provide an independent classification for comparison with the former. The Braun-Blanquet communities were found to be more homogeneous in terms o f previously defined habitat groupings and showed floristic relationships consistent with these groupings.
it is concluded that, with the type of sampling used, the synthetic phytosociological Braun-Blanquet method provides a more natural classification o f plant communities of the Reserve than does the monothetic divisive association-analysis method.

INTRODUCTION
When Acocks (1975) first wrote his Veld types of South Africa in 1953, little was known of the ecology of the complex fynbos vegetation of the Capensis region (Taylor, 1978). Acocks was therefore unable to subdivide fynbos to the same extent as he did the vegetation of other parts of South Africa. Though there had been general descriptions of fynbos by Marloth (1908) and Adamson (1938) and a few quantitative studies like those of Wicht (1948) and Rycroft (1951), there was still scant information on the response of different types of fynbos to treatments like veldburning and grazing. To determine the effects of such treatm ents, experi mental research was initiated locally but these isolated projects could not be satisfactorily com pared with one another, o r extrapolated to other areas, in the absence of a synoptic account of fynbos. To fulfil this need, it was decided in the early 1960's to conduct a primary survey of Cape Mountain Fynbos (Acocks's Veld Types 69 and 70) and, for this purpose, a suitable method had to be found. The survey of such a large area -some 37 000 km2 in rugged mountain terrain -would have to be divided into components, and the method would have to be sufficiently formalized and uniform to allow valid comparison of data from each component. In seeking a method suitable for a m ajor survey of this kind, a trial on a smaller area of fynbos was required.
The Cape of Good Hope Nature Reserve (Fig. 1) was chosen as the site for the trial, because its flora * Based on an M. Sc was comparatively well known (Adamson & Salter, 1950), and its area large enough (77 km2) and its vegetation sufficiently diverse to provide a represen tative sample of fynbos. The work was begun by the author in 1966 and presented as a thesis to the University of Cape Town (Taylor, 1969).