Mohamed Idrissa Cissé
Ecologist, ILCA, Bamako – Mali
Most of the studies devoted to browse in the Sahel limit themselves, as regards characterizing the feed contribution of trees, bushes and shrubs, to drawing up a list of flora. Production is a factor which is generally little understood, although the by-no-means inconsiderable role of browse in livestock feeds is recognized. During the dry season browse is very important in terms of its protein supply, which is often the factor enabling livestock to survive. After proximity to water points, browse can also be considered as a factor enhancing the feed value of grazing.
It is therefore imperative to arrive at a better appreciation of the feed product of ligneous plants, but in 1975 Peyre de Fabregues highlighted the problems posed by an evaluation of this kind.
As the direct measurement of biomass is usually a long and tedious task, research into relationships between the foliage and various physical parameters which are easy to measure appeared to be an interesting approach. Research focussed on the following species: Acacia albida Del, A. Seyal Del. Balanits aegptiaca (L.), Commiphora africana (A. Reeh.) Engl.; Pterocarpus lucens, Lepr. ex Guill. and Perr. and Ziziphus mauritiana Lam., which were selected on account of their known forage value and their abundance in ligneous populations in the ILCA/Mali project area.
Annual foliage production (and fruit, if any) was determined by weighing the foliage harvested after felling, followed by drying for five individuals selected according to stem circumference.
After felling, the height of the tree and the circumference of its trunk 40 cm above the ground were measured. The crown of the tree was taken as a circle with an average diameter worked out from the measurements of the two most extreme dimensions of the crown. Measurements were taken on 50 samples of A. albida, 44 stocks of P. lucens, 40 of Z. mauritania and 46 of A. seyal, 50 of C. africana and 48 of B. aegyptiaca.
The data collected will be used to establish the relationship between the biomass and each of the physical parameters measured. The relationship between the biomass and the area of the crown was worked out only for the first four species, and in Acacia seyal only 30 trees, the circumferences of which varied between 5 and 61 cm, were involved.
The foliage biomass was correlated in turn with the circumference of the trunk, the height of the tree and the area of the crown. Before adjusting the regression curves to the scatter of points observed, the variations were subjected to first a single and then a twofold logarithmic transformation. It was the latter which gave the best linear regression showing the allometric relationships between the biomass and each of the parameters. The principal results are given in Tables 1, 2 and 3.
Table 1. Relationship between biomass (p) and circumference (c) of trunk.
Species |
Linear regression equation for P (in g) and C (in cm) |
Correlation coefficient (r) |
Adjustment curve equation |
Acacia albida |
log P=2.08 log C + 0.00 |
0.98 |
P=C2 08 |
Acacia seyal |
log P=2.25 log C–0.22 |
0.98 |
P=0.60C2..25 |
Pterocarpus lucens |
log P=2.07 log C– 0.03 |
0.96 |
P=93.C2..07 |
Ziziphus mauritiana |
log P=1.91 log C+0.14 |
0.98 |
P=1.38.C1.91 |
Commiphora africana |
log P=1.78 log C+0.18 |
0.98 |
P=1.51.C 1.78 |
Balanites aegptiaca |
log P=1.50 log C+0.81 |
0.86 |
P=6.46. C 1.50 |
Table 2. Relationship between biomass (b) and height (h)
Species |
Linear regression equation for B (in g) and H (in cm) |
Correlation coefficient (r) |
Adjustment curve equation |
Acacia albida |
Log P=2.77 log H–4.01 |
0.95 |
P=98.10–6 H 2.77 |
Acacia seyal |
Log P=3.06 log H–4.70 |
0.96 |
P=20.10–6 H 3.06 |
Pterocarpus lucens |
Log P=2.83 log H–4.19 |
0.96 |
P=65.10–6H 2.83 |
Ziziphus mauritiana |
Log P=3.21 log H–5.51 |
0.90 |
P= 3.10–6H 2.83 |
Commiphora africana |
Log P=2.63 log H–3.81 |
0.95 |
P=155.10–6 H3 21 |
Balanites aegyptiaca |
Log P=2.97 log H– 4.42 |
0.95 |
P=38.10–6H 2.97 |
Table 3. Relationship between biomass (b) and crown area (a)
Species |
Linear regression equation for B (in g) and A (in dm2) |
Correlation coefficient (r) |
Adjustment curve equation |
Acacia albida |
log P=1.26 log S–0.50 |
0.96 |
P=0.32 S 1.26 |
Acacia seyal |
log P=1.22 log S–0.68 |
0.92 |
P=0.21 S l..22 |
Pterocarpus lucens |
log P=1.81 log S–0.22 |
0.97 |
P=0.60 S 1.22 |
Ziziphus mauritiana |
log P=1.10 log S–0.24 |
0.95 |
P=0.58 S 1.10 |
For all species we recorded a strong positive correlation (r) which was significant at 1% between the logarithm of foliage production and that of the different physical parameters. On the basis of r the relationship between biomass and trunk circumference is better than with other parameters, except in Balanites where a better correlation is obtained with height. By way of example, figures 1(a, b and c) illustrate the relationship between the biomass and each of the parameters in Acacia albida.
Figure 1. Curves showing the relationship between the foliage biomass (P) and the circumference of the trunk (C), the height (H) and the area of the crown (A) in Acacia albida
Figure 2. Scatter diagram and adjustment curves for the leaf production (P) of Pterocarpus lucens as a function of trunk circumference (C) (2a) and height (H) (2b).
The establishment of a relationship between the foliage biomass and the physical parameters of ligneous plants was the subject of publications by Pressland (1975) and Poupon (1976), who investigated, amongst other things, the relationship of foliage biomass of trunk circumference in Acacia aneura F. Muell (Australia) and Acacia Senegal in the Sahel. The allometric relationship discovered between B and C in the species studied here tallies with the one found in their studies.
However, reservations should be expressed as to the applicability of these relationships to greater circumference and height classes. Growth, in terms of both thickness and height, is not indefinite, just as leaf production has a maximum limit. The curves should show a plateau, which however has not become evident (see Figures 1a and 1b). This leads us to the conclusion that we have not worked on the circumference classes at which leaf production reaches its highest level. As a matter of fact we frequently rejected the larger trees, either because they had been damaged or else because they had deformed trunks.
The sampling methods also give rise to reservations. The process by which the trees were selected avoided abnormal cases and shapes. This would explain the high correlations obtained. It is possible that if the trees had been randomly selected the correlations would be less good and the search for a multiple correlation between foliage production and the various physical parameters would have been justified.
To test the validity of the relationship between B and C we compare the estimated production, calculated from the established formulae, and the average production measured per circumference class. The results, shown in Table 4, reveal that estimated production is to a great extent always within the limits of the interval of confidence (95%) of the average production actually measured. From these results we are able to gauge the field of application of the relationships established for each of the species studied. The field is not a particularly limited one except in Balanites, where we were only able to arrive at a relatively poor correlation coefficient between log B and log C. For this species it would be desirable to calculate the biomass as a function of height.
Table 4. Comparison of leaf production measured with leaf production calculated from the ratio of p (m/kg of dm) to c (in cm) for some trees of the Sahel
Species |
Circumference class in cm |
Range of aplication of rate of P to C | ||||||||||
1–10 |
11–20 |
21–30 |
31–40 |
41–50 |
51–60 |
61–70 |
71–80 |
81–90 |
91–100 |
|||
Acacia |
(5.5) |
(14.0) |
(25.0) |
(36.3) |
(44.2) |
(55.5) |
(65.2) |
(74.0) |
(83.6) |
(93.4) |
||
P. measureda |
0.4 ± |
0.31± |
099± |
1.80± |
3.11± |
4.2 ± |
6.45± |
9.02± |
10.18± |
14.01± |
3 to 97 cm | |
P. Calculated |
0.04 |
0.24 |
0.81 |
1.76 |
2.65 |
4.25 |
5.94 |
7.73 |
9.96 |
12.54 |
||
(7.2) |
(15.1) |
(24.8) |
(35.5) |
(44.6) |
(54.8) |
(67.2) |
(75.1) |
(83.6) |
||||
Acacia seyal 2.25 P = 0.60C. 10–3 |
||||||||||||
p. measureda |
0.05 |
0.34± |
1.21± |
2.33± |
3.61± |
3.41± |
7.24± |
10.17± |
13.44± |
12,5 to 8 cm | ||
P. calculated |
0.05 |
0.27 |
0.82 |
1.85 |
3.08 |
4.90 |
7.76 |
9.96 |
12.68 |
|||
(8.3) |
(14.0) |
(23.8) |
(33.1) |
(43.0) |
(55.0) |
(67.1) |
(78) |
(84.4) | ||||
Pterocarpus lucens 2.25 P = 0.93. 10–3 |
||||||||||||
P. measureda |
0.05± |
0.26± |
1.08± |
2.11± |
3.98± |
3.23± |
5.05± |
6.30± |
6.25± |
7.5 to 79 cm | ||
P. Calculated |
0.07 |
0.22 |
0.66 |
1.30 |
2.24 |
3.72 |
5.62 |
7.23 |
9.04 |
|||
Ziziphus mauritiana 1.91 P = 1.38C. 10–3 |
(6.0) |
(14.4) |
(26.0) |
(35.4) |
(44.4) |
(55.2) |
(65.8) |
(74.6) | ||||
P. measureda |
0.05± |
0.22± |
0.90± |
1.36± |
1.42± |
3.69± |
4.70± |
4.32± |
4 to 99 cm | |||
P. calculated |
0.04 |
0.23 |
0.70 |
1.25 |
1.93 |
2.93 |
4.10 |
5.21 |
||||
(6.7) |
(15.0) |
(24.9) |
(35.0) |
(45.2) |
(55.3) |
(65.2) |
(76.1) |
(85.6) |
(95.0) | |||
Commiphora africana 1.50 P =1.51C. 10–3 |
||||||||||||
p. measureda |
0.06± |
0.16± |
0.43± |
1.08± |
1.82± |
2.00± |
2.62± |
3.18± |
4.23± |
3.92 |
4 to 99 cm | |
P. calculated |
0.04 |
0.19 |
0.46 |
0.85 |
1.33 |
1.91 |
2.56 |
3.37 |
4.16 |
5.00 | ||
(6.4) |
(15.7) |
(25.2) |
(35.6) |
(45.0) |
(55.6) |
(66.0) |
(74.9) |
(86.4) |
(95.0) |
|||
Balanites aegyptiaca 1.50 P = 6.46C. 10–3 |
||||||||||||
p. measureda |
0.07± |
0.35 |
0.96± |
1.82± |
2.96± |
3.58± |
6.08± |
6.66± |
6.44± |
9.44± |
21 to 60 cm | |
P. calculated |
0.10 |
0.40 |
0.82 |
1.37 |
1.95 |
2.68 |
3.46 |
4.19 |
5.19 |
5.98 |
||
a Figures in brackets indicate the average circumference of the individuals felled in the circumference class under consideration.
Figure 3. Scatter diagram and adjustment curves, for the leaf production (P) of Ziziphus mauritiana as a function of trunk circumference (C) (3a) and height (H) (3b).
Figure 4. Scatter diagram and adjustment curves for the leaf production (P) of Acacia seyal as a function of trunk circumference (C) (4a) and height (H) (4b).
Moreover, the calculations presented do not take into account interannual variations in foliage production as linked with the climatic hazards indicated by Bille (1977) and Poupon (1976). As regards seasonal variation, supplementary measurements are in progress which will enable the results to be improved by showing the forage supply available for each season.
This study shows the existence of allometric relationships between the maximum foliage biomass of various browse species and some of their physical parameters (circumference of the trunk measured 40 cm above ground, height and area of the crown). These relationships enable the annual browse potential for an area to be evaluated from an inventory showing the structural aspects of each species (height, numbers in each circumference class, cover, etc.) in a ligneous formation.
Of the three parameters, trunk circumference is the one showing the best correlation with biomass, except in the case of Balanites. Furthermore, circumference measurements are easier to carry out in the field. However, the determination of biomass as a function of crown area might possibly be carried out from aerial photographs, although it would be necessary to be in a position to recognize the different species, a problem of a technical nature concerned with photo-interpretation.
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Pressland, A.J. (1975). 'Productivity and Management of Mulga in South-Western Queensland in relation to tree structure and density'. Aust. J. Bot. 23: 965–76.
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