The Journal of Indian Botany/Volume 3/October 1922/The Oecology of some Plant Communities in the Savannah Formation
THE OECOLOGY OF SOME PLANT COMMUNITIES IN THE SAVANNAH FORMATION
BY
R, H. Dastur, M.Scj., E.L.S., and W. T. Saxton, M.A., E.L.S.
1. Introduction
During the last fifteen years great strides have been made in the study of Oecology. A large amount of work has been done, both in Europe and America, to increase our knowledge of plant formations and associations. In most cases, and especially where large areas have been studied including various different formations, we are of opinion that the analysis of the vegetation has not been as exact as it should be, while in other cases it has seldom been possible to decide the precise relation between the plant and the various differing factors of the habitat. For the tropics the recent papers by McLean (4) give some interesting analysis of physical factors but no detailed analysis of the vegetation is attempted.
It is of course abundantly clear that the different aspects of vege- tation depend ultimately on physical factors such as water content of the soil, humidity of the air, light and temperature, but it is not always clear which of these factors is dominant in particular cases.
So far as the authors are aware, not very much work has been done with a view to elucidating the causes,, often rather obscure, which lead to somewhat marked differences of vegetation, in an area where the physical factors at first sight appear uniform, and where the differ- ences are, in the main, rather floristic than definitely oecological ; that is to say where the whole area must be held to belong to the same formation and association, although the communities * are different.
Such cases are very frequent in the vicinity of Ahmedabadf and it appeared to the writers that a more or less detailed study of one typical case might throw light on the problem as a whole. An area was selected which had been under casual observation for about 6 years. During that time nothing had been done to disturb the natural development of the vegetation and no marked changes seamed
The term “ Community ” is used here to denote a unit next below that generally understood by an “Association.** It thus corresponds approximately to “ Variety of Association ** as defined by Warming (10) or to ^Society” in the sense understood by Clements (2).
t Ahmedabad is about 50 miles N.E. of the Gulf of Cambay. Lat. 23°.2' N. Long. 72*39' E. to be taking place in tbe distribution of tbe species. It has always, during these years, shown a fairly sharp distinction into four parts passing from North-west to South-east and approaching the bank of the Sabarmati River. In some cases the dividing line between the communities is very abrupt but between the third and fourth parts it is more gradual.
It was apparent from the small size of the whole area, which is about half a mile from North-west to South-east, and about 100 yards from North-east to South -west, that no perceptible differences exist in the climatic factors, humidity of the air, light and tempera- ture. The altitude was also practically constant except for a very slight depression in the second part. It seemed therefore that the area was admirably adapted to an investigation of the differences between these four parts and their causes.
A preliminary general account of the oecology of Northern Gujarat was published by Saxton and Sedgwick (7) in 1918 in which a description of the Savannah associations is given. In that paper, the area we have studied is included under the heading of “other types of Savannah on Sandy Soil " (loc. cit. p. 233) and its relation to other types is shown.
2. Investigation
A . General . — The area selected is a plot of waste ground about 3 miles South-west of Ahmedabad on the North-west bank of the Sabarmati River. Its approximate dimensions have already been mentioned. Eor the greater part of its length it is bounded both North-east and South-west by cultivated land, but to the South-east it merges in a stripe of waste land extending along the river bank.
The four parts of the area, mentioned above, will be distinguished as Areas (I), (II), (III) and (IV), passing from North-west (l) fc to South-east (IV).
Area I is largely dominated by Cassia auriculata Zb, Area II by Acacia arabica Willd . and Cassia auriculata Z/. in similar proportions, Areas III and IV by Saccharum Munja Roxb., its dominance being more marked in Area IV.
In order to arrive at any definite conclusion five lines of research seemed to be desirable (a) a detailed analysis of the vegetation of the areas ; (b) an analysis, chemical and physical, of the soils, including the water contents; (c) a study of the physiological anatomy of the plants concerned ; (d) an investigation of the root systems of some of the more dominant plants and (e) experimental determination of the wilting point of the important plants, and their rate of transpiration under controlled conditions. Of these five lines the last is still awaiting investigation. The present account refers to the first four, and they will now be taken up in the order named.
2-£. Analysis of Vegetation . — Owing to the great diversity in size of the plants on each of the four areas the ordinary method of quadrat analysis appeared to be impracticable and we did not consider it necessary to chart any permanent quadrats, since no obvious changes (as pointed out above) were taking place in these areas. A modified form of list quadrat was therefore employed which was designed to meet the special case of the diversity in size of the plants* These can be roughly graded into three sizes — (l) trees (almost absent except in area II) ; (2) shrubs and large erect herbs, which dominate the whole area ; and (3) a carpet of low-lying creeping plants. The latter, owing to their small size, can only conveniently be studied in a quadrat less than twelve feet square, while it is necessary to employ a square of one hundred times this area to obtain an accurate figure for the trees.
To meet the difficulty we decided to employ a quadrat twelve feet square as our unit area, and to reduce all figures to correspond to this unit. Eor the trees a quadrat of 120 feet square was utilized, and the figures obtained, were divided by 100. Eor the undergrowth of small creeping plants quadrats varying from one to two feet square were used, and the figures multiplied by 144 or 36 as the case might be.
Eor obtaining a figure approximately representing the volume of a species we utilize the formula recommended by Clements (2) 2 n H IT r 3 , where n represents the number of plants per unit area, 3 the height and r half the width. It is obvious that this formula is based upon the assumption that the plant has the form of a cylinder of height 3 and diameter 2 r, which is seldom or never the case, but on the other hand few plants depart widely from this form, and no better formula suggested itself.
As it seemed uncertain how far two quadrats in the same area could be expected to agree with one another, figures were compared in two or three cases, for the same plant in two or more quadrats located in the same area. In each case where this was done it was found that the figures obtained did not differ by more than 5 per cent. We regard 21 per cent, therefore as the probable error in the figures shown below : —
We have shown the actual figures for plants amounting to not less than 0*01 per cent, of the whole, and plants below this percentage are merely given in order, without actual figures.
The lists are as shown in Tables 1 to 4.
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Table I
Abba I
Percent.
Cassia auriculata Linn .
44*90
Sida cordifolia Linn.
Ziyzphus rotundifolia Lamlc .
12’50
Cassia Tora Linn.
Cenchrus biflorus Boxb.
10*96
Tribulus terrestris Linn.
Indigofera enneaphylla Linn .
8*85
Perofcis latifolia Ait .
Saccharum Munja Boxb . ...
4*04
Peristrophe bicalyculata Nees,
Indigofera cordifolia Heyne.
3 84
Euphorbia pilulifera Linn.
Evolvulus alsinoides Linn.
3*27
Cyperus niveus Betz.
Commelina nudiflora Linn,
3*27
Eragrostis ciliaris Link, var ;
Achyranthes aspera Linn .
2*54
brachystachya Boiss .
Eleusine aegyptiaca Desf. ...
1*53
Boerhaavia diffusa Linn.
Panicum ramosum Linn, ...
1*34
Justicia diflusa Willd.
Spermacoce hispida Linn
1*15
Cenchrus catharticus-IteZite.
Sida grewioides Quill ,
■96
Eragrostis tremula Hochst.
Pavonia zeylanica Cay.
- 38
Convolvulus pluricaulis Choisy
Cassia obtusa Boxb.
- 19
Leptadenia reticulata W. d; A .
Crotalaria rnedicaginea Lamlc
- 19
Table 2
Area
II
Per cent.
Per cent.
Cassia auriculata Linn. ...
48*71
Oldenlandia corymbosa Linn. *12
Acacia arabica Willd .
42*71
Heliotropium marifolium Betx, *08
Evolvulus alsinoides Linn .
1*8
Euphorbia pilulifera Linn. *08
Saccharum. Munja Boxb . ...
1*37
Eieusine aegyptiaca Desf. ... *08
Eragrostis tenella Room, <&
Euphorbia microphylla Heyne. *04
Scfoult . var; pluraosa Stapf.
1*2
Indigofera cordifolia Heyne. *03
Crotalaria rnedicaginea Lamk
1*15
Striga euphrasioides Benth. *02
Calotropis procera £. Br. ...
•87
Vandeliia Crustacea Benth. *02
Cenchrus biflorus Boxb.
- 68
Chloris virgata Sw. ... *02
Sida cordifolia Linn.
- 53
Bonnaya brachiata Link. & Otto .
Eragrostis ciliaris Link, var ;
Digitaria royleana Brain.
brachystachya Boiss.
- 26
Leptadenia reticulata W. S A.
Table 3
Area
in
Per cent.
Per cent.
Saccharum Munja Boxb. ...
91*16
Indigofera linifolia Bets. ... *087
Calotropis procera B . Br . ...
3*68
Spermacoce hispida Linn.
Cassia auriculata Linn .
3*67
Peristrophe bicalyculata Nees.
Sida cordifolia
3*49
Perotis latifolia Ait.
Alysicarpus vaginalis Be. ...
3*48
Eragrostistciliaris Link, var ;
Indigofera enneaphylla Linn.
- 262
brachystachya :Boiss.
Phaseolus trilobus Ait.
- 262
Boerhaavia : diffusa Linn .
Indigofera cordifolia Heyne .
£"*
oo
O
Cenchrus catharticus Delile .
Tephrosia tenuis Wall.
- 087
Eragrostis' tremula Hochst,
2554—2
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Table 4 Area IV
Per cent.
Saccharum Munja Roxb. ...
96-6
Eleusine aegypfciaca Desf.
Indigofera linifolia Betz. ...
1*36
Eragrastis ciliaris Link, var
Indigofera cordifolia Heyne .
- 68
brachystachya Boiss.
Phaseolus trilobus Ait.
- 34
Perotis latifolia Ait.
Orotalaria Burhia Ham.
- 34
Panicum ram o sum Linn .
Indigofera enneaphylla Linn .
- 34
Cyperus niveus Reis.
Calotropis procera R. Br . ...
- 34
Eragrostis tremula Hochst . Cenchrus catharticus Delile .
2.C. Soil Analysis . — ‘The soil throughout; the four areas is an almost pure sand, but in Area II there is a perceptible proportion of humus. Treatment of the soils with dilute acid indicated the presence of carbonates. No complete analysis was attempted, but magnesium carbonate seemed to be present in greater quantity than the calcium salt.
By treatment of weighed samples with measured volumes of standard acid, and titrating the remaining acid with standard alkali, carbonates were estimated as C0 2 . Again by calcining weighed sam- ples, allowing them to cool in an atmosphere free from C0 2j and weighing again, it was assumed that the loss of weight would represent 00 2 plus humus. By subtracting from these figures those obtained previously for C0 2 alone, the amount of humus was estimated. The figures were as follows : —
Table 5
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Humus plus CO s
CO 2
Humus (by difference)
Area
Per cent.
Per cent.
Per cent.
I ...
2*12
0*77
1*35
II ...
4*37
1*50
2*87
Ill ...
... 2*48
1*38
1*10
IV ...
3*38
2*56
0*82
The general aspect of the vegetation, and the prevalence of families usually regarded as halophytic, suggested that sodium chloride might be present to an appreciable extent, but repeated tests of filtered soil extracts with dilute silver nitrate, gave negative results in each case.
Erom these results it did not appear, with the possible exception of area II, that the differences in the vegetation could be caused by differences in the chemical nature of the soil. Attention was next directed to the water content of the soil. In all our earlier estimates samples were taken to a depth of 8 inches only. On May 13th samples were also taken from each area at a depth of 18 inches, but the results were identical in each case with those taken at 8 inches. Erom the study of root systems to be des- cribed below, it afterwards appeared that it might have been better to study the water content at much greater depths, and we hope to complete a study of this question at a later date. Nevertheless the study of water content to a depth of 8 inches only, yielded results of considerable interest, and we think that the figures obtained are likely to bear a close relation to the water content at lower levels, especially in view of the identical results obtained at a depth of 18 ' inches. The samples were usually taken in air-tight, screw-top aluminium jars 1 and the water content calculated, after drying at 100 C., as percent- ages of the wet soil.
On some occasions the soil temperatures were recorded at the same time, but no appreciable differences were found in any of the areas and these records were therefore discontinued. The records of water content are as follows : —
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As rain fell on 13-5-1920, after No. 6, the records were dis- continued.
An examination of these figures indicates (l) that the ratio of water content in the four areas remains similar at all seasons, the order being always II, I, III, IV, and (2) that in the hot weather the actual water content in each case appears to converge to a minimum, with the possible exception of area II. The next step was to en- deavour to connect the water content with the physical structure of the soil, and this proved a more difficult problem than was anticipated. In the first place the soils were separated into three grades of fineness by passing air-dry weighed samples of each through sieves of 30 and 60 meshes to the inch. As in each case the proportion of small stones, etc., too large to pass through the 30 mesh sieve was very small and appeared unlikely to have any possible effect; on the water content this portion was neglected.
The size of the holes in the sieves varies with the thickness of the wire, so camera lucida drawings of a few meshes were made in each case under the microscope and carefully measured by comparison with drawings of a standard stage micrometer. In this way the average diameter of the holes was accurately measured, and this was found to eliminate some discrepancies noticed between results obtained with different sets of sieves. In the following tables, there- fore, we show only the average diameter of the holes and neglect references to the number of meshes to the linear inch.
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As so large a proportion passed through the sieve of smaller mesh another set of three sieves was employed of 30, 60 and 90 meshes to the inch, with the following results
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It was obvious that the water content could not be explained in terms of these figures since the proportion of smallest particle in table 7 was in the order II, III, IV, I, and that of table 8 in the order II, III, I, IV, while the order of the water content was II, I III, IV. It seemed likely therefore that the water content must depend on the proportion of particles of a decidedly smaller size than those passing through a sieve of 90 meshes. No finer sieve being obtainable, and as it appeared doubtful whether it could be made sufficiently accurate in any ca%e, some other means had to be devised
to solve the problem. ■ fvV/ 
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Literature Cited
1* Gannon, W. A. — Hoof; Systems of Desert Plants. Carnegie Inst. Wash. No. 131, 1931.
% Clements, E. E. — Plant Physiology and Oecology. Henry Holt and Co., 1907.
3. Haas, P. and Hill, T. G. — An Introduction to the Chemistry of
Plant Products. Longmans, Green and Co., 1913, p. 219.
4. McLean, E. C. — Studies in the Oecology of Tropical Eain Eorest. Journal of Oecology, 7, pp. 5 — 54 and 121 — 172, 1919.
5. Markle, M. S. — Boot Systems of Desert Plants. Bot. Gaz. 64* pp. 174-205, 1917.
6. Sabnis, T, S. — The Physiological Anatomy of the Plants of the Indian Desert, This Journal, 1. p. 244, 1920.
7. Saxton, W. T. and Sedgwick, L. J. — Plants of Northern Gujarat. Eecords Bot. Survey Ind. 6, 1918.
8. Solereder, H. — Systematic Anatomy of Dicotyledons. (English translation by Boodle and Britsch). Vol. II. p. 64?. 1908.
9. Thomas, H. Hamshaw — Some observations on plants in the Libyan Desert. Journal of Oecology. 9* pp* 75 — 89. 1921.
10. Warming, J. E.-— Oecology of Plants. (English translation by Groom and Balfour). Clarendon Press, 1909.
11. Weaver, J. E. — Oecological relations of Boots. Carnegie Inst. Wash. No. 286, 1919.
This investigation has been carried out in the Botanical Laboratory of the Madhavlal Banchhodlal Science Institute, Ahmedabad.