Sociological and ecological studies on the tropical weed vegetation of Pasuruan (the island of Java)
Recueil des travaux botaniques néerlandais , Volume 24 - Issue 1/3 p. 1- 256
1. The plain of Pasuruan (the island of Java), bearing on my investigation, is a young volcanic formation of the Quaternary period. (Chapter I and II B, a.) As the region is geologically young and the rivers continually supply large quantities of silt, as an ingredient of the irrigation-water, all food-material for plants is generally present in a sufficient amount. (Chapter II B, b.) A natural, undisturbed flora is to be found anywhere in this district, which is exclusively agricultural (rice, sugar-cane, maize etc.). The qualities of the crop, and its influence on the soil and weed-vegetation were described, together with the methods of cultivating. (Chapter II C). The climate of the district may be typified as a monsoon-climate with a fairly keen separation between a nearly rainless period of 4—5 months and a wet period with a total of about 1300 mm. of rain in about 100 rainy days. The monthly average of temperature is high and very equal (25,4°—27,1° C.): the sunshinepercentage, with an average of 75%, is very high. (Chapter II A). 2. In the weed-vegetation on culture-fields of the district, a number of communities, with a definite floristic constitution and physiognomy, could be determined. The weed-communities on sufficiently irrigated secondcrop-fields, not older than four to six weeks, give indications, in the examined area, about the agricultural value of the soils, agreeing with the views obtained in the cultivation of sugar-cane. The Polanisia-viscosa-community typifies the good light E K 28-soils, in which this variety is not affected by root-rot, when treated normally. The Panicum reptans-Polanisia wscosa-community occurs on light soils, on which E K 28 does not give its best produce, but on which root-rot is rare. They are very good soils for D I 52, if there is an ample water-supply. The Panicum reptans- Polanisia viscosa- Merremia emarginata-community typifies light and fairly light, less good soils, which soon dry up. E K 28 often goes wrong here, owing to root-rot. D152 can yield a sufficient produce, provided the soil is kept sufficiently moist. The Panicum reptans- Polanisia viscosa- Portulaca oleracea-community typifies light and fairly light soils, which do not dry up quickly. The variant A of this community occurs on soils very suitable for D I 52. E K 28 gives very changeable results, S real failure from a cultural point of view is exceptional, however. The variant B typifies soils, unsuitable for E K 28 ; D I 52 and S W 3 often deteriorate quickly and show signs of dying-off at the setting-in of the eastmonsoon in the year of reaping ; the P. O. J.-varieties are best suited for these soils. The Paniacum reptans-Gymnopetalum leucostictumcommunity grows on medium-heavy to heavy soils. The variant A is typical of soils, which are most suitable for strongly-rooting cane-varieties. Where the variant B occurs, only the hardiest canevarieties will grow ; at the setting-in of the east monsoon the condition generally quickly deteriorates. The variant C typifies soils which are yet suited for S W 3, for the rest, on these soils too, the strong P. O. J.-varieties are preferable, because they are less subject to risk. The Polanisia Chelidonii-community occurs only on fairly heavy clay with a high level of ground-water. The presence of the variants A, B or C is dependent on the higher or lower level of the ground-water and the greater or smaller permeability of the clay: on soils with the variant B, S W 3 and D I 52 may be planted, for the rest the strong P. O. J.-varieties yield the best produce on this heavy clay. The Dentella repens-Lippia nodiflora-community grows on heavy soils which, also superficially, dry up but slowly ; they are only suited for strong P.O.J.-varieties. The Moschosma Polystachyum-Sphaeranthus africanus-community is met with on damp, medium-heavy and fairly heavy soils, which have not sufficiently dried up and are therefore poor in oxygen. The Lippia nodiflora-Phaseolus trilobus-community is typical of heavy soils of inferior quality, which are damp at first, but dry up quickly. Even the strongest P.O.J.-varieties yield but moderately here, for in the year of reaping drying-up of the cane soon begins. (Chapter V). 3. The weed-communities described, which are studied statistically at so early an age that mutual competition was excluded, are typified by a great number of “constants” and are, generally, sharply delimited. The constitution-diagram of these communities shows the typical curve of homogeneity (Nordhagen). It was concluded that germination of definite plants only takes place within a definite ecological amplitude. Some sowingexperiments in experimental squares and other discussed observations confirmed this view. With a gradual course of ecological conditions too, the transition-zones between these pioneer-communities are very narrow; a possible explanation of this fact was given. I emphasized the fact, that later on the habitat and the communities are often mutually unrelated, even though, the vegetation was determined ecologically; it was pointed out that the dissimilar conditions during the arising of the communities should be taken into account to explain this. The corresponding qualities of my “communities” and the “associations” were discussed (Chapter VI). 4. A few succession-communities were described. It appeared that the ecological amplitude of the successioncommunities is wider than that of the communities preceding them (Chapter VII). 5. A number of communities on fallow rice-fields, after reaping the rice, were studied. The poverty in oxygen of the substratum is the chief determining factor for these communities. It appeared that on light and medium-light soils the occurrence of definite communities does not depend only on the quality of the soil, but also on the duration and the intensity of the irrigation of the preceding rice-fields (Chapter IV). 6. On saline soils a number of communities sharply delimited occur, depending among other things on the salt-amount of the soil; on soils, containing calciumcarbonate-concretions, a few communities were found which pointed to calcium-carbonate. On germinating the halophytic communities are already determined as such; in other words, the common view that the exclusive occurrence of halophilous- plants depends only upon competitors, is not confirmed here. In special, bare experimental squares on saline soils, the germination was experienced of halophytic communities, in whose immediate neighbourhood non-saline vegetation was abundant. Nevertheless not a single plant, typically confined to a halophobous community, germinated in the open saline vegetation, to be superceded later on by victorious halophilous competitors, as it is usually believed: the reverse too, was investigated, and with the same result. When the flora grows older and competition sets in, the struggles are fought between the halophilous plants or between the nonsaline ones; an intermingling of these opponents, however, followed by a selective competition, leading to the conquest of halopholous or halophilous plants, dependent on soil conditions, was never observed by me. The condition of sugar-cane and maize is correlatively connected with the occurrence of definite salt-indicating communities. Besides with the chemical character of soils, the occurrence of halophilous as well as calciphilous plants is connected with a definite combination of external conditions. The obligatory calciphilous plants are weak competitors in this district, in consequence the calcareous character of a soil is no more to be recognized from the successions. 7. The sugar-cane appears to react on the differences of the soil within the amplitude of a community, as was spoken of under 2. The Polanisia viscosa-community was closer examined in this respect. The rate at which and the way in which, the Polanisia viscosa-community attains the first succession, in different experimental squares, under equal climatic conditions, appears to be a basis for a closer division of the soils within the ecological amplitude of this community, agreeing with the experiences of the cultivation of sugar-cane with regard to the agricultural value of these soils. 8. The rains in the west-monsoon, combined with the high temperature in the tropics, enables plants to grow very rapidly. For that reason, the west monsoon-squares gave an opportunity to study the way in which plants compete with one another. The chief results may be summarized in the following “competition-rules”, which were experimentally composed. a. The way in which plants mutually compete and the results of this struggle, in an area of a definite floristic composition, are determined, beforehand by the local ecological conditions. Accidentally floristic fluctuations, not determined ecologically, are unable to influence the course of the struggle essentially. h. Plants, having a quick and luxuriant growth of their superterranean parts in conditions favourable to them, are the keenest competitors. They cast their shade over weaker competitors or supersede the others in the struggle for space. Root-competition too occurs, more especially in the east monsoon. c. The vigorous competitors, which finally form the succession, appear to be at the same time droughtresistant plants; inversion of the rule is not allowed, however. d. The keenest competitors within the communities, preceding the successions, are always constants at the same time. Reversely, constancy in itself, is not a condition for a strong competitor, even if the plant is drought-resistant (Polanisia viscosa). e. As an effect of mutual competition some plants are entering upon a resting-period in the west monsoon; their leaves are shed and sometimes their superterranean parts die off totally. These plants are capable of regeneration in the height of the east monsoon; for some plants, however, a sufficient amount of light is an essential condition. (Chapter IX). 9. In the beginning of the east monsoon the vegetation in most west monsoon-squares appears to be chiefly constituted of drought-resistant plants and to have attained a relative equilibrium, only disturbed for a short time during the successive west monsoons (Chapters IX and X). 10. In experimental squares, laid out towards the end of west monsoon, the east monsoon-vegetation was studied, with the following results: a. Practically, the soil on non-irrigated area at a depth of more than 20 cm. below the surface, does not decrease in moisture-content, even not during the very prolonged, entirely rainless east monsoon of 1925. b. The top-layer of the soil, however, after the setting-in of the drought dries up so quickly, that after a short time not a single plant germinates any more; the vegetation maintaining itself in the east monsoon, almost exclusively originated during the west monsoon. c. There is generally no correlation between the minimal value of moisture-content for germinating and the drought-resistance. The strongly drought-resistant of the plants Tridax procumbens, however, still germinates, with a very low moisture-percentage of soil. d. In the east monsoon several plants shed their leaves and often their secondary shoots, in connection with the shooting forth of new, very small leaves; apparently, this is a way to diminish their transpiring surface. e. A number of plants have a resting-period in the east monsoon, displaying itself by the shedding or shrivelling of leaves and shoots and their reviving, a long time before the rainy season sets in again, without any obvious change in the external conditions. f. As the east monsoon continues, a number of plants reduce their deeper-lying root-system, which dries up and often dies off; they form at the same time more and more superficial secondary roots. Polanisia viscosa develops on the superficial secondary roots, which were formed later on, small hypertrophic tubercles, which seem to serve as water-storing organs. g. As a rule the moisture-content of soil does not fall below the wilting-point of plants, rooting deeper than 20—30 cm. below the surface. (Chapter XI). 11. Of four plants (Mesophytes) the wilting-point was determined in two soil-types, typified by the Polanisia viscosa-community and in another one, typified by the Panicum reptans-Polanisia viscosa-Merremia emarginata-community. a. Results of experiments obtained with potted plants of a length of 20 cm. on an average: 1. The wilting-points of the four plants in soil, typified by the Panicum reptans-Polanisia viscosa-Merremia emarginata-community, were about 4 % higher than the wilting-points in the two other soils, typified by the Polanisia viscosa-community. On the mutual difference of these latter two soil-types, the four plants reacted differently. The greatest mutual difference of wilting-point of the four plants in one single soil-type. was about 20 %; and of the wilting-points of one single plant in the three soil-types about 50%. 2. It appeared that, at the time of permanent wilting, the cells of leaves and shoots are not plasmolysed. If no water is supplied again, plasmolysis is observed; this should be considered, however, as a dying- off-phenomenon. Permanent wilting weakens the drought-resistance of a plant. This appears from the fact that a plant, watered again after permanent wilting, under similar external conditions, wilts quicker now than a plant, which had not permanently wilted before. A plant, which had permanently wilted two or more times successively, can finally recover its turgidity only partly or not at all, even if itVs watered abundantly several times. 3. Potted plants do not generally admit of conclusions concerning the degree of their drought-resistance under natural conditions. (See sub. 11 b, 2). b. Results of experiments obtained with more or less full-grown plants in bottomless boxes of one meter by one meter by 0,80 meter, totally dug in the soil. 1. The plants displayed in a more or less degree similar phenomena of reducing their transpiring surface and their deeper-lying roots, as observed under natural conditions (see sub 10 f.) and most pronounced with Polanisia viscosa and Vernonia chinensis. 2. The individuals of one single species, under similar external conditions, wilt at a totally different moisturecontent of soil, dependent upon their degree of adaptation to the drought and induced by it. Specimens of Polanisia viscosa and Vernonia chinensis, which obtained the typical east monsoon-physiognomy, showed a wilting-point. more than 50 % lower than specimens wilting with their west monsoon-aspect. A few specimens of Polanisia viscosa renewed twice their foliage in a period of three months. Whether different individuals of one single species are fitted to assume the typical east monsoon-aspect, seems chiefly determined by mutual, internal, structural differences. The great significance of the diminishing of the transpiring surface lies in the fact that it is connected with a decrease of the wilting-point. 3. There is, generally, no correlation between the size of the plant, its absortion-sphere (the depth of soil-layer with roots still functioning), the length of the deepest root and the wilting-point. It appears, however, that plants with the typical east monsoon-aspect and which are the last to wilt (sub 11 b 2) proportionately root most superficially; nevertheless they are the most droughtresistant individuals. The proportion of the length of the plant to the absorption-sphere of the roots, changes with Vernonia chinensis from 1,2 on an average, for west monsoon-specimens, to 4.5 on an average for plants with typical east monsoon-aspect; for Polanisia viscosa these values are 1,3 and 4,7. From this it appeared that the decrease of the value of the wilting-point is far more significant in the struggle against drought, than the damage by the reduction of deeper descending roots. 4. The differences in the qualities of soil within the amplitude of a community can better be typified by analysing the course of competition (sub 7) than by determining witing-points. (Chapter XII).
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W.J.C. Kooper. (1927). Sociological and ecological studies on the tropical weed vegetation of Pasuruan (the island of Java). Recueil des travaux botaniques néerlandais, 24(1/3), 1–256.
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