METHOD OF CONTROL HABITATS
Fig. 83. Simple culture of floating ecads of Ranunculus sceleratus.
361. Competition cultures. Although it is quite possible to carry on experiments in invasion and succession in the planthouse, the limited space usually available makes this undesirable, except in a few problems where control is necessary. Competition cultures, on the other hand, yield better results in the planthouse than in the field, since the physical factors and the appearance of unwelcome migrants are much more easily controlled. The possibilities of the culture method in the study of competition seem inexhaustible, and the author has found it necessary to confine his own investigations to a few of the fundamental problems. In this work, he has distinguished several kinds of cultures, based chiefly upon the species concerned and the arrangement of the individuals. Simple cultures are those in which a single species is used. The resulting group is a family, and the competition is between like individuals. In such cultures, the problem of the factors in competition is reduced to its simplest terms. Mixed cultures are based upon two or more species, and the problem is correspondingly complicated. As a rule, all the seeds have been sown at the same time in both simple and mixed cultures, but it has been found desirable to make some heterochronous cultures, in which seeds are also sown after the plants have appeared. Mixed cultures are distinguished as layered cultures, when the species are of very different height. Thus, rosettes have been grown with stemmed plants, tall slender forms with low branching ones, erect plants with twining and climbing plants, etc. Further evidence as to the nature of competition has been sought by means of ecad cultures, and factor cultures. In the former, plants of different response to water and light are grown together under the same conditions, in order to evaluate the part played by the nature of the plant. In a factor culture, the area is divided into two or more parts which are given different amounts of water or of light, in order to determine the influence of slight variations upon the same competitors. In somewhat similar fashion, an attempt has been made to ascertain the bearing of biotic factors upon competition. Cultures are easily made in which Cuscuta or parasitic fungi are used to place certain species at a disadvantage. Permanent cultures are obtained by allowing the plants to ripen and drop their seeds for several generations, just as in nature. They are indispensable for determining the final outcome of the competition between different species.
Fig. 84. Mixed culture of Solidago rigida and Onagra biennis.
362. Details of culture methods. All competition cultures have been made 1 meter square. In other words, they are quadrats, and they are treated exactly as denuded quadrats in the field with respect to factor readings, charts, and photographs. In the writer’s studies, germination tests were made of a large number of species, and those selected which showed a high per cent of germinability. Since this was the first experimental study of competition, this test was deemed necessary, but it is quite evident that no such selection is made in nature. Consequently, when the seeds used are known to be fresh, a germination test is usually superfluous. Considerable care was taken also to select species known to be vigorous growers, with the result that practically all the species used for experiment were ruderal or subruderal. The species employed, and the kinds of cultures in which they were grouped were as follows:
Fig. 85. Heterochronous culture of Helianthus annuus and Datura stramonium. Family culture of Datura, Verbascum, etc., in the foreground.
1. Simple culture of Helianthus annuus. The culture plot was divided into four equal parts; 12 seeds were planted in one, 25 in another, 50 in the third, and 100 in the fourth.
2. Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus canadensis. Twenty-five seeds each of Helianthus and Panicum were planted alternately at equal distances in one-half of the plot, while the other half was planted similarly with Helianthus and Elymus.
3. Mixed culture of Solidago rigida and Onagra biennis. Over one-half of the plot were scattered 50 seeds of Solidago and 100 of Onagra; over the other, 100 and 200 seeds respectively.
4. Layered culture of Laciniaria punctata, Bidens frondosa, Salvia pitcheri, Cassia chamaecrista and Kuhnia glutinosa. Fifty seeds of each species were scattered more or less uniformly over the entire plot.
5. Layered culture of Silphium laciniatum, Datura stramonium and Lactuca ludoviciana. Fifty seeds of Datura and Lactuca, and 25 of Silphium were sown uniformly in one-half of the plot. In the other half, 25 holes were made at equal intervals, and one seed of each of the three planted in each hole.
6. Ecad culture of Oenothera rhombipetala (xerophytic), Verbascum thapsus (mesophytic), and Penthorum sedoides (hydrophytic). One hundred seeds of Oenothera and 200 each of Verbascum and Penthorum were scattered over the plot.
7. Heterochronous culture of Helianthus annuus and Datura stramonium. One hundred seeds of Helianthus were scattered over one half, and the same number of Datura seeds over the other half of the plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25 seeds in a second circle at some distance. A month later, 100 seeds of Helianthus were sown in the Datura plot, and vice versa.
8. Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra, Datura, Penthorum, Solidago and Verbascum. The plot was divided into 9 squares and in each were sown 50 seeds of one of these plants.
9. Community culture. The sowing was made exactly as for the family culture, except that 20 seeds of each plant were used. In the middle of each square, 5 seeds of a different species were planted. For the Helianthus, Kuhnia, and Panicum groups, Onagra was used; for Bidens, Onagra, and Datura, Helianthus was used, and for Penthorum, Solidago, and Verbascum, Panicum.
At the time the cultures were started, check plants were sown in pots. The most vigorous seedlings were transplanted singly to large pots, and grown under conditions of water, light, and soil as similar as possible to those of the competition plots. Photographs of check plants and plots were made at the proper intervals, and the plots were charted in quadrats to show the course of competition. The factors which control competition were sought in a critical study of water-content and light values, which is still in process. This work has gone far enough to indicate the correctness of the view[[45]] that competition is purely physical in character. It has, moreover, been demonstrated that “room” in competition is merely a loose expression for the relation between the number of individuals in a given space, and the amount of water, light, and temperature available in the same space.