Research methods in ecology

by Frederic E. Clements

The permanent quadrat is regularly 1 meter square, a size determined both by the exigencies of charting and photographing. When ecograph batteries are used, the quadrat is located as close to the latter as is possible. Otherwise, the quadrat itself should constitute a station for making factor observations. This connection is absolutely essential, since the quadrat is used expressly to determine the structural changes, which are produced by physical factors, and the reaction of vegetation upon them. Permanent quadrats are established in different formations or stages of a succession to trace the invasion of new species and the dropping out of old ones in response to competition. They serve to distinguish the proper formation, which represents a particular stage of development, from the mixed formations which precede and follow, and also to determine the exact course as well as the rapidity of the change that follows each reaction. When applied to different examples of the same stage, and to all the different stages of a succession, the whole development of the latter may be minutely traced and definitely recorded. The importance of following the changes from aspect to aspect is much less, since these are periodical rather than dynamic. They are an essential feature of structure, however, and it has been the practice to make at least one series of aspect charts from each permanent quadrat.

For tracing the invasion and competition of lichens and mosses, which play a primary role in initial formations, a subquadrat is used. The size varies, but it is usually smaller than the quadrat, although the latter is entirely available in the case of the large foliose lichens. For the crustose and smaller foliose forms, a subquadrat 2 decimeters square is used, and for the larger forms and tufted mosses, one of 5 decimeters. In the case of ground forms, tapes are employed, and the quadrat is permanently staked. On rocks and cliffs, where moss and lichen stages are most common, tapes are impracticable, and the quadrat is permanently outlined with paint. Charts of lichen quadrats are made to the usual scale of 10 : 1.

212. Manner of use. Permanent quadrats are mapped and photographed in exactly the same way as chart quadrats. As soon as this has been done, a labeled stake is driven at the upper left-hand corner, so that its edge indicates the exact position of the quadrat stake, and a smaller one is placed at the opposite corner to facilitate the task of setting the tapes accurately in later readings. The label stake bears merely the number of the quadrat and the date when it was first established. It is firmly fixed and allowed to project just enough to enable it to be located readily. Its position requires careful landmarking when the quadrat is to be visited year by year. In forest formations, this is readily done by blazing, but in grassland it is necessary to have recourse to compass and pacing, or to erect an artificial landmark. After several charts have been made, a permanent quadrat attains a high value, and every precaution must be taken to prevent losing its exact location. At the second reading of a quadrat, whether in the succeeding aspect or year, the tapes are placed with reference to the stakes, and a chart and photograph are made in the usual manner. These are labeled and dated like the original ones, but they are numbered to indicate both the quadrat and the series, e. g., 15² indicates the second chart, and photograph made of quadrat 15. The date indicates whether the readings are by the aspect or the year, though this may be shown also in the name of the series itself. It is clearly an advantage to have the two successive charts of a quadrat upon the same sheet, and to file all the charts and photographs of the same permanent quadrat together, and in the proper order.

Since much of the value of a permanent quadrat depends upon its use as a station for observing physical factors, it is unprofitable to establish a large number. The results of invasion and competition can be ascertained by the quadrat alone, but these should be merely preliminary to seeking for their causes. Clearly, a quadrat should be established for each battery of instruments, while additional ones should be located only in so far as they can be visited often enough to give an insight into the factors that control them. In view of the fact that the most important factors, water-content and light, are less variable than humidity, temperature, and wind, it will suffice if visits are made once a week. This is especially true when it is possible to refer the more variable factors to the continuous records of a base station. While all the results determined for permanent quadrats are preserved in the field record, a record of them is also kept on the reverse of the chart sheet for convenience in interpreting the different charts.

The Denuded Quadrat

213. Description. This is primarily a permanent quadrat from which the plant covering has been removed, after it has been charted and photographed. What is practically the same thing is obtained by establishing a permanent quadrat in a new soil, or in one recently laid bare and not yet reclothed with plants. These, however, are merely permanent quadrats, in which the first chart and photograph furnish a record of the habitat alone. They are of great importance in succession, and will be more fully discussed under experimental vegetation. The denuded quadrat is of the usual size, 1 meter, though the smaller lichen quadrats are also denuded. The location is subject to the conditions already indicated, especially with reference to physical factors. The denuded quadrat, however, is particularly adapted to the study of invasion and the resulting competition. Consequently, when migration is markedly from one direction, a series of denuded quadrats throws a flood of light upon the actual steps in invasion. Denuding is a valuable aid in succession, but it must be clearly recognized that, while permanent quadrats register the exact course of the succession, denuded ones can merely furnish facts as to the probable courses of stages not now in evidence.

Fig. 54. Denuded quadrat; this is the quadrat shown in figure 53; photographed September 7, 1904.

214. Methods of denuding and recording. Permanent quadrats may be denuded at any time during the time they are under observation. The best results, however, are to be obtained by establishing the two side by side, or at least close together. In this way, they are mutually supplementary, and furnish the most evidence possible with regard to the procedure of invasion and competition. Another advantage is found in that the same observations of climatic factors will do for both, though water-content and soil temperatures are necessarily different. A quadrat which is to be denuded is first mapped, photographed, and labeled exactly like a permanent quadrat. The vegetation is then destroyed. This is usually done by removal, though it may also be burnt, destroyed by flooding, or in some other manner. The method will depend upon the use which the quadrat is to serve. If it is to throw light upon the vegetation of an area in which denudation has affected the surface alone, the aerial parts only are removed by paring the surface with a spade. When the disturbance is to be more profound, the upper seed-bearing layer is removed, and the underground parts dug up. In the interpretation of a secondary succession, the denuding cause is made use of in a fashion as nearly natural as possible. Ordinarily, the plants are removed just below the top of the ground by a spade, leaving the underground parts undisturbed. This method has yielded very interesting results.

Quadrats have been denuded in the fall after the majority of the plants have completed their growth. This is largely owing to the fact that other field work is less pressing at this time. Denudation can be done as well in the spring, though the invasion will be slower in this case, since the seeds which have accumulated will be partly or entirely removed. During the first season the denuded quadrat should be mapped every month, and, if the invasion be rapid, photographed also. In open formations, especially those of a xerophytic nature, a single chart and photograph made at the end of the season are sufficient. In a few cases of this sort, indeed, no invaders have appeared until the second year. Beginning with the second season, a single record taken near the close of the growing period will suffice. Denuded quadrats are labeled, dated, and filed exactly as other permanent quadrats, but it should be noted that the first member of the chart and photograph series is that which records the original vegetation of the area denuded.