Research methods in ecology

by Frederic E. Clements

TRANSECTS

217. The transect is essentially a cross section through the vegetation of a station, a formation, or a series of formations. It is designed primarily to show the order of arrangement of species in zones and societies, but it also serves as a record of the heterogeneity of any area. In the form of the layer transect, it furnishes a graphic method of representing the spatial relations of the species in layered formations, e. g., forests, ponds, and lakes. It is merely a logical extension of the idea underlying the quadrat, and the transect is, indeed, little more than an elongated quadrat. An important difference, however, lies in the fact that the former normally traverses areas more or less unlike, while the latter is always located in a homogeneous one. Furthermore, the transect is plotted with especial reference to the topography. With respect to dimension, transects are classified as line, layer, and belt transects, and the latter may also be permanent or denuded.

The Line Transect

218. Description and method. A simple transect is sometimes made by establishing the points between which it is to be run, and then recording the plants pace by pace along this line. This is satisfactory where the striking changes in structure are desired. A more accurate method is ordinarily used, since it gives detailed results, and at the same time brings out the more general features. For this, use is made of a tape of proper length which is divided into decimeters. Tapes of 10, 50, and 100 meters are used, and if they are furnished with eyelets, transects of intermediate lengths may be run with them. When longer transects are desired, as in the case of forest formations, tapes of 500 or 1,000 meters should be used with eyelets a meter apart. The transect is located in the area to be studied by running the tape from one landmark to another, fastening it here and there by means of quadrat stakes. Previous to this, the shortest distance between landmarks is ascertained when the transect runs through a depression or upon a level surface. In the case of an elevation, the height is ascertained by a barometer, the length and angle of the two slopes obtained, and the length of the base line determined from these data. The field record of the arrangement of the plants is made entirely without reference to the surface line. The vertical lines on the centimeter sheet are taken to correspond with the tape, and the individual which touches the latter on either side is recorded to the right or left respectively and within the proper square. The species are indicated as for quadrats. A single row on either side may be taken alone, but the double series serves as a desirable check. After the record is made, the topography of the transect is drawn carefully to scale. This drawing is made upon the scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for those that are longer. The combination of this drawing with the line series of plants can not be made advantageously in the field. For the shorter transects, meter sizes of centimeter plotting paper can often be used to advantage. In this event, the topographic line is drawn to the scale of 10 : 1 and the series of plants transferred directly to it. In the case of transects between 10 and 100 meters, the scale of the drawing is increased from 1000 : 1 to 100 : 1, so that each decimeter of the original series is compressed into a centimeter. For the longest transects, corresponding reductions must be made, but in these it will be remembered that the series is plotted by meter instead of decimeter.

Fig. 55. Line transect running east and west in the Picea-Pinus-hylium, showing the relation of the herbaceous layer to the Carex-Catha-helium, invading along the brook; ecotones at e.

219. The location and size of line transects are determined by the purpose for which they are designed. Short transects are valuable for detail, but they can be used to advantage only where changes in arrangement are taking place rapidly. They are especially adapted to the study of minute alternations and to the zonation of small ponds, streams, ditches, roads, blowouts, etc. Longer transects can not furnish the same detail, on account of the amount of time necessary, but they are invaluable for the zonation and alternation of larger areas, such as the consocies, formation, and formation series. They are of particular importance for the record of zonation, since they afford a clue to the topographic symmetry of the area. The location of a transect depends upon the area to be studied, though it should always run through a portion as typical as possible. The general direction is ascertained by means of the compass, and when there is a measurable difference in elevation it is taken by the barometer or otherwise.

The points at which ecotones cross the transect are carefully indicated upon the chart. They serve as stations for simultaneous readings of physical factors, though in the majority of cases water-content readings alone will determine the reason for the ecotone. Photographs of line transects should be made while the tape is in position, in order that the superficies of the series may be as evident as possible.

The Belt Transect

220. Details. This differs from the line transect in that it is wider, and consequently affords a more accurate record of the arrangement of plants. While both give the actual facts of distribution, the line transect necessarily ignores the minor lateral deviations in position. These are brought out in a strip of some width, and the belt transect thus gives a more correct view of the variations which result from competition in an area physically homogeneous. The width of such transects depends upon the length, and the character of the vegetation. The standard width is one decimeter in herbaceous formations, and one meter in the long transects which are used in woodlands. In open vegetation, especially in the initial stages of successions, the width may often be increased to advantage, but ordinarily the amount of work necessary to run a belt transect of some length limits the width to one decimeter.

The location of a belt transect, the choice of landmarks, the determination of direction and elevation are made exactly as for the line transect. The topographic map is made in precisely the same way also, the scale used depending upon the length. Two tapes, however, are employed, and these are placed so that they mark off a strip just one decimeter wide. Every few meters, or oftener if need be, they are checked by a decimeter rule, and fixed firmly in place by quadrat stakes. The arrangement of the plants is recorded as for the line transect, except that the record covers a decimeter strip just as in quadrat work. Accordingly, an interval of a centimeter is left on the sheet between the successive portions of the strip, in order that the latter may be put together without confusion when the topographic map and the plant series are combined. The record should invariably start in the upper left-hand corner and read down. The map and the centimeter strip recording the plants of the transect are combined on a common scale as already indicated for the line transect.

The ecotones of zones are shown on belt transects by single cross lines, and those of consocies by parallel cross lines. In taking photographs of the transect, it is desirable to use guidons to mark these points clearly. The same device may also be used to indicate the course of the transect, when the tapes are completely hidden by the plants. Physical factor readings should always be taken, and, as before, they are best made at the intersections of the ecotones.

The Permanent Transect

221. Advantages. Both line and belt transects, after they have been recorded, should be rendered permanent, in order that they may serve to indicate the changes of a heterogeneous area from year to year in the same detailed fashion that the permanent quadrat does for homogeneous ones. For historical as well as for physical reasons, the ecotones of zones and of consocies are subject to change from year to year, and the amount and direction of this change can only be ascertained from annual records made in exactly the same spot. By means of the permanent transect alone the very origin of such areas can be followed from one stage to another of the succession. Moreover, the transect is equally valuable with the quadrat in making it possible to follow every step of the minute changes wrought by competition.

222. Details. The transect is made permanent by blazing the landmarks at either end, if these already exist, or by erecting them when it is necessary. A label stake is driven at each end, on which is painted the number and date of the transect and its length. Each stake should also indicate the exact direction in which the other lies. The position of the ecotone is indicated by smaller stakes bearing the number of the transect and the date when the ecotone was found at that point. These are left in place, and in a few years show very graphically the change in position of the zones. For the first season, permanent transects afford results of great value when recorded for each aspect, but after this an annual visit will suffice. The details of mapping, plotting, etc., are identical with those indicated above, with the addition that all charts and photographs must bear the number of the reading as well as that of the transect. Physical factor observations are taken as often as the charts are made, and the results noted on the back of the chart sheet for purposes of ready comparison.

The Denuded Transect

223. The denuded transect bears exactly the same relation to a permanent one as that which exists between the denuded and the permanent quadrat. While the permanent transect records the actual mutations due to changing physical factors or to competition, the denuded transect throws needed light upon the mobility and ecesis of the various species, and upon the nature of the competition between them. Denuded transects may be established wherever it seems desirable, after the strip has been properly charted and photographed. The most valuable results, however, are secured by locating each one alongside of a permanent one. The best plan is to locate and chart two permanent transects a meter apart. A single view is then made of the two. One of them is denuded together with a strip 2 decimeters on either side, resulting in a denuded transect 5 decimeters wide. In charting this during succeeding years, the entire width may well be plotted as long as the vegetation is open, but after it has again become well established, it is necessary to save time by confining one’s attention to the central decimeter strip. Photographs can be made either of the permanent and denuded transects singly, or of the two together. The latter method has certain obvious advantages. Climatic factor readings can be made for both transects in common, but all those factors which are affected by the exposure of the soil surface must be observed in each.

The Layer Transect

224. This is a modification of the line transect, by means of which the vertical relations of plants are also shown, especially the tendency to form layers which is so regular a feature of forest formations. Owing to the difficulty of charting in three planes, belt transects do not lend themselves to this purpose. Because of the greater complexity, layer transects can rarely exceed ten meters in length except in those formations where layering is little or not at all developed. The simplest method is to establish a line transect in the ordinary way, and then to record the height of each plant as its position is noted. This is done by means of a measuring stick ruled in decimeters, which can be moved from interval to interval along the tape, or better, by two such sticks connected by tapes a meter long at every five decimeters of the sticks. These should be two meters high for woodland, and one meter for grassland. Layer transects often run on even surfaces, but if this is not the case, the usual data for a topographic map should be taken. The final chart is constructed on the scale of 10:1, the height of each plant being indicated by a vertical line equal to .1 of the observed height. A photograph of a representative meter of the transect is taken when the measuring sticks and rods indicated above are in position. Physical factor readings, principally of light, but often also of humidity, temperature, and wind are made at the height of the various layers when these are present.