The Preparation of Plantation Rubber :: CHAPTER III THINNING OF AREAS

CHAPTER III THINNING OF AREAS

On this subject there is unanimity regarding the necessity for the operation. Divergence of opinion exists only as to a matter of degree.On the one hand there is the school of planters who would advocate the advisability of planting up to, say, 200 trees per acre, with subsequent thinning out by selection. At the other extreme there is the opinion that we should plant only a few more trees per acre than it is intended eventually to maintain, the argument being that by this method the growth and development of individual trees will be so much greater than in close planting that the necessity for drastic thinning out will not arise.

Unfortunately for the latter school, a very important point is overlooked—viz., that size and general development are not criteria of yielding capacity. It might thus follow that a stand of ninety well-grown trees per acre might give very disappointing yields per acre. In a few instances this has been noted with 30 by 30 feet planting, but it is doubtful whether the factor influencing such results has been appreciated.

The apostles of close-planting have this in their favour: that if the trees to be removed are selected on proper lines, it is possible to have all remaining trees of comparatively high-yielding strain. This is a very sound argument, but its practicability is limited very largely by the question of early growth and development. It would seem the sane course in any event not to plant more trees per acre than may grow normally, and without branch or root interference up to the fifth year (the normal first year of tapping).Before this stage has been reached, stunted or deformed trees will have been noted and removed, so that in the first year of tapping thinning proper can be commenced. In the past this has been effected wholly by selection of trees according to their general appearance and situation; but it is now safe to predict that future operations will be based upon sounder and more scientific lines.Trees will be selected for removal according to their individual yields, a standard which we have been advocating for years without much practical success. In Java and Sumatra much good work has been done in this direction, and recently a commencement has been made in the F.M.S.

Typical Young Planted Area

Typical Young Planted Area.

Heavy original jungle timber.

It is within the daily observation of all planters that certain trees regularly give greater yields than others, and that such trees are not to be distinguished by size or general development. Moreover, with slight variations, it has been found that a good yielding tree is consistently a good yielder, and the converse holds true.If, therefore, measurements of individual yields are taken at intervals, and the results recorded during the first year of tapping of an area, an excellent guide is obtained for the first round of thinning. It is found in actual practice that five, or even three, readings during the year are sufficient to give the indication required. It is not essential that simultaneous readings should be taken over a large area; in fact, such a step is really impracticable at first. The simplest method is to employ either—

(a) A small uniform vessel in which the latex is measured by means of a thin slip of bamboo upon which graduations are marked.

(b) A glass measure graduated regularly.

Another example of a Recently Planted Area

Another example of a Recently Planted Area.

In both cases it is immaterial what units are represented by the graduations—whether cubic centimetres, quarter ounces, half-ounces, or ounces, as long as the unit is not too large. It is preferable to employ a fairly small unit, so that in taking readings from young trees a wider range may be obtained between poor yields and good yields. In the case of older trees a larger unit may be taken.

The first stage in the operations is to number all trees in the field to be tested, and to prepare a rough register, with three or five vacant columns opposite each tree number.

It is not advisable to commence the record of yields until the panel of bark has been under tapping for a month or two. It is found that an intelligent coolie can be taught the method of measuring and rough recording. The latter is accomplished by means of marks made upon the virgin bark of the tree above the tapped area. The marks may be made with a tapping knife, by means of paint, or with a lead pencil. The simplest form of record consists in putting one mark for each graduation of reading.

In practice it is found that, commencing about an hour after the first tree has been tapped (in the case of young trees) and following the course taken by the tapper, the measurer of yields is able to do about 2 full tasks (650 to 750 trees) per diem. Each day progress is made through the field.

Obviously on such a small scale and utilising only one measuring coolie the comparison is restricted very much; but in any case this is immaterial as, owing to the personal equation of the tapper, comparison strictly should be limited and internal—i.e., it should really be confined to one task only at a time. In this way the worst trees in any task are indicated.

The keeping of the records may be entrusted to a field clerk, but is better placed in the hands of a European. The register is taken into the field and the rough records found on the trees are noted in the columns against the tree number.

Most planters are aware in a general way of the disparity between the yields of individual trees, but they would probably be surprised if they undertook the institution of such records.

The following figures must not be taken as typical. They represent the average results from several tasks in a young field from which all ill-grown and deformed trees had been removed. It is immaterial what the units represent, as they are purely arbitrary and were selected for the purpose of obtaining a fairly wide range.

Any trees which failed to yield sufficient latex to reach the first mark were registered at zero. The following percentages were obtained:

Zero			3	per cent.
Above	mark	1	6	„
„	„	2	16	„
„	„	3	42	„
„	„	4	12	„
„	„	5	14	„
„	„	6	6	„
„	„	7	0	„
„	„	8	1	„
„	„	9	0	„
			100	„

It may be remarked that, judging by ordinary standards, it was impossible to discriminate between good yielders and others, and if thinning were to be done on the usual lines it is quite possible that some of the best yielding trees would be removed.

Taking the mark No. 5 as the datum line, it will be noted that 79 per cent. of the trees come below and 21 per cent. above. In the latter proportion the majority lie close to the datum line. It will be seen that there are outstanding yielders even amongst these young trees, and that it would be possible to mark about 10 per cent. of the stand per acre at once for removal in the first round of thinning.

In the case of old trees it is possible that one would encounter greater extremes of yields than those shown in the foregoing table, especially if a certain amount of thinning had been done previously on empirical lines. Sufficient has been written to show that the only reasonable basis for selection of trees in thinning is that of yields; and it is obvious that if the method be adopted the future yield per acre of any area is bound to be in excess of the same area as thinned on rule-of-thumb lines.Yields per Tree.—A great feature is made in estate reports of the figure showing the average yield per tree per annum. Assuming an area to be yielding at the average high rate of 540 lbs. per acre per annum, with an average stand of ninety trees per acre, the yield per tree per annum averaged over all trees is 6 lbs. Keeping in mind the test-figures on a previous page, it will be obvious that some of these trees may have given very much more than 6 lbs. during the year, and some less. In view of present information it would not be surprising to find that a few might have been yielding upwards of 15 lbs. per annum. Unfortunately this information is only to be obtained by individual tests, and under normal estate conditions the facts escape notice. Cases are known in which out-standing individual trees have been known to yield at the rate of 25 lbs. and more per annum.

Widely Planted Young Area, just ready to be brought
into Tapping.

Future Yields per Tree.—It has been shown that by selective methods based on yields, poor trees can be eliminated. Whether by a process of seed-selection or by means of propagation based on bud-grafting and marcotting, it needs no great stretch of imagination to forecast future conditions under which trees may be bred which will be capable eventually of giving an average yield of 25 lbs. per annum over any given area. Yields of 1,000 lbs. per acre per annum should be obtained easily.

Field of Old Rubber Trees in which Thinning had been delayed too long

Field of Old Rubber Trees in which Thinning had been
delayed too long.

Note height and comparative lack of girth.

Trees per Acre.—This brings us to the question as to how many trees one should leave to the acre after thinning operations. Figures have been given by various authorities, but it appears to the writer at the present time to be impossible to lay down a general rule. So much depends upon conditions. In certain cases where the soil is admittedly poor, the average growth below normal, and thinning has been postponed too long, the writer has been forced to the conclusion that it would be most inadvisable, and commercially unsound, to reduce the stand of trees below 120 per acre. In such instances the average yield per tree equalled only 3 lbs. per annum, and although the trees were upwards of nine or ten years old the crowns were small and sparse. It is doubtful whether such trees will ever exhibit any further development, and to thin them further would probably lead only to a diminution in the crop per acre.

Under normal conditions of growth an arbitrary figure of eighty trees per acre has been selected as a standard by many estates. In these cases it would probably be correct to state that thinning was undertaken on almost purely empirical lines—i.e., that trees were not selected by tests of individual yields. As far as such a method retained the apparently most vigorous trees it was successful; but in view of what has been written it might explain some of the disappointing results which have followed upon such a system of thinning.

It will be clear that any decision regarding the number of trees to be retained must be derived from a study of the detailed results of individual tests. If the large majority of the trees appear to be fairly uniform in yields the first thinning must be confined to comparatively few trees. Where there is, on the other hand, a good percentage of high-yielding trees the final stand per acre may be appreciably less. Unless and until such information is available, one cannot give any definite opinion as to the requisite number of trees to be retained per acre.

Similarly, intelligence must be displayed in deciding which of several uniformly-yielding trees should be removed. In the average sense of this consideration one must pay no attention to symmetry of spacing, but when dealing with trees of fairly uniform yields one needs to study the characteristic development of the trees individually, in order to retain those which would appear to be most favourably situated with regard to surrounding trees.

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