In various systems of tapping, a distance of one foot between successive cuts has been very widely adopted. Here, again, prolonged and laborious experiments are required in order to ascertain what is the most profitable distance to adopt in the case of each system and under various conditions. The evidence contained in the following paragraphs, which bears partly on this question, requires to be greatly elaborated.
When a fairly old tree is first tapped by a herringbone or multiple V system, adopting the usual interval of a foot between the cuts, it is found that the latex flows in nearly equal quantity from each cut of die vertical series. As the bark between the several cuts becomes used up, a relatively larger proportion of latex is found to flow from the lowest cut; and some of the upper cuts may even cease altogether to yield latex, whilst there is still an inch or more of bark remaining untapped. It is probably best in all cases to leave the last inch or so of bark untapped, and to continue tapping the lowest cut only, if the time has not arrived for passing on to a fresh area of bark. When the same section of the tree comes to be tapped a second time, it may be marked out an inch higher than before, in order that the unused slips of bark may now be tapped first.
In an experiment at Henaratgoda 29 old trees were tapped daily on six wide V's, spaced at vertical intervals of a foot During the first month's tapping the three upper cuts yielded together a daily average of 558 c.c. of latex (nearly a pint), and the three lower cuts a daily average of 561 c.c. The quantities were therefore almost exactly equal, although the latex from the lower cuts contained a slightly higher proportion of rubber. During the six months' tapping, at the end of which the bark between the cuts had been almost entirely removed, the average daily yield from each of the six cuts was as follows:
Thus, when the bark between the cuts was nearly exhausted, the lowest cut was yielding as much latex as all the other five put together. In addition, the latex from the lowest cut was more concentrated and contained a higher percentage of rubber.
The trees described above were old, and possessed a good thickness of bark. In the case of younger trees we should probably expect the difference to be still greater. On the other hand, the rate of tapping was exceedingly rapid, the whole of the bark being removed from one side of the tree in six months. The evidence on the whole, however, seems to point to the conclusion that the distance between successive cuts may profitably be made greater than 12 inches. In other words, a reduction in the number of cuts on a given area of bark, as compared with the systems now generally adopted, would probably lead to more satisfactory results.
The above experiment shows further that in the case of mature trees there is little difference between the initial yields of latex from areas of bark situated at a height of 1—3 feet and 4—6 feet from the ground respectively. Generally speaking, there is a slight falling off in yield as we pass up the trunk from ground level, owing to the diminished thickness of bark and the smaller girth of the tree In the lower part of the trunk of mature trees this falling off is not very rapid, and the fact that tapping is often confined to the lowest six feet of the trunk is largely a matter of convenience.
It must be remembered that the lowest cut of all drains a larger area of bark than any of the other cuts, a fact which partly accounts for the very much larger yield obtained from it.
The effect of different intervals between successive tappings on old trees has been discussed at some length in Chapter IV. So long as the yield from a young plantation continues to increase steadily, we have not at present sufficient grounds for recommending any change from the interval of one or two days which is usually adopted; in fact there is evidence that in the case of young trees the yield per tapping may decrease as the interval between successive tappings is extended. In the case of older plantations of closely planted trees, if the yield shows a tendency to remain stationary or to fall off, we would suggest the trial of twice as long an interval without any other alteration of the system in use. We should not be greatly surprised if the planter were to find after some months that he is harvesting as much rubber as before, with an expenditure of only half the labour.