It is well known that rubber trees possess a marked individuality as regards the amount of latex which can be drawn from them. Tapping coolies, if left to themselves, soon discover these differences, and confine their attention to the best-yielding trees. The differences are often great, and to a large extent independent of the girth of the trees. Among a group of 29 trees of uniform age tapped daily, the highest and lowest average yields for the first 30 tappings were respectively 166 and 8 cubic centimetres. The circumferences of these two trees were 52 and 32 inches respectively, and they were not the largest and smallest trees of the group. The yield per inch of bark removed was in the ratio of 317 to 25, or more than 12 to 1. Such differences afford good grounds for anticipating that markedly increased yields could be got from plantations derived from the seed of trees selected for their high yielding capacity. As we shall point out at greater length in Chapter V, the selection of seed-bearers may be expected to play an important part in the future development of the rubber-planting industry.
The increased yield associated with wound response is clearly manifested when the interval between successive tappings is anything between one and ten days1. The longest interval over which this effect can persist is not known, and would doubtless be found to vary in the case of different trees. It was observed, however, in the case of a very large tree which was tapped daily, that, when daily tapping was renewed after an interval of a month during which tapping had been suspended, the first day's yield was much smaller than the average, and was followed by a rapid increase on the days immediately following. A longer interval than a week between successive tappings is not likely to enter into the calculations of the practical planter. Which is the most profitable among intervals shorter than seven days, ije. which will lead to the largest permanent yield, is a question that requires to be made the subject of further discussion and experiment.
The view generally accepted at the present time is that, with a view to obtaining the largest possible yield without permanent damage to the tree, tapping should proceed at such a rate that the paring of the whole area of bark accessible from the ground occupies not less than four years. Under these circumstances it will not be necessary to touch the oldest renewed bark until the beginning of the fifth year. The system of tapping is generally so arranged that this object is achieved by tapping either daily or on alternate days. There is evidence however that, in the case of old trees closely planted and close planting is the rule on the older plantations a better result can be obtained by increasing the interval between successive tappings.
1 Except perhaps in the case of very young trees.
In this connection reference may be made to Table XII, which shows the annual yields from 70 old and closely planted trees, divided into groups of 10, which were tapped at different intervals. The rate of tapping of the different groups was Such that the original bark would be exhausted at approximately the following rates.
Although Group I gave the highest total yield, the bark was so much injured by the rapid tapping that no further extraction of latex was possible after May, 1911, and it is anticipated that some years must elapse before tapping can be resumed. All the other groups show increased yields in 1911 and 1912, but the relative increase is greater in the case of the groups tapped at longer intervals. In illustration of this difference, details are here given of the yields during the first four months of 1912.
Number of tappings
Yield of dry rubber
Yield per tapping
The above figures may be compared with those in Table XI. A more convenient comparison is given however in the following table.
Average tapping interval (days)
Average yield per tapping per inch of bark, 1st 40 tappings (grms.)
Average yield per tapping per inch Jan.—Apr., 1912 (grammes)
It is to be observed that January to April is usually a period of comparatively low yield, whilst the first 40 tappings of groups V, VI and VII extended over the whole of the period of highest yield. The increase in the later yields per tapping is therefore certainly not exaggerated by the conditions of the experiment. Under these conditions, after the tapping has been in progress for three and a half years, it appears that the longer the interval between successive tappings up to an interval of about a week the greater is the yield per tapping.
If we consider the total yields of rubber per month, this yield is greatest at first from the trees tapped at more frequent intervals. The relative yield from the trees tapped at longer intervals however gradually increases. After three and a half years' continuous tapping, the yield from trees tapped once a week may become as great as or greater than that from trees tapped at any shorter interval. A discussion of the practical importance of these conclusions may be reserved for a later page.
The results just recorded naturally lead on to a discussion of the amount of tapping which can be performed without injury to the tree. Overtapping may be considered either in relation to the excision of bark or to the removal of latex. It is usual to discuss only the former kind of loss in this connection, but there is no doubt that a tree can be overtapped by pricking only, without any removal of bark tissue. In many systems of pricking the damage done to the bark is at least as great as in the case of paring, and with the best possible method of incision some damage is inevitable. Removal of latex without injury to the bark is in fact impossible, but even if this were possible there would still be a limit to the amount of latex which could be extracted without injury to the tree. For the manufacture of latex necessarily uses up a certain amount of food and energy, and the supply of these in the tree is not unlimited.