Moderate tapping seems to encourage not only increased latex formation but also an increased rate of growth in thickness over the area tapped. It seems probable that this increase must take place at the expense of the growth of other parts of the tree. So long as the only recognisable effect of this kind is a reduction in the quantity of seed produced, no harm is done from the planter's point of view. It may even be suggested that artificial removal of the young fruits might prove profitable, if it could be carried out at a small expense, as leading to the conservation of food supplies in the tree.
Severe tapping has a reverse effect in every way. If the trees are tapped to excess, growth is checked, and in particular the proper renewal of the bark is interfered with. The latex moreover becomes poor in quality and contains a smaller percentage of caoutchouc. Most serious of all is the effect upon the general health of the plant. The tree may be so weakened that it is unable to withstand the attacks of fungus diseases, which would not have been able to gain a footing if the tree had been preserved in a condition of perfect health. Canker and similar diseases seldom attack the trees unless the latter are either overtapped or very closely planted.
In Hevea brasiliensis as grown under plantation conditions, repeated tapping on a moderate system at intervals varying from one to ten days leads to an immediate steady increase in the yield of latex and rubber obtained at each tapping. In the case of young and vigorous trees, this increase may continue for an indefinite period, subject to certain seasonal variations. The increase is found to take place even with daily tapping if the system adopted is such as to allow four years for the renewal of the bark. In the case of old trees, closely planted, after tapping has continued for some time, the yield from tapping at longer intervals increases relatively to the yield from tapping at shorter intervals.
This fact may be regarded as one aspect of the more general phenomenon that moderate tapping leads to a steady increase in yield, whilst overtapping ultimately leads to a relative falling off in yield. The amount of excision which constitutes overtapping depends upon the conditions and upon the individuality of the particular tree.
The bulk of latex which can be extracted in a year is probably often quite as great as the total volume which all the latex tubes of the tree could contain at any one time. Although the latex can pass gradually from one part of the tree to another, especially in the vertical direction, the greatest part of the latex removed is probably secreted at no great distance from the wounded area.
Under the comparatively uniform conditions of the climate of Western Ceylon, the yield of latex at certain seasons may be nearly twice as great as at other seasons. Seasonal differences of yield are partly determined by climatic conditions, but probably also in part by the quantity of food materials available in the tree.
The yield from different trees of the same age and girth varies greatly. A difference of iooo per cent is not at all uncommon.
The yield is greatest near the foot of the tree, and decreases gradually on passing upwards. In all probability the yield is roughly proportional to the volume of the bark at any given level.
The percentage of rubber in the latex shows variations similar to those displayed by the total yield. The percentage is highest during the period of highest yield. Severe tapping leads to a marked falling off in the percentage of rubber present.
It may be concluded that in practice the rate of tapping should be reduced either
(1) if the bark is being used up at a greater rate than one quarter of the available amount annually, or
(2) if the concentration of rubber in the latex falls much below thirty per cent, or
(3) if the yield of latex fails to show an increase over the amount obtained at the corresponding period of the preceding season.