Given a uniform and moderate system of tapping, the yield from a Hevea tree should increase steadily with its age and girth. Too drastic tapping will certainly reduce the rate of increase of yield, and may even lead to a falling off in the absolute yield of latex and of rubber. On the other hand, there is reason to believe that the yield after a given number of years will be greater from a tree which has been regularly tapped in moderation than from one which has been left untapped. It should be observed, however, that the definite experimental evidence upon this point is very limited. Assuming that moderate tapping stimulates the tree to increased activity in the production of latex, it should be die aim of the planter to discover just that rate of tapping which will lead to the most rapid increase of yield under the conditions affecting each particular field.
1 Circulars R. B. G. Peradeniya, Vol. v. No. 16.
Just as in die case of many other physiological functions, the yield of latex at any given age would appear to be controlled by a number of limiting factors. The system and rate of tapping represent one such factor. Others are the available space, soil and water supply, and climatic conditions generally. These conditions severally react upon what is probably the most important factor of all, namely the individual character of the tree.
The rate of increase of yield will be reduced if the trees are planted too closely. But the most profitable number of trees which can be borne by any given acre at any given age, is a problem which can only be solved by prolonged trial and experience. When the soil is rich, more space is required for the full development of the trees than when the soil is poor. Similarly, more space is required at any given age at low than at high elevations.
The condition of the water supply seems to have an even more immediate effect upon the yield of latex. Where the water level is close to the surface of the soil, as in parts of the Federated Malay States, large yields are obtained at an early age. When the water level is deep below the surface, the roots may not penetrate to the requisite depth for some years, and during this period the yield is more dependent upon the seasonal rainfall. From old trees which have penetrated to the water level, larger and more continuous yields are obtained. This seems to be the case with the old trees at Henaratgoda, where the water level is upwards of 25 feet below the surface.