The problem of the physiological effect of paring upon the tree is therefore a complicated one. In addition to the rate of removal of the bark, both the amount of latex taken from the tree and the frequency of this extraction have to be considered. In the experiment with seven groups of trees described above, Group I and some of the trees of Group II may be said to have been overtapped, because at the time when the whole of the outer bark had been removed up to the greatest height convenient for tapping, the renewal of the first area tapped was still imperfect. This result was no doubt partly due to defects in tapping, but with the most perfect tapping there must be a limit to the possible rate of removal of the bark which will permit of proper renewal.
Let us assume a system by which the whole of the outer bark is removed to a certain height in a given period. Then if at the end of this period the bark of the area first tapped has not renewed sufficiently to allow of a second tapping, it is clear that the proper period for renewal has been underestimated. From the point of view of bark-removal the trees have been over-tapped. This is the criterion of overlapping which is most often adopted in practice. It would probably be better however to extend the idea of overlapping somewhat further, and to recognise that it is possible for a tree to be overtapped owing to the excessive removal of latex, although there may be a considerable area of untapped bark still available for tapping.
In the case of young growing trees planted with plenty of space, the yield of latex may be expected to increase steadily from year to year, so long as the bark is preserved in good condition. The ideal rate of tapping may be defined as that rate which is associated with the greatest increase in yield as time goes on. Any quicker rate may consequently be regarded as involving overtapping from the point of view of latex removal In the case of the old trees at Henaratgoda, the rate of tapping associated with the greatest increase in yield proved to be considerably slower than the rate required for a degree of bark renewal which would generally be regarded as complete.