Four systematic spacing trials were planted between 1969 and 1973 on three islands in the Republic of Kiribati which have contrasting regimes of rainfall, in order to investigate the spacing requirements of coconuts in relation to rainfall (under atoll conditions). Two of the trials are on Kiritimati, which has an annual average rainfall of only 870 mm; one is on Tarawa which receives 2043 mm, and the fourth is on Butaritari which receives 3185 mm. Three of the trials are in the form of an almost complete wheel, whilst the fourth consists off our fan-shaped sectors. The range of planting densities in all trials was from 126 to, 632palms per hectare, in 12 treatments.

Due to discontinuity of staff only one of the trials on Kiritimati was recorded during the period in which the coconuts came into, bearing but this showed fairly clearly that there is a relationship between precocity and planting density, with the palms coming into production earlier at the wider spacing.

During the period of maximum cropping of the palms the optimum planting density under conditions of 870 mm annual rainfall is outside of the range of densities covered by the trials, and somewhat less than 126 palms per hectare.

At 2000 mm rainfall, and for fertilized coconuts there is nearly equal yield per unit area of land within the range 169 to 304 palms per hectare, due to an inverse relationship between productivity per palm and number of palm per hectare. Number of fronds per palm and the rate of frond (and therefore bunch) production were inversely proportional to planting density, whilst trunk heights were directly proportional to planting density. The spacing to be chosen under these conditions will therefore be dependent upon other factors, such as the labour involved in digging the planting holes, whether the palms are to be intercropped, used for toddy or drinking nuts, whether the fronds are of significant value, and whether or not labour for subsequent maintenance is in short supply. In most situations, however, the lowest planting density which gives maximum productivity per unit area of land would be desirable.

There were shifts in the optimum planting density in response to rainfall during the previous season, from which it might be inferred that on islands which receive significantly less than 2000 mm rainfall per annum, and especially if the coconuts are not fertilized, the optimum population density is somewhat less than 169 palms per hectare. A convenient round figure under these circumstances is a spacing of 9 meters triangular, which gives a planting density of 143 palms per hectare.

Results from the trial on Butaritari were inconclusive due to only one year's full recording being obtained coupled with an unfortunate choice of site, but from the shifts which were observed in the trial on Tarawa in response to annual variations in rainfall it can be inferred that under high rainfall conditions a relatively high population density is optimal (but probably within the range 169 to 304 palms per hectare).