Changes Over Twenty-One Years in the Kolombangara Rain Forests

(1) Six floristic types of lowland tropical evergreen rain forest on Kolombangara island have been monitored for 21.3 years. (2) Canopy structure and the populations of the twelve dominant canopy-top tree species were recorded up to twelve times, on sample plots of 0.63 ha each, initially twenty-two plots but eventually only twelve. (3) The twelve species have differing requirements of canopy disturbance for regeneration, and can be given a pioneer index on the scale 1-4. (4) The pioneer index of the forests as a whole fell by 0.21 during the 21.3 years, showing that disturbance was not sufficient to maintain their initial composition. (5) Individual forest types differed from the mean and within the types the component plots differed from one another, in percentage canopy disturbance, in pioneer index and in gains and losses to individual species. (6) Usually, but not always, extensive canopy disturbance was followed by an increase in tree numbers of species with a high pioneer index. (7) Before the survey began the north coast forests (types IV-VI) had been more extensively disturbed than those on the west (types I-III), with type VI most disturbed of all. During the survey greatest disturbance occurred to types II, IV and V and least to I and VI. Forest type VI had the biggest drop in pioneer index (0.42). (8) This is the most comprehensive survey yet available of tropical rain forest dynamics with its combination of large area, long time, the monitoring of total populations of the studied species, and the independent record of canopy disturbance. The results show strong spatial and temporal differences in population size and turnover of species and in extent of canopy disturbance; and that generalizations on forest species composition and its change with time are highly dependent on timescale and space scale. Average values or those based on limited data conceal substantial differences.