ECOLOGICAL CONSTRAINTS ON RAIN-FOREST MANAGEMENT AT BAJO CALIMA, WESTERN COLOMBIA

A forest harvesting system using skyline cables was evaluated for its effect on secondary forest structure and tree species richness of lowland rain forests in the Bajo Calima Concession, western Colombia. Forests were sampled using six 0.1 ha plots in mature primary forest and sites 0.4, 4, 8 and 12 years since logging. Clear-cutting reduced overstory (trees greater than or equal to 10 cm dbh) basal area, biomass and richness to 7%, 4%, and 17%, respectively, of primary forest levels. By the twelfth year 46% of basal area, 37% of biomass, and 38% of richness had returned. However, 63% of biomass and 50% of richness were composed of 'core pioneer' species. The replacement of primary (climax) species dominants by pioneer species indicates that the early process of regeneration is more appropriately described as secondary succession. Age since clear-cutting during the first 12 years of growth was a significant linear predictor of both pioneer and climax species biomass and basal area, but not climax richness. Extrapolation of these trends beyond 12 years suggests that overstory basal area and biomass would equal that of mature rain forests by 30 years (the proposed rotation time). However, the decline in pioneer growth rates over time, the slow climax growth rates, and the failure to predict observations in an 18-year-old stand indicate that such a model is unrealistic. Climax species need longer than 30 years to recover, and biomass may decline between 15 and 30 years if many short-lived pioneer trees die before climax trees are well established. More consideration needs to be given to understanding the regeneration of climax species if conservation and harvesting are to be combined. Putting part of the concession under a longer rotation time may permit climax trees to regenerate successfully.