Sustainable timber harvesting in Venezuela: a modelling approach.
Reliable data on the growth and yield of logged-over forest, to determine sustainable cutting cycles, are widely missing for the tropics. We used the process-based model FORMIND2.0 to analyse the growth and yield of logged-over forest in Venezuela under different logging scenarios over a period of 240 years, and compared results with unlogged stands. The performance of the model was evaluated with a detailed stability and sensitivity analysis. In the absence of further logging, the logged-over stand approached the stand structure of mature forest in terms of bole volume and basal area after about 50-100 years. 30-year cutting cycles with conventional logging methods and net extraction volumes of 45 and 60 m3 ha-1 cycle-1 did not provide sustainable yields under either of 2 minimum felling diameters (35 and 50 cm) that were applied. Only the 60-year cutting cycle provided sustainable yields under conventional and reduced-impact logging, with the different minimum felling diameters and a range of net volumes extracted (30-60 m3 ha-1 cycle-1). With the longest cutting cycle (60 years), bole volume recovered to levels similar to the mature unlogged stand, but the species composition was very different. Scenarios with reduced-impact logging provided a significantly higher timber volume than under conventional logging. The conservation of forest resources will only be possible with long cutting cycles (at least 60 years) in combination with reduced-impact logging.