Big-leaf mahogany Swietenia macrophylla, due to exploitation for its timber, has undergone a dramatic decrease in its native South American range. This Bolivian study evaluated whether recovery of over-exploited mahogany populations may be enhanced by managing the species and surrounding forest; four different interventions varying in logging and silvicultural treatment intensities were assessed.
Data were gathered over 4-years in 12, 27-ha plots of the Long Term Silvicultural Research Program in the Agroindustria Forestal La Chonta forestry concession (15°47′S, 62°55′W). Plots received one of four treatments:
i) an unharvested control;
ii) logged following practices stipulated under Bolivian forestry law ('normal treatment');
iii) logged as (ii) but with additional silvicultural treatments ('intermediate treatment');
iv) double logged with more intensive silvicultural treatments ('intensive treatment').
In the plots, mahogany saplings and trees over 1.3 m tall were mapped and measured in March-April 2002, July-August 2003 and March-May 2006. Seedlings and saplings (< 1.3 m tall) were measured around 58 adult mahogany trees (> 50 m apart); seedlings within a 2 m radius around each tree base and within six (1 × 48 m) transects radiating from them, were counted.
Population growth rate was simulated by computer modelling based on field observations.
Treatments represented a gradient in logging intensity and silvicultural treatments, resulting in a gradual increase in the area disturbed by logging activities and light levels as treatment application progressed.
Seedling and sapling survival was lowest in unlogged forest and highest under intermediate treatment. High seedling counts in all treatments in 2002 were probably due to high rainfall (70% higher than average in 2001) rather than increases in light levels. In 2002 and 2005 rainfall was 19 to 33% lower than average; even in the controls (no increase in light levels) there were higher densities of seedlings in 2002 than in 2006.
Seedling and sapling survival was higher under normal and intermediate treatments, in comparison with intensive treatment (probably because the higher logging intensity resulted in greater competition from other plants due to canopy opening). Survival of mahogany trees over 1.3 m tall was not affected by treatment (almost 100% among trees of > 50 cm diameter at breast height (DBH). Growth rate of larger trees tended to increase with management intensity; removal of lianas and other competing trees had a positive effect on growth.
Model simulations suggest that mahogany recruitment is enhanced by intermediate levels of silvicultural treatments, and that population recovery is dependent on retaining trees (> 70 cm DBH) that produce large numbers of seeds.