Management implications of long-term tree growth and mortality rates: A modeling study of big-leaf mahogany (Swietenia macrophylla) in the Brazilian Amazon

Abstract

Knowledge of tree age-size relationships is essential towards evaluating the sustainability of harvest regulations that include minimum diameter cutting limits and fixed-length cutting cycles. Although many tropical trees form annual growth rings and can be aged from discs or cores, destructive sampling is not always an option for valuable or threatened species. We used an individual-based population model developed for big-leaf mahogany (Swietenia macrophylla, Meliaceae) in southeast Amazonia, Brazil to simulate stem age-size relationships and examine forest management implications of mortality and diameter increment growth. Growth trajectories of 10,000 simulated mahogany trees were consistent with growth trajectories reconstructed from 32 mahogany discs with annual rings. Trajectories were highly variable and strongly autocorrelated; diameter was a poor predictor of tree age even when accounting for up to 10years of previous growth history. Commercial-sized trees (60cm diameter) ranged in age from 33 to 180years (74yr median). Only 12.5% of seedlings survived to this size, but survival and time to reach commercial size depended strongly on early growth history (first 10years of life). A tree grown at the 75th percentile growth rate throughout its lifetime requires 70years to attain commercial size, but Brazilian forest management regulations imply a rotation length of 60years. These results demonstrate that individual-based models parameterized with typical census data can incorporate individual variation and growth autocorrelation and realistically simulate tree growth and mortality. In the absence of tree ring data, such models can be used to evaluate the consequences of long-term growth and mortality for sustainable management. In the case of mahogany, our results suggest that non-detrimental harvests cannot be achieved without lengthening cutting cycles, increasing commercial tree retention rates, and regularly applying silvicultural treatments designed to increase stem diameter growth rates. Forest managers can have the greatest effect on the rate of commercial recruitment in the first 10years after a harvest by implementing treatments ensuring that adequate numbers of new stems establish and recruit to dominant positions in recovering canopy gaps. Regrettably, sustainable mahogany management systems developed based on understanding of the species’ ecology will not be as simple as current harvest regulations in Brazil imply.