The following page describes forest management practices that can encourage protection of natural mahogany populations and accelerate growth rates for improved timber yields. Use the links below to jump to the section of interest.
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Since beginning fieldwork in southeast Pará in 1995, our research objective has been to establish a biological foundation for the sustainable management of mahogany in natural forests based on long-term studies of mortality, growth, reproduction, and regeneration. By answering basic questions about mahogany’s life history, we can recommend silvicultural practices aimed at maintaining healthy, productive populations within the ecosystems where they naturally occur. For an comprehensive guide to managing mahogany in plantations see Mayhew & Newton (1998).
Managing mahogany populations for sustained production is uniquely challenging because mahogany’s great value encourages removal of any individual tree that can pay its way out of the forest. A second problem is that most remaining populations occur in remote Amazonian forests where access is difficult and infrastructure is minimal. Finally, these remaining populations tend to occur at very low densities, on the order of 1 commercial-sized tree (> 60 cm diameter) in 5 to 10 hectares or more. Under these circumstances, financial investments in forest management can tip marginally profitable harvest activities into the red.
BEST FOREST MANAGEMENT PRACTICES
Brazilian federal regulations governing industrial timber production require that logging companies develop and submit management plans to the Brazilian Forest Service for prior approval. Current best forest management practices emphasize planning and reducing damages to residual forest stands, or reduced-impact logging (RIL). Commercial trees must be identified, measured for stem diameter, and mapped on a grid of parallel forest trails; the locations of primary and secondary logging roads, logyards, and skidtrails must be planned to minimize forest entry by heavy machinery; and vines must be cut during the year before timber harvests to reduce damages to neighboring trees as large crowns are felled (individual vines often grow from tree crown to crown, tying them together, and can pull neighboring trees to the ground during felling if not cut beforehand).
As discussed in Harvest Regulations, management plans including mahogany must respect the minimum diameter cutting limit of 60 cm, retain at least 20% of commercial-sized trees to serve as seed trees between harvests, and ensure that landscape-scale population densities do not fall below 0.05 commercial trees per hectare (5 trees per 100 hectares). This last requirement may place severe constraints on harvests in remote forests if landscape-scale densities are already between 10 and 20 commercial trees per 100 hectares, allowing only 5 to 15 trees per 100 hectares to be harvested.
Mapping mahogany trees before harvests provides baseline information that can inform management decisions. Where adult trees do and do not occur on the landscape indicates where seedling enrichment plantings should be located, if these will be part of the management plan. Like all species, mahogany occurs where it does for specific reasons. For example, at Marajoara mahogany inhabits low-ground forests adjacent to seasonal streams. Experimental studies have shown that understory light levels and soil nutrient availability are, on average, higher in these areas, providing the conditions necessary for mahogany’s successful regeneration and recruitment to adult size. By this logic, attempting to establish mahogany seedlings in enrichment plantings on high ground where adult trees rarely occur is likely to fail because growing conditions there are not optimal. Many forest tree species will outcompete mahogany in drier, nutrient-poor growing conditions on high ground.
Measuring the diameters of mahogany trees before harvests allows forest managers to see the distribution of tree sizes within a population, and to estimate harvest yield. Population structure can suggest several things about current and historic growing conditions at a given site. When commercial populations are dominated by a few very large trees, with relatively few smaller stems close to the minimum commercial size, we can infer that disturbance events fostering regeneration and recruitment are rare in general and rarer still in recent decades. This means that silvicultural practices aimed at increasing next-generation tree densities may face great difficulty on this landscape, for the same reasons that adult mahogany trees occur at such low densities. On the other hand, if population structures are dominated by smaller trees occurring at relatively high densities, with few very large adults, it may mean that disturbance events fostering regeneration and recruitment are relatively common, and that management efforts will be easier than under the opposite scenario. See Ecology > Populations.
At Marajoara, once mahogany trees have grown larger than pole size, that is, to 20 cm diameter or larger, annual mortality rates average approximately 1% per year regardless of stem size. Similar mortality rates have been reported from populations in Central America. This simple fact has extremely important implications for what happens to surviving populations between harvests (in Brazil, harvests occur at 25- to 35-year intervals). That is, over a 30-year cutting cycle roughly 26% of surviving trees will die of natural causes, reducing yields by the amount that these dead trees cannot be recovered during the second harvest.
As noted under Ecology > Adults, forest managers can attempt to reduce mortality rates between harvests. Since vines account for roughly 50% of observed tree deaths, and mortality risk increases as the degree of crown coverage by vines increases, cutting vines should reduce mortality rates.
DIAMETER GROWTH RATES
At Marajoara and other field sites in southeast Pará, mahogany trees larger than 20 cm diameter grow, on average, approximately 0.65 cm diameter per year. Trees growing at 75th percentile rates, that is, faster than 75% of all other trees, increase diameter at the rate of ~1 cm per year. This 75th percentile growth rate is what we call the ‘optimal’ or target growth rate for forest managers, whose ultimate objective is to grow high-value timber as fast as possible during the years between harvests.
In a perfect world we would know everything we need to know about any mahogany tree growing in the forest, including its growth history (which can tell us about wood quality and tree health) and current diameter growth rate. This information would be useful because one commercial tree in five (20%) must be retained for seed tree and future timber production purposes. That is, choosing between two trees that are each 75 cm diameter, we would rather harvest the slow-grower and retain the fast-growing tree to maximize timber yield during the second harvest.
As discussed elsewhere on this website (Ecology > Adults), the best predictor of how fast a given tree will grow next year – and during the years after that – is how fast it grew last year, a phenomenon called growth autocorrelation. This means that a single year of diameter growth data offers a powerful tool for making decisions about which trees to harvest and which to leave standing. Choosing between two equal-sized commercial trees, one that grew 1 cm during the previous year and one that grew 0.2 cm, is easy: we should harvest the slow-growing tree and retain the fast grower to maximize timber yield during the next harvest. Having two or three years of growth data before the harvest further increases the precision of this management tool.
The degree of crown vine loading is the next most important factor determining diameter growth rates by individual mahogany trees. It is not uncommon to encounter mahogany trees whose crowns are so completely covered by vines that they are barely alive, literally starving to death for lack of sunlight. While these trees obviously are not growing, almost any level of crown vine coverage can impact diameter growth rates. This means that vine cutting, the simplest silvicultural practice available for increasing long-term timber production, is also the most cost-effective because it both reduces mortality rates and boosts diameter growth rates.
We have found that mahogany trees released from heavy vine loads require roughly 5 years to recover average growth rates characteristic of trees without vines. We also have found that it is important to return to trees where vines were cut after one or two years to make sure that all vines have been eliminated from tree crowns. Because vines frequently ‘travel’ from tree crown to tree crown in the forest canopy, it is often necessary to locate their points of origin at some distance from the target tree. For this reason some logging companies may cut vines from future crop trees to a radius of up to 50 meters.
Amaral P, Veríssimo A, Barreto P & Vidal E (1998) Floresta para Sempre; um Manual para a Produção de Madeira na Amazônia. IMAZON, Belém, Pará, Brazil (http://www.imazon.org.br/publicacoes/livros).
Grogan J (2002) Some simple management guidelines could help the sustainable management of bigleaf mahogany in the neotropics. Tropical Forest Update, ITTO Newsletter 12(4): 22-23.
Grogan J, Barreto P & Veríssimo A (2002) Mogno na Amazônia Brasileira: Ecologia e Perspectivas de Manejo (Mahogany in the Brazilian Amazon: Ecology and Perspectives on Management). IMAZON, Belém, PA, Brazil (http://www.imazon.org.br/publicacoes/livros).
Grogan J, Ashton MS & Galvão J (2003) Big-leaf mahogany (Swietenia macrophylla) seedling survival and growth across a topographic gradient in southeast Pará, Brazil. Forest Ecology and Management 186: 311-326.
Grogan J & Barreto P (2005) Big-leaf mahogany on CITES Appendix II: big challenge, big opportunity. Conservation Biology 19: 973-976 (http://www.treesearch.fs.fed.us/pubs/30087).
Grogan J, Jennings SB, Landis RM, Schulze M, Baima AMV, Lopes JCA, Norghauer JM, Oliveira LR, Pantoja F, Pinto D, Silva JNM, Vidal E & Zimmerman BL (2008) What loggers leave behind: impacts on big-leaf mahogany (Swietenia macrophylla) commercial populations and potential for post-logging recovery in the Brazilian Amazon. Forest Ecology and Management 255: 269-281 (http://www.treesearch.fs.fed.us/pubs/29979).
Grogan J & Landis RM (2009) Growth history and crown vine coverage are principal factors influencing growth and mortality rates of big-leaf mahogany Swietenia macrophylla in Brazil. Journal of Applied Ecology 46: 1283-1291 (http://www.treesearch.fs.fed.us/pubs/36898).
Grogan J, Peña-Claros M & Günter S (2011) Managing natural populations of big-leaf mahogany. In: Günter S, Stimm B, Weber M, Mosandl R (Eds.), Silviculture in the Tropics, pp. 227-235. Springer-Verlag, Berlin – Heidelberg, Germany.
Grogan J, Schulze M, Lentini M, Zweede J, Landis RM, Free CM (2013) Managing big-leaf mahogany in natural forests: Lessons learned from an ITTO-CITES Programme project. Tropical Forest Update, ITTO Newsletter 22(1): 12-15, 19.
Kelty MJ, Cámara-Cabrales L & Grogan J (2011) Red oak in southern New England and big-leaf mahogany in the Yucatan Peninsula: can mixed-species forests be sustainably managed for single-species production? Journal of Sustainable Forestry 30: 637-653.
Mayhew JE & Newton (1998) The Silviculture of Mahogany. CABI Publishing, New York, NY, USA.
Snook LK (1998) Sustaining harvests of mahogany (Swietenia macrophylla King) from Mexico's Yucatan forests: past, present and future. In: Primack B, Bray DB & Galletti H (Eds.), Timber, Tourists and Temples: Conservation and Development in the Maya Forests of Belize, Guatemala and Mexico, pp. 61-80. Island Press, Washington, DC, USA.
Stevenson NS (1927) Silvicultural treatment of mahogany forests in British Honduras. Empire Forestry Journal 6: 219-227.
Veríssimo A, Barreto P, Tarifa R & Uhl C (1995) Extraction of a high-value natural resource in Amazonia: the case of mahogany. Forest Ecology and Management 72: 39-60.