By ‘Distribution Patterns’ we mean two interrelated topics: First, where on the landscape can mahogany typically be found? Second, at what density do mahogany populations occur, that is, how many trees can we expect in a given area where it grows?
During the early years of colonial exploitation in Central America mahogany was known as a riverine species growing along coastal riverways on the Atlantic seaboard of British Honduras (now Belize). Its proximity to flowing water enabled early loggers to exhaust local populations as timber was exported to England for ship-building and fine furniture; these relatively high-density riverine populations are long gone from this region.
Moving into interfluvial and upland forests in Belize, mahogany occurs at highest densities in transition zones where different forest communities grade into each other, characterized by high light availability and soils with medium to good drainage. Examples include the sequelar, between savanna and forest, and botanal-escobal palm associations on well-drained soils that are transitional communities between wooded swamps and upland climax vegetation. Mahogany also is common in this region in dense stands of poor form in bajos (wooded swamps), where forest canopies are low and thin and soil moisture conditions are seasonally extreme, in deep, gleyed, acidic clays that crack on drying. It invades stands of Pinus caribaea (Pinaceae) on drier sites where understory light levels are high. But in successionally ‘advanced’ forests with closed canopies on rich, well-drained alluvial or granitic soils mahogany cannot regenerate in deep shade.
Lamb (1966) described mahogany growing at low densities in Central America’s Petén region on upland plateaus in zapotal-ramonal associations where Manilkara zapote (Sapotaceae) and Brosimum alicastrum (Moraceae) dominate on shallow, well-drained, alkaline clay soils. His caobal associations (caoba being the Spanish word for mahogany) boast the highest densities of mahogany on deep, well-drained valley soils and moist slopes just above them. Noting that little to no advance regeneration of mahogany could be found in these associations, Lamb concluded that they were late-successional, and that mahogany would be eliminated from them without further disturbance. He further noted that areas with high disturbance frequencies or steep topography seem to favor mahogany.
In South America, early descriptions of mahogany’s distribution patterns emphasized its association with river floodplains in the upper reaches of the western Amazon Basin. From Ecuador, Peru, Bolivia, and Brazil many observers described its tendency to grow at highest densities on drier, firmer soils slightly above seasonally inundated floodplains, where floods occurred infrequently. In lowland Peru, mahogany was reported to survive flooding but was usually found above lower areas with prolonged inundation. In Bolivia, Irmay (1949) wrote that mahogany “prefers lowlands and river margins with periodic inundation and humid soil year long… [it grows] in dense forests found on permeable, firm soil above but not near river margins”. Elsewhere in lowland Bolivia mahogany has been found perched atop steep erosion gullies in floodplain ecosystems. Similar reports from Brazil have described mahogany’s distribution in south Pará as concentrated “in low and more humid places near the streams”.
In southeast Pará, Brazil where we have studied mahogany since 1995, we find three patterns of landscape distribution. First, riverine populations along rivers large enough to flow year-round were the first to be logged out in the 1970s when this region began to be opened up to market infrastructures by gold miners, loggers, ranchers, and small-scale agriculturists. These mahogany populations vanished before scientists could document them, so we know little about them. Second, populations in terra firme forests aggregate along the banks of seasonal streams that flow only during the rainy season. These areas are characterized by fast draining, sandy gray to white hydromorphic or ‘water-formed’ soils that have relatively high nutrient status compared to fine-textured soils on slightly higher ground. And third, populations occur at relatively low densities on the slopes to the tops of steep inselbergs or ‘island mountains’ that punctuate this region’s landscape at irregular intervals. These fire-prone slopes are dry and exposed, with shallow sandy soils. Mahogany rarely grows to large stature on inselbergs but thrives there nonetheless in highly disturbed and open conditions.
In Acre, we have found similar though less distinct distribution patterns. Very few riverine populations survived two phases of heavy exploitation, the first strictly by water transport (to sawmills in Manaus in central Amazonia) during the 1930s and 1940s, and the second in the 1980s and early 1990s when overland transport finally became feasible. Terra firme populations survive in limited areas. These again tend to grow near seasonal streams though terrain is much steeper than in southeast Pará. But physiographic and physiological mechanisms for these associations have not been studied.
Mahogany also can be found growing at elevations as high as 1400 m in the Andean foothills of Ecuador, Peru, and Bolivia. Trees growing above 500 m elevations in Bolivia tend to be smaller than lowland trees, with denser, higher quality wood. This could be a function of generally drier soil conditions due to rapid subsurface drainage on steep slopes.
The general pattern, then, is that we find mahogany most commonly in forests near water, whether along the banks of permanent (aseasonal) rivers or beside seasonal streams that stop flowing during the dry season. Otherwise we find it in stressed or disturbed transition zones between forest communities where canopy disturbance is frequent and light levels at ground level are persistently high.
What about density? Mahogany trees are known for occurring in clumps or aggregations of several tens to hundreds of mature stems, often with expanses of ‘empty’ forest many kilometers wide separating aggregations. Densities in Mexico and Central America tend to be higher than those reported from South America, though of course surviving populations do not necessarily reflect historic natural densities after centuries of exploitation, especially of riverine populations. In the Petén district of northern Guatemala densities of 12 trees per hectare were once found over large areas. Nearly pure stands of limited extent – a few hectares – with densities up to 55–70 trees per hectare have been reported from Panama, Nicaragua, Guatemala, Belize, and Mexico. On Mexico’s Yucatan Peninsula densities of 2.5–6.1 trees per hectare ≥ 15 cm diameter were reported during the 1990s in previously logged forested areas.
In Bolivia and Brazil, recent large-scale inventories indicate that mahogany occurs at lower densities in both riverine and terra firme forests. In floodplains of lowland Bolivia adjacent to aseasonal rivers, aggregations of 0.1–0.2 commercial-sized (> 80 cm diameter) trees per hectare have been found. Outside areas of occurrence mahogany may not reappear for distances up to 10 km. These densities are similar to inventory results from Acre and Amazonas in west Brazil, where we found 0.12 trees > 20 cm diameter per hectare at our Sena Madureira field site (equivalent to 12 trees in 100 hectares, that is, 12 mahogany trees in a square of forest 1 km long by 1 km wide). Compare this to Marajoara in southeast Pará where we found 0.65 trees > 20 cm diameter per hectare (65 trees per 100 hectares), while a nearby research site in this region reported 1.18 trees this size per hectare (118 trees per 100 hectares). Even these ‘high’ densities are low by almost any standard: imagine trying to find 65 trees of a single species in and area of dense tropical forest 1 km on a side, many of them smaller in diameter than your waist!
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