Amazon Silviculture: The Science of Trees

In tropical areas of the Amazon basin with large areas of natural forest, indigenous peoples have practiced various forms of silviculture, or forest management, often in combination with shifting agricultural cultivation. In modern times, mahogany (Sweitenia macrophylla) and Spanish cedar (Cedrela odorata) have traditionally been the most valuable timber species, but are currently depleted from much of their natural range (see mahogany). In the southern Amazon, common species include Bagassa guianensis, Vataeropsis speciosa and Goupia glabra. In Bolivia, Hura crepitans and Pseudolmedia laevis are commonly logged in areas where mahogany is gone. In the northern Amazon state of Para, Bagassa guianensis, Hymenaea courbaril, Manilkara huberi, Dipteryx odorata, and Symphonia globulifera are commonly harvested. Throughout the Amazon, ipê (various species of Tabebuia) is a valuable wood used in the wood decking market.

In the Amazon forest, most logging is single tree selection, where only selected species with timber value are harvested, usually a few trees or 20-40 m³ / hectare. In other areas of the tropics, such as Southeast Asian diptocarp forests, for example, a much larger percentage of mature trees are harvested for valuable timber. However, many trees are accidentally damaged during logging from felling, entangling with lianas, and cutting of skid trails; studies of logging operations in Para found that canopy cover was reduced to 45% in logged sites. Other studies estimate that 10-40% of total forest biomass is lost in the logging process, and that an even greater amount is vulnerable to desiccation, fire, and other fragmentation effects. In the western Amazon, most timber harvest is conducted in the dry season, usually July-November, which can exacerbate drought conditions.

Long term timber management in the Amazon basin is not well studied. Growth rates are unknown for most species, so harvest rotations are rarely modeled. Nutrient analyses find that Calcium, Magnesium, and Potassium may be limiting nutrients in heavily logged sites. Some studies find that conventional logging rotations of 30 years are insufficient to regenerate commonly logged species, and that longer rotations of greater than 60 years are necessary.  Other studies find that enrichment planting can assist long term yields of commercially viable species. However, repeated clearing or tending is needed to free commercial species from aggressive pioneers and lianas.

In logged areas, Amazon forest is usually left to regenerate naturally. In most areas, gaps are colonized by lianas and trees of low commercial value. In Para, logged gaps were dominated by Jacaranda copaia, a low-value commercial species. In the 1980s and 1990s, forestry scientists prescribed selective logging techniques such as strip silviculture, which attempts to imitate the natural small canopy gap disturbance present in many tropical forests. Arguments for these practices rested on ecological theory of gap disturbance and its positive effect on overall biodiversity. More recently, however, researchers argue that in many areas, medium and large scale disturbance from storms or flooding may be necessary for regeneration of many tropical trees such as mahogany, cedar, and Brazil nut. They argue that larger scale disturbance was a natural part of Amazon forests, from indigenous peoples that practiced active management. Nevertheless, larger scale logging disturbance may result in excessive damage from fire, mortality, and other deleterious edge effects.


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Region: 
Amazon