The behaviour of the tropical rain forest of the Brazilian Amazon after logging / J.N.M. SIlva.

Tese (Ph.D.) University of Oxford, 1989

A logged over Brazilian Amazon "terra firme" tropical forest situated in Santarém region, State of Pará, was studied for its natural regeneration dynamics and stand development after logging. Experimental logginf was carried out in 1979, removing 75 m³ ha-¹ from an average of 16 trees per ha and 63 species. Malayan Linear Regeneration Samplings carried out on three occasions, one before logging in 1975, and two others in 1981 and 1985, two and six years after logging, revealed that the stocking of commercial species was 41%, slightly over the minimum required according to the Malayan standards. Opening the canopy had induced natural regeneration of desirable species. The number of stocked quadrats rose to 76% six years after logging, an increase of 85%. Stocking with potentially commercial species was always very high (over 90%) in all assessments, reaching nearly 100% six years after logging. Regeneration and growth of light demanding commercial and potential species accounted for a considerable proportion (over 30%) of the total stocking. Stand development was monitored through 36 permanent sample plots of 0.25 ha each, established and measured two years after logging, and remeasured four times since them. Assuming a polycyclic management system with felling cycles of 30-35 years, the results, led to the conclusion that logging intensity applied was too heavy to produce a new economic harvest at the end of the projected regeneration period, based on the stocking of medium sized commercial trees left standing. During, the period monitored, the average diameter increment of all species were 0,5 cm year-¹, which are in the range of published figures for other tropical rain forests with similar disturbance status. Commercial species had the same average diameter growth as for all species. Logging boosted the average diameter growth, but its beneficial effect lasts only for 3-4 years after canopy opening. Solar radiation had a strong influence on tree growth. Trees with crowns fully exposed to sun light showed significantly higher increments than those receiving only sidelight or completely shadded. Mortality rates were relatively high just after logging, but averaged 2.5% per year during the period monitored, which is only slightly higher than the average reported elsewhere for undisturbed forests. Ingrowth of commercial species has fallen dramatically since the first enumeration (neraly 60%) due to mortality and canopy closure which jeopardizes sustainability. To assure replenishment, more light is needed for the population of seedlings and saplings. Projection of the stand over a period of 33 years confirmed the conclusions from six years' data that the stand left after logging was not sufficient to produce an economic output at the end of the projected cutting cycle. Simulations of management regimes showed that an economic harvest would only be possible if silvicultural treatments were applied at ten year intervals and if some species of the potential group entered the market in the course of the present regeneration period. The projections, are, however, restricted by the arbitrary assumptions made for the behaviour of the parameters used by the simulation model.