The role of forest plantations in the restoration of tropical forests.

摘要

A wide diversity of native and exotic woody species colonized the Pinus patula and Grevillea robusta plantations established at the Shume reforestation project and Meru reforestation station in Tanzania, demonstrating the potential for using plantations to enhance tropical forest restoration. The presence of more shade-tolerant species suggests that succession under plantations proceed faster than in an unmanaged secondary succession. Seed transfer from natural forests and fallows, mainly by birds and mammals, aided the accumulation of numerous tree species. The absence of some pioneer species in the seedling regeneration category, especially at the Meru reforestation station, suggests that the structural complexity of the Grevillea robusta understory eliminates the conditions needed for pioneer species.;The location of a plantation relative to natural forest, its age, size and its stocking level all appear important in mediating natural regeneration. At both sites, proximity to a rich and diverse natural forest appeared to be central to the development of the plantation understory. In this study, the size of the plantation, though confounded with age, also had an impact on natural regeneration success.;The most abundant tree species identified in natural forest transects were poorly represented in plantations. The degree of plantation isolation relative to a seed source was a major constraint to a given species' ability to colonize the plantations. The mobility of the main seed dispersal agents (birds and mammals) contributed to undergrowth enrichment by bringing in diverse native woody species from distant locations. Most native species recorded under distant plantations were bird-dispersed, followed by wind-dispersed exotics from fallow fields.;Unmanaged plantations did not support native species in the same densities as is found in the thinned plantations. The number of intermediate and shade tolerant species increased with increasing stocking levels before declining in more dense plantations, while pioneer species, more common in poorly stocked plantations, declined with increasing stocking levels. At the plantation level, most regeneration was highly concentrated in first few hundred meters directly adjacent to natural forest. There was also a greater association of woody species with steeper slopes, low parts of the hill and with west- and north-facing slopes (eastern wind shadows) within the plantations, illustrating the importance of the spatial organization and topography on natural regeneration success.