Forest recovery and carbon sequestration under shifting cultivation in the Eastern Arc Mountains, Tanzania: Landscape and landuse effects.

摘要

The traditional practice of shifting cultivation in many parts of Africa results in a matrix of secondary forests of varying age. The biodiversity value and carbon sequestration potential of forest in shifting cultivation systems is largely unknown. Newly developed carbon marketing mechanisms provide novel opportunities for considering fallows in the context of broad-reaching, landscape level conservation strategies. The objective of this dissertation study was to quantify the biodiversity and carbon potential of fallows, and to develop means by which to predict recovery based on landscape factors and historical and ongoing landuse. The study was conducted in South Nguru Mountain, a part of the globally recognised biodiversity rich Eastern Arc Mountains, Tanzania. Soil and vegetation surveys were conducted in 0.1 ha (20 x 50 m) plots to derive biodiversity indices and carbon stock woody biomass and soils. Along a 300--1600 m elevation gradient more than 120 farms and fallows at different stages of abandonment were surveyed and their cultivation and fallow histories obtained. The findings show that secondary forests in the Ngurus are rich in tree diversity and can harbour some endemics species, but that persistence of forests beyond 30 years remains a challenge. Carbon response to landscape and landuse factors was variable and complex but some general patterns were evident. Elevation was the most important gradient driving C stock in fallows; it required stratifying the landscape into lowland and submontane zones. Within these zones historical and landuse generated some predictable outcomes for C. While it is too soon to develop mechanistic models for C dynamics in the area, estimates can be made. Different sources of uncertainties in the estimates of C were identified, least of which was the need for wood specific densities for most tree species in the Eastern Arc Mountains. This study provides a useful assessment of the value of secondary forests for biodiversity conservation and carbon storage in a region that needs to explore alternative ways to meeting conservation objective in human-dominated landscapes.