Predictable waves of sequential forest degradation and biodiversity loss spreading from an African city

Abstract

Tropical forest degradation emits carbon at a rate of approximately 0.5 Pgxy(-1), reduces biodiversity, and facilitates forest clearance. Understanding degradation drivers and patterns is therefore crucial to managing forests to mitigate climate change and reduce biodiversity loss. Putative patterns of degradation affecting forest stocks, carbon, and biodiversity have variously been described previously, but these have not been quantitatively assessed together or tested systematically. Economic theory predicts a systematic allocation of land to its highest use value in response to distance from centers of demand. We tested this theory to see if forest exploitation would expand through time and space as concentric waves, with each wave targeting lower value products. We used forest data along a transect from 10 to 220 km from Dar es Salaam (DES), Tanzania, collected at two points in time (1991 and 2005). Our predictions were confirmed: high-value logging expanded 9 kmxy(-1), and an inner wave of lower value charcoal production 2 kmxy(-1). This resource utilization is shown to reduce the public goods of carbon storage and species richness, which significantly increased with each kilometer from DES [carbon, 0.2 Mgxha(-1); 0.1 species per sample area (0.4 ha)]. Our study suggests that tropical forest degradation can be modeled and predicted, with its attendant loss of some public goods. In sub-Saharan Africa, an area experiencing the highest rate of urban migration worldwide, coupled with a high dependence on forest-based resources, predicting the spatiotemporal patterns of degradation can inform policies designed to extract resources without unsustainably reducing carbon storage and biodiversity.

Fig. 1.

Map of the degradation waves of dominant forest use in the study area in 1991 and 2005. The numbered forests were those used in the study (with the exception of no. 9 because of dangerous field conditions). Charcoal burning has moved a road distance of 30 km from DES in this time period, and medium-value timber logging has moved 160 km. The outer boundary of high-value timber logging was already outside the study area in 1991. Muhoro and Nyamwagne, the two forests south of the Rufiji River (green), do not follow the general degradation pattern because they are plantation forests.

Fig. 2.

Patterns in forest use and condition at increasing distance from DES. (A) Forest use at increasing distance from DES, quantified as the numbers of stumps of trees used for charcoal burning (R² = 0.73) or as low-, medium-, or high-value timber (R² = 0.01, 0.41, and 0.74, respectively), presented as interpolated lines using a Loess function (span = 2). (B) Forest condition with distance from DES measured using forest structure (basal area, average stump diameter, standing timber, and above-ground live tree or standing carbon) and economic (average stump value) variables. (C) Estimated tree species richness at differing distances from DES using observed species richness (Mao Tau Index) and total species richness (Chao 1 and Jaccard 2 Indices) estimators, each randomized 50 times over eight transect sections (total sample area = 0.4 ha). (D) Kisiju forest at a distance of 90 km from DES has almost been destroyed since 1991, and all woody resources have been converted to charcoal. (E) Illegally logged high-value timber harvested at a distance of 200 km from DES. For the production of A, B, and C, n = 8 independent data points (forests), respectively.