Agricultural fallows are the main driver of natural forest regeneration in Tanzania

Abstract

Rates and drivers of natural forest regeneration are areas of uncertainty for policy, forest management and climate change mitigation. In this study, the rate of deforestation and the rate and drivers of natural regeneration are described for 56 million hectares of village land in Tanzania, a country undergoing rapid deforestation. To determine the regeneration and deforestation rates, remote sensing (RS) data for 500 randomly selected points were reviewed for a 34 year period from 1987 to 2021 using Google Earth Engine. Over this period, regeneration, involving a transition from forest to non-forest and back to forest was detected on 4.8% of village land (95% CI: 3.1%-7.1%), while 0.8% of land transitioned from non-forest to forest (95% CI: 0.2%-2.04%). 22% of village land was deforested (95% CI: 18.6%-26.1%), equivalent to a mean annual net loss of 0.35 million hectares of forest. Using a combination of RS data, field plots and structured interviews, the land cover change trajectories of 180 regenerating plots, in 10 sampling clusters, were assessed to identify regeneration drivers and assess biomass and tree species accumulation rates. Agricultural fallows are the regeneration driver in 47% of plots (95% CI: 39.8%-54.8%). Other common regeneration drivers include abandonment of cultivated areas for reasons apart from fallowing, conservation and post wood-extraction abandonment in 19% (95% CI: 13.9%-26%), 18.3% (95% CI: 13%-24.8%) and 12.8% (95% CI: 8.3%-18.6%) of plots, respectively. The mean carbon sequestration rate was 1.4 Mg C ha−1 y−1, equivalent to 4.3 Tg C y−1 (95% CI: 3.9-4.7 Tg C y−1) across the 3.15 million hectares of regenerating village land forest. The mean species accumulation rate was 1.08 species y−1 (95% CI: 1.0-1.2). Regeneration time, location and precipitation have the greatest influence on biomass and species richness. The study highlights the potential for natural regeneration to contribute to global and national climate and biodiversity goals and to sustainable, productive forest management. The importance of cooperation and policy-alignment between the forest, agriculture and land sectors are under-scored.