Abstract
African savannas and dry forests represent a large, but poorly quantified store of biomass carbon and biodiversity. Improving this information is hindered by a lack of recent forest inventories, which are necessary for calibrating earth observation data and for evaluating the relationship between carbon stocks and tree diversity in the context of forest conservation (for example, REDD+). Here, we present new inventory data from south-eastern Tanzania, comprising more than 15,000 trees at 25 locations located across a gradient of aboveground woody carbon (AGC) stocks. We find that larger trees disproportionately contribute to AGC, with the largest 3.7% of individuals containing half the carbon. Tree species diversity and carbon stocks were positively related, implying a potential functional relationship between the two, and a ‘win–win’ scenario for conservation; however, lower biomass areas also contain diverse species assemblages meaning that carbon-oriented conservation may miss important areas of biodiversity. Despite these variations, we find that total tree abundance and biomass is skewed towards a few locally dominant species, with eight and nine species (5.7% of the total) accounting for over half the total measured trees and carbon, respectively. This finding implies that carbon production in these areas is channelled through a small number of relatively abundant species. Our results provide key insights into the structure and functioning of these heterogeneous ecosystems and indicate the need for novel strategies for future measurement and monitoring of carbon stocks and biodiversity, including the use for larger plots to capture spatial variations in large tree density and AGC stocks, and to allow the calibration of earth observation data.
Highlights
Dry tropical forests and woodlands are the dominant vegetation cover in southern Africa, extending over 4 million km2 across 10 countries (Mayaux and others 2004)
Our landscape-level estimates of aboveground carbon (AGC) stocks (24 ± 16 tC ha-1) are similar to those recorded using similar approaches in Mozambique by Ryan et al (2011) (21 ± 11 tC ha-1) and Woollen and others (2012) (21 ± 10 tC ha-1), but lower than the regional average (28.7 ± 19.1 tC ha-1) (Ryan and others 2016) which includes many plots from protected areas which are unlikely to be representative of the wider miombo eco-region
The results highlight the obvious importance of maintaining a low DBH threshold in lower biomass stands in order to capture and quantify the majority of aboveground woody carbon stocks (AGC) stocks
Summary
Dry tropical forests and woodlands are the dominant vegetation cover in southern Africa, extending over 4 million km across 10 countries (Mayaux and others 2004). The range of species supported by the ecosystem helps to underpin the livelihoods of an estimated 150 million rural and urban dwellers who rely heavily on the timber, food, medicine and construction materials that the woodlands and forests provide (Ryan and others 2016). Despite their scale and importance for local livelihoods, the ecology and functioning of these seasonally dry ecosystems remain poorly studied in comparison with the more carbon dense moist tropical forests in South America (Fauset and others 2015; Poorter and others 2015), and to a lesser extent, those in Central Africa (Lewis and others 2013). Many important ecological questions remain poorly resolved, for example, around the magnitude and distribution of aboveground woody carbon stocks (AGC) across these heterogeneous landscapes, and how this relates to patterns in vegetation structure, tree species diversity and composition
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