Does shifting Carbon Use Efficiency determine the growth rates of intact and disturbed tropical forests? Gathering new evidence from African forests

Why do trees in different tropical forests grow at different rates? Why do some trees within a site grow faster than others? At first impression, It seems a reasonable assumption that the 'visible productivity' (e.g. wood production and canopy litterfall) is somehow related to how much carbon and energy the forest or the individual tree captures from photosynthesis, the Gross Primary Productivity (GPP); this assumption is implicit in much of the forest ecology literature, as well as in many biosphere models. When we see explanations as why forests are increasing growth rates in response to global change, or increased productivity after disturbance, we tend to frame these explanations in the context of increased photosynthesis (either because of increased abiotic drivers - e.g. increased light or carbon dioxide, or because of increased photosyntheric capacity, e.g leaf nitrogen content) However, our recent work in Amazonia has indicated that the site-to-site variability in net primary productivity (NPP) in lowland rainforests is not related to how much carbon and energy the forest captures through photosynthesis, but much more determined by how much of that captured carbon used by plants for their internal metabolism (Malhi et al., submitted to Nature), the autotrophic respiration, Ra. This tentative finding has consequences for much of tropical forest research, and global change vegetation models. Moreover, our early results suggest that disturbance is the main determinant of how much an ecosystem allocates to autotrophic respiration, with less autotrophic respiration in disturbed systems. We would now like to explore this topic further in five ways: (i) by exploring in greater detail the spatial and temporal variation of autotrophic respiration; (ii) by greatly increasing the number of sites investigated; (iii) by assessing the extent to which results from Amazonia are generalisable in another biogeographical realm, namely equatorial Africa; (iv) by explicitly exploring how disturbance affects carbon use and allocation by tracking these before and after selective logging; (v) by exploring how much interspecific variation in NPP is determined by autotrophic respiration. The underlying hypotheses we are exploring are that (i) there is no significant site-to-site variation in the GPP of moist tropical lowland forests (within Africa and in comparison to Amazonia), despite variation is soil properties, climate and tree species composition; (ii) there is substantial site-to-site variation in net primary productivity (NPP), and this is mainly driven by shifts in carbon use efficiency (CUE, the proportion of photosynthetic carbon converted to biomass), and (iii) forest CUE increases substantially after disturbance (logging) and subsequently declines over time, and (iv) this shift is driven by differing plastic variation in CUE within surviving individuals, rather than by community replacement. In the process, we will pioneer comprehensive carbon cycle assessment in intact and disturbed African tropical forests, replicated across two contrasting countries, Ghana (West Africa) and Gabon (Central Africa). Our sampling strategy will encompass plots in (i) wet primary forests (2 countries x 2 plots), (ii) moist primary forests (2 countries x 2 plots),(iii) tracking sites before, during and after logging disturbance (2 countries x 2 plots), and (iv) plots recovering from logging disturbance 10, 15 and 20 years ago (2 countries x 2 plots). At all sites we will collect 2.0-2.5 years of data. Our project will provide substantial scientific capacity building in Ghana and Gabon,we will train and utilise 6 student field researchers (3 full time, 3 part-time) in each country, and hold wider-reach training workshops in carbon cycle science in each country at the start and end of the project. this event.