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

为什么不同热带森林的树木生长速度不同？为什么一个站点内的一些树比其他树长得快？乍一看，这似乎是一个合理的假设，即“可见生产力”（例如木材产量和树冠凋落物）在某种程度上与森林或单个树木从光合作用中捕获的碳和能量有关，即总初级生产力（GPP）；这一假设隐含在许多森林生态学文献中，以及许多生物圈模型中。当我们看到为什么森林在应对全球变化时增长速度加快，或者在受到干扰后生产力提高的解释时，我们倾向于在光合作用增加的背景下构建这些解释（要么是因为增加的非生物驱动因素-例如增加的光或二氧化碳，要么是因为增加的光合作用能力，例如叶片氮含量）。我们最近在亚马逊地区的研究表明，低地雨林净初级生产力（NPP）的不同地点差异与森林通过光合作用捕获的碳和能量的多少无关，而更多地取决于植物内部代谢（Malhi等人，提交给Nature）、自养呼吸（Ra）所捕获的碳的多少。这一初步发现对许多热带森林研究和全球变化植被模型产生了影响。此外，我们的早期结果表明，干扰是生态系统分配给自养呼吸多少的主要决定因素，在受干扰的系统中自养呼吸较少。我们现在想从五个方面进一步探讨这一主题：(i)通过更详细地探索自养呼吸的时空变化；（ii）大幅增加调查地点的数目；（iii）评估亚马逊地区的结果在多大程度上可以推广到另一个生物地理领域，即赤道非洲；（iv）通过在选择性采伐前后跟踪干扰对碳利用和分配的影响，明确探索干扰对碳利用和分配的影响；(v)探索NPP的种间变异在多大程度上是由自养呼吸决定的。我们正在探索的基本假设是：(i)尽管土壤性质、气候和树种组成存在差异，但潮湿的热带低地森林（非洲境内和与亚马逊河流域相比）的GPP没有显著的地点差异；（ii）净初级生产力（NPP）在不同地点之间存在显著差异，这主要是由碳利用效率（CUE，光合碳转化为生物量的比例）的变化驱动的；（iii）森林CUE在受到干扰（伐木）后大幅增加，随后随着时间的推移而下降；（iv）这种变化是由存活个体内CUE的不同可塑性变化驱动的，而不是由群落替代驱动的。在此过程中，我们将率先在完整的和受干扰的非洲热带森林中进行综合碳循环评估，并在两个截然不同的国家——加纳（西非）和加蓬（中非）——进行复制。我们的采样策略将包括以下地块：(i)潮湿原始森林（2个国家× 2个地块），（ii）潮湿原始森林（2个国家× 2个地块），（iii）伐木干扰之前、期间和之后的跟踪地点（2个国家× 2个地块），以及（iv）从10、15和20年前的伐木干扰中恢复的地块（2个国家× 2个地块）。在所有站点，我们将收集2.0-2.5年的数据。我们的项目将为加纳和加蓬提供大量的科学能力建设，我们将在每个国家培训和利用6名学生实地研究人员（3名全职，3名兼职），并在项目开始和结束时在每个国家举办更广泛的碳循环科学培训讲习班。这一事件。

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.