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？为什么一个地方的一些树比其他的长得快？第一印象似乎是一个合理的假设，即“可见生产力”（例如木材生产和树冠凋落物）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。当我们看到解释为什么森林在应对全球变化时增长率增加，或者在干扰后生产力增加时，我们倾向于将这些解释置于光合作用增加的背景下（要么是因为非生物驱动因素增加--例如光或二氧化碳增加，要么是因为光合能力增加，例如叶片氮含量）然而，我们最近在亚马逊河流域的研究表明，在低地雨林中，净初级生产力（NPP）的地点与地点之间的变化与森林通过光合作用捕获多少碳和能量无关，但更多地取决于植物将多少捕获的碳用于其内部代谢（Malhi等人，提交给Nature）、自养呼吸、Ra.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，我们的早期研究结果表明，干扰是多少生态系统分配到自养呼吸的主要决定因素，与较少的自养呼吸干扰系统。我们现在想从五个方面进一步探讨这一主题：（一）通过更详细地探讨自养呼吸的空间和时间变化;（二）通过大大增加调查地点的数量;（三）通过评估亚马逊河流域的结果在另一个地理学领域，即赤道非洲的推广程度;（iv）明确探索干扰如何影响碳的使用和分配，通过跟踪这些之前和之后的选择性记录;（v）通过探索有多少种间变异的NPP是由自养呼吸。我们正在探索的基本假设是：（i）潮湿热带低地森林的GPP没有显著的站点间差异（在非洲境内，与亚马逊河流域相比），尽管土壤性质、气候和树种组成存在差异;（ii）不同地点的净初级生产力差异很大，这主要是由碳利用效率的变化所驱动的（CUE，光合碳转化为生物量的比例），和（iii）森林CUE大幅增加后，干扰（伐木），随后随着时间的推移而下降，和（iv）这种转变是由不同的塑料变化CUE内幸存的个人，而不是由社区更换。在这一过程中，我们将率先对非洲完整和受干扰的热带森林进行全面的碳循环评估，并在加纳（西非）和加蓬（中非）这两个对比鲜明的国家进行复制。我们的抽样策略将包括以下地块：（一）潮湿原始林（2个国家x 2块地块），（二）潮湿原始林（2个国家x 2块地块），（三）在伐木干扰之前、期间和之后的跟踪地点（2个国家x 2块地块），以及（四）从10年、15年和20年前的伐木干扰中恢复过来的地块（2个国家x 2块地块）。在所有研究中心，我们将收集2.0-2.5年的数据。我们的项目将在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碳循环科学的广泛培训研讨会。大会及其他人

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.