NSFDEB-NERC: Addressing the plant growth source-sink debate through observations, experiments, and modelling

化石燃料燃烧导致大气中二氧化碳（CO2）浓度上升。然而，二氧化碳的上升速度远远低于排放量所显示的速度。陆地表面和海洋共同吸收了每年约50%的二氧化碳排放量。一个合理的解释是，土地吸收的发生是因为更高水平的二氧化碳增加了植物的光合作用，这意味着更多的植物有碳可用于支持生长。然而，另一种解释认为树木的生长并不受光合作用本身的限制，而是受其他因素的控制，如细胞分裂、营养或供水。树木受碳限制的程度以及在何种情况下，对于理解它们将如何应对大气中二氧化碳水平的上升至关重要。这个项目将提高我们对这一及时问题的认识。在生长季节的每个星期，研究人员都将在微观水平上量化木材生长和细胞发育的速率。将从马萨诸塞州的哈佛森林的主要树种中采集样本。这项工作将持续三年，以研究每年天气的变化如何影响木材生长的时间和数量，以及不同物种之间的差异。研究人员还将进行一项新的实验，以控制树木内的碳流，从而研究当地碳供应与木材生长之间的关系。因此，该项目将直接解决树木生长是否受碳限制的问题。这项研究有可能彻底改变我们对植被在全球碳循环中的作用，环境变化对植物的影响以及我们对树木年轮中记录的过去气候的解释的理解。该项目将支持博士后学者的高级研究培训，并涉及与英国科学家的合作。该项目将通过实地观察，实验和建模，调查树木生长对大气CO2增加的响应潜力。研究将侧重于有争议的问题，即树木的生长是由碳的供应（即来自光合作用和/或木材中的非结构性碳储存，因此“来源有限”）还是生长过程本身（即“库有限”）控制。研究人员将通过仔细测量生长在马萨诸塞州哈佛森林的三种广泛分布的温带物种（即白松，红橡树和红枫）的成熟树木内的碳的内部动态，首次在整棵树的尺度上梳理木材生长的控制。在一项观察性研究中，研究人员将在细胞水平上测量非结构性碳储存动态和木材生长的细节。在一项实验研究中，研究人员将使用冷块韧皮部冷却环操纵新的光合产物向生长区域的运输，从而能够得出关于木材生长和碳供应之间关键关系的新推论。随后，使用收集的数据和派生知识开发的模型将用于研究这些结果对历史和未来陆地碳汇的影响。

Fossil fuel burning is causing atmospheric concentrations of carbon dioxide (CO2) to rise. However, the rate of CO2 rise is much slower than emissions suggest it should be. It appears that the land surface and oceans are together absorbing about 50% of annual CO2 emissions. A plausible explanation is that the land uptake is occurring because higher levels of CO2 increase plant photosynthesis, meaning more plants have carbon available to support growth. However, a competing explanation proposes that tree growth is not limited by the rate of photosynthesis itself, but is instead controlled by other factors such as rates of cell division, nutrients, or water supply. The extent to which trees are carbon-limited, and under which circumstances, is fundamental to understanding how they will respond to rising levels of atmospheric CO2. This project will improve our understanding of this timely issue. Each week during the growing season, the researchers will quantify rates of wood growth and cellular development at the microscopic level. Samples will be collected from the dominant tree species at Harvard Forest, Massachusetts. This will be done for three years, to examine how year-to-year variation in weather influences the timing and amount of wood growth, and how this differs among species. The researchers will also conduct a novel experiment to manipulate the flow of carbon within the tree, and thus investigate the relationships between local carbon supply and wood growth. This project will thus directly address the question of whether or not tree growth is carbon-limited. The research has the potential to revolutionize our understanding of the role of vegetation in the global carbon cycle, the impacts of environmental change on plants, and our interpretations of past climates as recorded in tree growth rings. This project will support the advanced research training of a postdoctoral scholar, and involves a collaboration with UK scientists.Using field observations, experiments, and modeling, this project will investigate the potential for tree growth to respond to increasing atmospheric CO2. Research will focus on the contentious issue of whether tree growth is controlled by the supply of carbon (i.e. from photosynthesis and/or non-structural carbon stores in wood, hence "source-limited") or the growth processes themselves (i.e. "sink-limited"). The researchers will tease apart the controls on wood growth at the whole-tree scale for the first time through careful measurements of the internal dynamics of carbon within mature trees of three widely-distributed temperate species (i.e. white pine, red oak, and red maple) growing in Harvard Forest, Massachusetts. In an observational study, the researchers will measure non-structural carbon storage dynamics and details of wood growth at the cellular level. In an experimental study, the researchers will manipulate the transport of new photosynthetic products to growing regions using cold-block phloem chilling collars, and will thus be able to draw novel inferences concerning key relationships between wood growth and carbon supply. Subsequently, a model developed using the collected data and derived knowledge will be used to examine the implications of these results for the historical and future terrestrial carbon sink.