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

化石燃料的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比排放所显示的要慢得多。陆地表面和海洋似乎共同吸收了每年约50%的二氧化碳排放量。一种合理的解释是，土地吸收正在发生，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可用的碳来支持生长。然而，一种与之竞争的解释提出，树木的生长并不受光合作用本身的速度限制，而是受其他因素的控制，如细胞分裂的速度、营养物质或水的供应。树木的碳限制程度，以及在什么情况下，是了解它们如何应对大气中二氧化碳水平上升的基础。这个项目将提高我们对这个及时问题的认识。在生长季节的每个星期，研究人员将在微观水平上量化木材生长和细胞发育的速度。样本将从马萨诸塞州哈佛森林的优势树种中收集。这项研究将持续三年，以研究每年的气候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来控制树木内部的碳流，从而研究当地碳供应与木材生长之间的关系。因此，这个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记录的过去气候的解释的理解。该项目将支持一名博士后的高级研究培训，并与英国科学家合作。通过实地观测、实验和建模，该项目将调查树木生长对大气中二氧化碳增加的响应潜力。研究将集中在有争议的问题上，即树木的生长是由碳供应（即来自光合作用和/或木材中的非结构碳储存，因此“来源有限”）还是生长过程本身（即树木的生长）控制。“sink-limited”)。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树种（即白松、红橡树和红枫）成熟树木的碳内部动态，首次在整棵树的尺度上梳理出对木材生长的控制。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，研究人员将利用冷阻韧皮部冷却项圈来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，利用收集到的数据和获得的知识开发的一个模型将用于检查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.